Routing, Routed, and Non-Routable Methods

ROUTING Methods

A normal term that describes an equation, or protocol, utilized by a router to look for the appropriate path that information is sent. The routing protocol also identifies how hubs inside a network share information with one another and report changes. The routing protocol allows a network to create dynamic changes to the conditions, so routing choices don't have to be predetermined and static.

Routing, Routed and Non-Routable Methods

ROUTING ROUTED NON-ROUTABLE

ROUTING Methods

ROUTING Methods would be the software that permit hubs to dynamically advertise and learn routes, pick which routes can be found and what are most effective routes to some destination. Routing methods utilized by the web Protocol suite include:

· Routing Information Protocol (RIP and RIP II).

· Open Least Path First (OSPF).

· Intermediate System to Intermediate System (IS-IS).

· Interrior Gateway Routing Protocol (IGRP).

· Cisco's Enhanced Interior Gateway Routing Protocol (EIGRP).

· Border Gateway Protocol (BGP).

Routing is the procedure of moving data across several systems. Inside a network, all hosts are directly accessible since they're on a single

ROUTED Methods

ROUTED Methods are simply data being moved over the systems. Routed methods include:

· Ip Address

o Telnet

o Remote Procedure Call (RPC)

o SNMP

o SMTP

· Novell IPX

· Open Standards Institute networking protocol

· DECnet

· Appletalk

· Banyan Vines

· Xerox Network System (XNS)

Outdoors a network, specialized products known as ROUTES are utilized to carry out the routing procedure for sending packets between systems. Hubs are attached to the edges of several systems to supply connectivity together. These products are often devoted machines with specialized software and hardware to accelerate the routing process. These products send and receive routing information to one another about systems that they'll and can't achieve. Hubs examine all routes to some destination, pick which routes possess the best metric, and place a number of routes in to the IP routing table around the router. By preserve a present listing of known routes, hubs can quicky and effectively send your data coming to you when received.

You will find a lot of companies that leave hubs: 'cisco', Juniper, Bay, Nortel, 3Com, Cabletron, etc. Each company's method is different in how it's set up, but many will interoperate as long as they share common physical and knowledge link layer methods ('cisco' HDLC or PPP over Serial, Ethernet etc.). Before buying a router for the business, check together with your Internet provider to determine what equipment they will use, and select a router, that will interoperate together with your Internet provider's equipment.

NON-ROUTABLE Methods

NON-ROUTABLE Methods cannot survive being routed. Non-routable methods presume that computer systems they'll ever contact are on a single network (to obtain them your routed atmosphere, you have to bridge the systems). The modern modern systems are not so loving toward methods that don't understand the idea of a multi-segment network and many of these methods are dying or receding useful.

· NetBEUI

· DLC

· LAT

· DRP

· MOP

RIP (Routing Information Protocol)

RIP is really a dynamic internetwork routing protocol primary utilized in interior routing conditions. An engaged routing protocol, instead of a static routing protocol, instantly finds out routes and develops routing tables. Interior conditions are usually private systems (autonomous systems). In comparison, exterior routing methods for example BGP are utilized to exchange route summaries between autonomous systems. BGP can be used among autonomous systems on the web.

RIP uses the length-vector formula produced by Bellman and Ford (Bellman-Ford formula).

Routing Information Protocol

Background

The Routing Information Protocol, or RIP, because it is more generally known as, is among the most long lasting of routing methods. RIP can also be one of the most easily confused methods because a number of RIP-like routing methods grown popular, most of which even used
exactly the same title! RIP and also the myriad RIP-like methods were in line with the same group of calculations which use distance vectors to mathematically compare routes to recognize the very best road to a destination address. These calculations emerged from academic research that goes back to 1957.

Present day open standard form of RIP, sometimes known to as IP RIP, is formally defined in 2 documents: Request Comments (RFC) 1058 and Internet Standard (STD) 56. As IP-based systems grew to become both more numerous and greater in dimensions, it grew to become apparent to the web Engineering Task Pressure (IETF) that RIP must be up-to-date. Consequently, the IETF launched RFC 1388 in The month of january 1993, that was then superceded in November 1994 by RFC 1723, which describes RIP 2 (the 2nd form of RIP). These RFCs referred to extra time of RIP's abilities but didn't make an effort to obsolete the prior form of RIP. RIP 2 enabled RIP messages to hold more details, which allowed using a simple authentication mechanism to secure table updates. More to the point, RIP 2 supported subnet masks, a vital feature which was unavailable in RIP.

This chapter summarizes the fundamental abilities featuring connected with RIP. Subjects range from the routing update process, RIP routing metrics, routing stability, and routing timers.

Routing Updates

RIP transmits routing-update messages at regular times so when the network topology changes. Whenever a router gets to be a routing update which includes changes for an entry, it updates its routing table to mirror the brand new route. The metric value for that path is elevated by 1, and also the sender is indicated because the next hop. RIP hubs maintain just the best route (the path using the cheapest metric value) to some destination. After upgrading its routing table, the router immediately starts transmitting routing updates to share with other network hubs from the change. These updates are sent individually from the regularly scheduled updates that RIP hubs send.

RIP Routing Metric

RIP utilizes a single routing metric (hop count) to determine the length between your source along with a destination network. Each hop inside a path from source to destination is designated a hop count value, that is typically 1. Whenever a router gets to be a routing update that consists of a brand new or transformed destination network entry, the router adds 1 towards the metric value suggested for the update and makes its way into the network within the routing table. The Ip from the sender can be used because the next hop.

RIP Stability Features

RIP prevents routing loops from ongoing indefinitely by applying a restriction on the amount of hops permitted inside a path in the source to some destination. The utmost quantity of hops inside a path is 15. If your router gets to be a routing update that consists of a brand new or transformed entry, and when growing the metric value by 1 causes the metric to become infinity (that's, 16), the network destination is recognized as unreachable. The down-side of the stability feature is it limits the utmost diameter of the RIP network to under 16 hops.

RIP includes many other stability features which are present with many routing methods. These functions are made to provide stability despite potentially rapid alterations in a network's topology. For instance, RIP implements the split horizon and holddown systems to avoid incorrect routing information from being propagated.

RIP Timers

RIP uses numerous timers to manage its performance. Included in this are a routing-update timer, a route-timeout timer, along with a route-flush timer. The routing-update timer clocks the interval between periodic routing updates. Generally, it's set to thirty seconds, having a small random period of time added whenever the timer is totally reset. This is accomplished to assist prevent congestion, that could derive from all hubs concurrently trying to update their neighbors. Each routing table entry includes a route-timeout timer connected by using it. Once the route-timeout timer expires, the path is marked invalid but is maintained within the table before the route-flush timer expires.

Packet Formats

The next section concentrates on the IP RIP and IP RIP 2 packet formats highlighted in Figures 44-1 and 44-2. Each illustration is then explanations from the fields highlighted.
RIP Packet Format

· Command&mdashIndicates if the packet is really a request or perhaps a response. The request asks that the router send any a part of its routing table. The response is definitely an unrequested regular routing update or perhaps a answer a request. Reactions contain routing table records. Multiple RIP packets are utilized to convey information from large routing tables.

· Version number&mdashSpecifies the RIP version used. This area can signal different potentially incompatible versions.

· Zero&mdashThis area isn't really utilized by RFC 1058 RIP it had been added exclusively to supply backward compatibility with prestandard types of RIP. Its title originates from its past due value: zero.

· Address-family identifier (AFI)&mdashSpecifies the address family used. RIP is made to carry routing information for many different methods. Each entry comes with an address-family identifier to point the kind of address being specified. The AFI for IP is 2.

· Address&mdashSpecifies the Ip for that entry.

· Metric&mdashIndicates the number of internetwork hops (hubs) happen to be traversed within the visit to the destination. This value is between 1 and 15 for any valid route, or 16 to have an unreachable route.

Note: As much as 25 occurrences from the AFI, Address, and Metric fields are allowed in one IP RIP packet. (As much as 25 locations could be listed in one RIP packet.)

RIP 2 Packet Format

· Command&mdashIndicates if the packet is really a request or perhaps a response. The request asks that the router send any part of its routing table. The response is definitely an unrequested regular routing update or perhaps a answer a request. Reactions contain routing table records. Multiple RIP packets are utilized to convey information from large routing tables.

· Version&mdashSpecifies the RIP version used. Inside a RIP packet applying the RIP 2 fields or using authentication, this value is placed to two.

· Unused&mdashHas something set to zero.

· Address-family identifier (AFI)&mdashSpecifies the address family used. RIPv2's AFI area functions in the same way to RFC 1058 RIP's AFI area, with one exception: When the AFI for that first entry within the message is 0xFFFF, the rest of the entry consists of authentication information. Presently, the only real authentication type is straightforward password.

· Route tag&mdashProvides a technique for distinguishing between internal routes (learned by RIP) and exterior routes (learned using their company methods).

· Ip&mdashSpecifies the Ip for that entry.

· Subnet mask&mdashContains the subnet mask for that entry. If the area is zero, no subnet mask continues to be specified for that entry.

·Next hop&mdashIndicates the Ip from the next hop that packets for that entry ought to be submitted.

· Metric&mdashIndicates the number of internetwork hops (hubs) happen to be traversed within the visit to the destination. This value is between 1 and 15 for any valid route, or 16 to have an unreachable route.

Note: As much as 25 occurrences from the AFI, Address, and Metric fields are allowed in one IP RIP packet. That's, as much as 25 routing table records could be listed in one RIP packet. When the AFI identifies an authenticated message, only 24 routing table records could be specified. Considering that individual table records aren't fragmented into multiple packets, RIP doesn't need a mechanism to resequence datagrams bearing routing table updates from neighboring hubs.

Summary

Despite RIP's age and also the emergence more sophisticated routing methods, it's not even close to obsolete. RIP is mature, stable, broadly supported, and simple to configure. Its simplicity is perfect for use within stub systems as well as in small autonomous systems that don't have enough redundant pathways to warrant the expenses of a more elaborate protocol.

Review Questions

Q&mdashName RIP's various stability features.

A&mdashRIP has several stability features, probably the most apparent being RIP's maximum hop count. By putting a finite limit on the amount of hops that the route may take, routing loops are frustrated, it not exclusively removed. Other stability features include its various timing systems which help be sure that the routing table consists of only valid routes, in addition to split horizon and holddown systems that prevent incorrect routing information from being disseminated through the network.

Q&mdashWhat is the objective of the timeout timer?

A&mdashThe timeout timer can be used to assist purge invalid routes from the RIP node. Routes that are not rejuvenated for any given time period are most likely invalid due to some alternation in the network. Thus, RIP keeps a timeout timer for every known route. Whenever a route's timeout timer expires, the path is marked invalid but is maintained within the table before the route-flush timer expires.

Q&mdashWhat two abilities are based on RIP 2 although not RIP?

A&mdashRIP 2 allows using a simple authentication mechanism to secure table updates. More to the point, RIP 2 supports subnet masks, a vital feature that's unavailable in RIP.

Q&mdashWhat may be the maximum network diameter of the RIP network?

A&mdashA RIP network's maximum diameter is 15 hops. RIP can count to 16, but that value is recognized as a mistake condition as opposed to a valid hop count.

Network Routing Methods - IGRP, EIGRP, OSPF, ISIS, BGP

Overview

The purpose of routing protocols is to learn of available routes that exist on the enterprise network, build routing tables and make routing decisions. Some of the most common routing protocols include RIP, IGRP, EIGRP, OSPF, IS-IS and BGP. There are two primary routing protocol types although many different routing protocols defined with those two types. Link state and distance vector protocols comprise the primary types. Distance vector protocols advertise their routing table to all directly connected neighbors at regular frequent intervals using a lot of bandwidth and are slow to converge. When a route becomes unavailable, all router tables must be updated with that new information. The problem is with each router having to advertise that new information to its neighbors, it takes a long time for all routers to have a current accurate view of the network. Distance vector protocols use fixed length subnet masks which aren't scalable. Link state protocols advertise routing updates only when they occur which uses bandwidth more effectively. Routers don't advertise the routing table which makes convergence faster. The routing protocol will flood the network with link state advertisements to all neighbor routers per area in an attempt to converge the network with new route information. The incremental change is all that is advertised to all routers as a multicast LSA update. They use variable length subnet masks, which are scalable and use addressing more efficiently.

Interior Gateway Routing Protocol (IGRP)

Interior Gateway Routing Protocol is a distance vector routing protocol developed by Cisco systems for routing multiple protocols across small and medium sized Cisco networks. It is proprietary which requires that you use Cisco routers. This contrasts with IP RIP and IPX RIP, which are designed for multi-vendor networks. IGRP will route IP, IPX, Decnet and AppleTalk which makes it very versatile for clients running many different protocols. It is somewhat more scalable than RIP since it supports a hop count of 100, only advertises every 90 seconds and uses a composite of five different metrics to select a best path destination. Note that since IGRP advertises less frequently, it uses less bandwidth than RIP but converges much slower since it is 90 seconds before IGRP routers are aware of network topology changes. IGRP does recognize assignment of different autonomous systems and automatically summarizes at network class boundaries. As well there is the option to load balance traffic across equal or unequal metric cost paths.

Characteristics

• Distance Vector

• Routes IP, IPX, Decnet, Appletalk

• Routing Table Advertisements Every 90 Seconds

• Metric: Bandwidth, Delay, Reliability, Load, MTU Size

• Hop Count: 100

• Fixed Length Subnet Masks

• Summarization on Network Class Address

• Load Balancing Across 6 Equal or Unequal Cost Paths ( IOS 11.0 )

• Update Timer: 90 seconds

• Invalid Timer: 270 seconds

• Holddown Timer: 280 seconds

• Metric Calculation = destination path minimum bandwidth x delay (usec)

• Split Horizon

Enhanced Interior Gateway Routing Protocol (EIGRP)

Enhanced Interior Gateway Routing Protocol is a hybrid routing protocol developed by Cisco systems for routing many protocols across an enterprise Cisco network. It has characteristics of both distance vector routing protocols and link state routing protocols. It is proprietary which requires that you use Cisco routers. EIGRP will route the same protocols that IGRP routes (IP, IPX, Decnet and Appletalk) and use the same composite metrics as IGRP to select a best path destination. As well there is the option to load balance traffic across equal or unequal metric cost paths. Summarization is automatic at a network class address however it can be configured to summarize at subnet boundaries as well. Redistribution between IGRP and EIGRP is automatic as well. There is support for a hop count of 255 and variable length subnet masks.

Convergence

Convergence with EIGRP is faster since it uses an algorithm called dual update algorithm or DUAL, which is run when a router detects that a particular route is unavailable. The router queries its neighbors looking for a feasible successor. That is defined as a neighbor with a least cost route to a particular destination that doesn't cause any routing loops. EIGRP will update its routing table with the new route and the associated metric. Route changes are advertised only to affected routers when changes occur. That utilizes bandwidth more efficiently than distance vector routing protocols.

Autonomous Systems

EIGRP does recognize assignment of different autonomous systems which are processes running under the same administrative routing domain. Assigning different autonomous system numbers isn't for defining a backbone such as with OSPF. With IGRP and EIGRP it is used to change route redistribution, filtering and summarization points.

Characteristics

• Advanced Distance Vector

• Routes IP, IPX, Decnet, Appletalk

• Routing Advertisements: Partial When Route Changes Occur

• Metrics: Bandwidth, Delay, Reliability, Load, MTU Size

• Hop Count: 255

• Variable Length Subnet Masks

• Summarization on Network Class Address or Subnet Boundary

• Load Balancing Across 6 Equal or Unequal Cost Paths (IOS 11.0)

• Hello Timer: 5 seconds on Ethernet / 60 seconds on Non-Broadcast

• Holddown Timer: 15 seconds on Ethernet / 180 seconds on Non-Broadcast

• Metric Calculation = destination path minimum bandwidth x delay (msec) x 256

• Split Horizon

• LSA Multicast Address: 224.0.0.10

Open Shortest Path First (OSPF)

Open Shortest Path First is a true link state protocol developed as an open standard for routing IP across large multi-vendor networks. A link state protocol will send link state advertisements to all connected neighbors of the same area to communicate route information. Each OSPF enabled router, when started, will send hello packets to all directly connected OSPF routers. The hello packets contain information such as router timers, router ID and subnet mask. If the routers agree on the information they become OSPF neighbors. Once routers become neighbors they establish adjacencies by exchanging link state databases. Routers on point-to-point and point-to-multipoint links (as specified with the OSPF interface type setting) automatically establish adjacencies. Routers with OSPF interfaces configured as broadcast (Ethernet) and NBMA (Frame Relay) will use a designated router that establishes those adjacencies.

Areas

OSPF uses a hierarchy with assigned areas that connect to a core backbone of routers. Each area is defined by one or more routers that have established adjacencies. OSPF has defined backbone area 0, stub areas, not-so-stubby areas and totally stubby areas. Area 0 is built with a group of routers connected at a designated office or by WAN links across several offices. It is preferable to have all area 0 routers connected with a full mesh using an Ethernet segment at a core office. This provides for high performance and prevents partitioning of the area should a router connection fail. Area 0 is a transit area for all traffic from attached areas. Any inter-area traffic must route through area 0 first. Stub areas use a default route injected from the ABR to forward traffic destined for any external routes (LSA 5,7) to the area border router. Inter-area (LSA 3,4) and intra-area (LSA 1,2) routing is as usual. Totally stubby areas are a Cisco specification that uses a default route injected from the ABR for all inter-area and external routes. The totally stubby area doesn't send or receive external or inter-area LSA's. The not-so-stubby area ABR will advertise external routes with type 7 LSA. External routes aren't received at that area type. Inter-area and intra-area routing is as usual. OSPF defines internal routers, backbone routers, area border routers (ABR) and autonomous system boundary routers (ASBR). Internal routers are specific to one area. Area border routers have interfaces that are assigned to more than one area such as area 0 and area 10. An autonomous system boundary router has interfaces assigned to OSPF and a different routing protocol such as EIGRP or BGP. A virtual link is utilized when an area doesn't have a direct connection to area 0. A virtual link is established between an area border router for an area that isn't connected to area 0, and an area border router for an area that is connected to area 0. Area design involves considering geographical location of offices and traffic flows across the enterprise. It is important to be able to summarize addresses for many offices per area and minimize broadcast traffic.

Convergence

Fast convergence is accomplished with the SPF (Dijkstra) algorithm which determines a shortest path from source to destination. The routing table is built from running SPF which determines all routes from neighbor routers. Since each OSPF router has a copy of the topology database and routing table for its particular area, any route changes are detected faster than with distance vector protocols and alternate routes are determined.

Designated Router

Broadcast networks such as Ethernet and Non-Broadcast Multi Access networks such as Frame Relay have a designated router (DR) and a backup designated router (BDR) that are elected. Designated routers establish adjacencies with all routers on that network segment. This is to reduce broadcasts from all routers sending regular hello packets to its neighbors. The DR sends multicast packets to all routers that it has established adjacencies with. If the DR fails, it is the BDR that sends multicasts to specific routers. Each router is assigned a router ID, which is the highest assigned IP address on a working interface. OSPF uses the Router ID (RID) for all routing processes.

Characteristics

• Link State

• Routes IP

• Routing Advertisements: Partial When Route Changes Occur

• Metric: Composite Cost of each Router to Destination (100,000,000/interface speed)

• Hop Count: None (Limited by Network)

• Variable Length Subnet Masks

• Summarization on Network Class Address or Subnet Boundary

• Load Balancing Across 4 Equal Cost Paths

• Router Types: Internal, Backbone, ABR, ASBR

• Area Types: Backbone, Stubby, Not-So-Stubby, Totally Stubby

• LSA Types: Intra-Area (1,2) Inter-Area (3,4), External (5,7)

• Hello Timer Interval: 10 seconds for Ethernet / 30 seconds for Non-Broadcast

• Dead Timer Interval: 40 seconds for Ethernet / 120 seconds for Non-Broadcast

• LSA Multicast Address: 224.0.0.5 and 224.0.0.6 (DR/BDR) Don't Filter!

• Interface Types: Point to Point, Broadcast, Non-Broadcast, Point to Multipoint, Loopback

Integrated IS-IS

Integrated Intermediate System - Intermediate System routing protocol is a link state protocol similar to OSPF that is used with large enterprise and ISP customers. An intermediate system is a router and IS-IS is the routing protocol that routes packets between intermediate systems. IS-IS utilizes a link state database and runs the SPF Dijkstra algorithm to select shortest paths routes. Neighbor routers on point to point and point to multipoint links establish adjacencies by sending hello packets and exchanging link state databases. IS-IS routers on broadcast and NBMA networks select a designated router that establishes adjacencies with all neighbor routers on that network. The designated router and each neighbor router will establish an adjacency with all neighbor routers by multicasting link state advertisements to the network itself. That is different from OSPF, which establishes adjacencies between the DR and each neighbor router only. IS-IS uses a hierarchical area structure with level 1 and level 2 router types. Level 1 routers are similar to OSPF intra-area routers, which have no direct connections outside of its area. Level 2 routers comprise the backbone area which connects different areas similar to OSPF area 0. With IS-IS a router can be an L1/L2 router which is like an OSPF area border router (ABR) which has connections with its area and the backbone area. The difference with IS-IS is that the links between routers comprise the area borders and not the router.

Each IS-IS router must have an assigned address that is unique for that routing domain. An address format is used which is comprised of an area ID and a system ID. The area ID is the assigned area number and the system ID is a MAC address from one of the router interfaces. There is support for variable length subnet masks, which is standard with all link state protocols. Note that IS-IS assigns the routing process to an interface instead of a network.

Characteristics

• Link State

• Routes IP, CLNS

• Routing Advertisements: Partial When Routing Changes Occur

• Metric: Variable Cost (default cost 10 assigned to each interface)

• Hop Count: None (limited by network)

• Variable Length Subnet Masks

• Summarization on Network Class Address or Subnet Boundary

• Load Balancing Across 6 Equal Cost Paths

• Hello Timer Interval: 10 seconds

• Dead Timer Interval: 30 seconds

• Area Types: Hierarchical Topology similar to OSPF

• Router Types: Level 1 and Level 2

• LSP Types: Internal L1 and L2, External L2

• Designated Router Election, No BDR

Border Gateway Protocol (BGP)

Border Gateway Protocol is an exterior gateway protocol, which is different from the interior gateway protocols discussed so far. The distinction is important since the term autonomous system is used somewhat differently with protocols such as EIGRP than it is with BGP. Exterior gateway protocols such as BGP route between autonomous systems, which are assigned a particular AS number. AS numbers can be assigned to an office with one or several BGP routers. The BGP routing table is comprised of destination IP addresses, an associated AS-Path to reach that destination and a next hop router address. The AS-Path is a collection of AS numbers that represent each office involved with routing packets. Contrast that with EIGRP, which uses autonomous systems as well. The difference is their autonomous systems refer to a logical grouping of routers within the same administrative system. An EIGRP network can configure many autonomous systems. They are all managed by the company for defining route summarization, redistribution and filtering. BGP is utilized a lot by Internet Service Providers (ISP) and large enterprise companies that have dual homed internet connections with single or dual routers homed to the same or different Internet Service Providers. BGP will route packets across an ISP network, which is a separate routing domain that is managed by them. The ISP has its own assigned AS number, which is assigned by InterNIC. New customers can either request an AS assignment for their office from the ISP or InterNIC. A unique AS number assignment is required for customers when they connect using BGP. There are 10 defined attributes that have a particular order or sequence, which BGP utilizes as metrics to determine the best path to a destination. Companies with only one circuit connection to an ISP will implement a default route at their router, which forwards any packets that are destined for an external network. BGP routers will redistribute routing information (peering) with all IGP routers on the network (EIGRP, RIP, OSPF etc) which involve exchange of full routing tables. Once that is finished, incremental updates are sent with topology changes. The BGP default keepalive timer is 60 seconds while the hold timer is 180 seconds. Each BGP router can be configured to filter routing broadcasts with route maps instead of sending/receiving the entire internet routing table.

Characteristics

• Path Vector

• Routes IP

• Routing Advertisements: Partial When Route Changes Occur

• Metrics: Weight, Local Preference, Local Originated, AS-Path, Origin Type, MED

• Hop Count: 255

• Variable Length Subnet Masks

• Summarization on Network Class Address or Subnet Boundary

• Load Balancing Across 6 Equal Cost Paths

• Keepalive Timer: 60 seconds

• Holddown Timer: 180 seconds

• Designated Router: Route Reflector

BGP Routing Table Components

• Destination IP Address / Subnet Mask

• AS-Path

• Next Hop IP Address

'cisco' Routing Protocol - Fundamental Stuff

CCNA Routing

Layer-3 devices are generically called routers. Routers basically have two functions:

1. To find a layer-3 path to a destination network
2. To move packets from one interface to another to get a packet to its destination

In order to accomplish the first function, a router will need to:

- Learn about routers it is connected to in order to learn the networks that they know about
- Find locations of destination network numbers
- Choose a best path to each destination
- Maintain the most up-to-date routing information about how to reach destination networks

Types of Routes

A router can learn a route via one of two methods: static and dynamic. A static route is a route that is manually configured on the router. Dynamic routes are routes that a router learns by running a routing protocol. Routing protocols will learn about routes from other neighboring routers running the same routing protocol.

Administrative Distance

Administrative distance ranks the IP routing protocols, assigning a value, or weight, to each protocol. Distances can range from 0 to 255. A smaller distance is more believable by a router, with the best distance being 0 and the worst, 255.

Static Route Configuration

Router(config)# ip route destination_network_# [subnet_mask] IP_address_of_next_hop_neighbor
[administrative_distance]
or
Router(config)# ip route destination_network_# [subnet_mask] interface_to_exit [administrative_distance]

Default Route Configuration

A default route is a special type of static route.Where a static route specifies a path a router should use to reach a specific destination, a default route specifies a path the router should use if it doesn't know how to reach the destination.

Router(config)# ip route 0.0.0.0 0.0.0.0 IP_address_of_next_hop_neighbor [administrative distance]
Or
Router(config)# ip route 0.0.0.0 0.0.0.0 interface_to_exit [administrative_distance]

Dynamic Routing Protocols

Dynamic routing protocols learn about destination networks from neighboring routers. Dynamic routing protocols fall under one of three categories: distance vector, link state, and hybrid. Because of the differences between the various routing protocol types, each has advantages and disadvantages.

Distance Vector Protocols

Distance vector protocols are the simplest. Distance vector routing protocols use the distance and direction (vector) to find paths to destinations. Most distance vector protocols use the Bellman-Ford algorithm for finding paths to networking destinations. Some examples of IP routing protocols that are distance vector are RIPv1 and IGRP. Distance vector protocols periodically use local broadcasts with a destination IP address of 255.255.255.255 to share routing information.

Processing Updates

When a distance vector protocol receives a routing update, it performs these steps:

1. Increment the metrics of the incoming routes in the advertisement (for IP
RIP, add 1 to the hop count).
2. Compare the network numbers in the routing update from the neighbor
to what the router has in its routing table.
3. If the neighbor's information is better, place it in the routing table and remove the old entry.
4. If the neighbor's information is worse, ignore it.
5. If the neighbor's information is exactly the same as the entry already in the table, reset the timer for the entry in the routing table (in other words, the router already learned about this route from the same neighbor).
6. If the neighbor's information is a different path to a known destination network, but with the same metric as the existing network in the routing table, the router will add it to the routing table along with the old one. This assumes you have not exceeded the maximum number of equal-cost paths for this destination network number.

The six steps are generally referred to as the Bellman-Ford algorithm.

Link State Protocols

Link state protocols use an algorithm called the Shortest Path First (SPF) algorithm, invented by Dijkstra, to find the best path to a destination. Examples of link state protocols include IP's OSPF. Whereas distance vector protocols use local broadcasts to disseminate routing information, link state protocols use multicasts. A distance protocol will send out its routing table religiously on its periodic interval whether there are changes or not. Link state protocols are smarter. They multicast what is called a Link State Advertisement (LSA), which is a piece of routing information that contains who originated the advertisement and what the network number is. LSAs are typically generated only when there are changes in the network. link state routers send out only incremental updates. Advantage that link state protocols have over distance vector protocols is that they support route classless routing [VLSM].

Hybrid Protocols

A hybrid protocol takes the advantages of both distance vector and link state protocols and merges them into a new protocol. Typically, hybrid protocols are based on a distance vector protocol but contain many of the features and advantages of link state protocols. Examples of hybrid protocols include RIPv2, EIGRP.

Problems with Distance Vector Protocols

Problem: Convergence

The term convergence, in routing terms, refers to the time it takes for all of the routers to understand the current topology of the network. Link state protocols tend to converge very quickly, while distance vector protocols tent to converge slowly.

Solution: Triggered Updates

You actually have two solutions that you can use in order to speed convergence: change the periodic timer interval and/or use triggered updates.

Problem: Routing Loops

A routing loop is a layer-3 loop in the network. Basically, it is a disagreement about how to reach a destination network. Packet Loops in the Network.
Solution: Maximum Hop Count: IP RIP set a hop count limit of 15, by default, and IGRP allows a hop count of 100.

Solution: Split Horizon

Split horizon is used with small routing loops. Split horizon states that if a neighboring router sends a route to a router, the receiving router will not propagate this route back to the advertising router on the same interface.

Solution: Route Poisoning

Whereas split horizon is used to solve small routing loop problems, distance vector protocols use two mechanisms to deal with large routing loop problems: route poisoning and hold down timers. Route poisoning is a derivative of split horizon. When a router detects that one of its connected routes has failed; the router will poison the route by assigning an infinite metric to it. In IP RIP, the route is assigned a hop count of 16 (15 is the maximum), thus making it an unreachable network.

When a router advertises a poised route to its neighbors, its neighbors break the rule of split horizon and send back to the originator the same poisoned route, called a poison reverse. This ensures that everyone received the original update of the poisoned route.

Hold-Down Timers

In order to give the routers enough time to propagate the poisoned route and to ensure that no routing loops occur while propagation is occurring; the routers implement a hold-down mechanism. During this period, the routers will freeze the poisoned route in their routing tables for the period of the hold-down timer, which is typically three times the interval of the routing broadcast update. 180 Sec's

RIP (Routing Information Protocol)

IP RIP (Routing Information Protocol) comes in two different versions: 1 and 2. Version 1 is a distance vector protocol. Version 2 is a hybrid protocol. RIPv1 uses local broadcasts to share routing information. These updates are periodic in nature, occurring, by default, every 30 seconds, with a hold-down period of 180 seconds. Both versions of RIP use hop count as a metric, which is not always the best metric to use. RIPv1 is a classful protocol. Another interesting feature is that RIP supports up to six equal-cost paths to a single destination. The default is actually four paths, but this can be increased up to a maximum of six.

RIPv2

One major enhancement to RIPv2 pertains to how it deals with routing updates. Instead of using broadcasts, RIPv2 uses multicasts. And to speed up convergence, RIPv2 supports triggered updates-when a change occurs, a RIPv2 router will immediately propagate its routing information to its connected neighbors. A second major enhancement that RIPv2 has is that it is a classless protocol. RIPv2 supports variable-length subnet masking (VLSM), which allows you to use more than one subnet mask for a given class network number.

Configuring IP RIP

Router(config)# router rip
Router(config-router)# network IP_network_#
Specifying RIP Version 1 and 2
Router(config)# router rip
Router(config-router)# version 1|2
Troubleshooting IP RIP
show ip protocols
show ip route
debug ip rip
debug ip rip

IGRP (Interior Gateway Routing Protocol)

The Interior Gateway Routing Protocol (IGRP) is a Cisco-proprietary routing protocol for IP. Like IP RIPv1, it is a distance vector protocol. However, it scales better than RIP because of these advantages:

- It uses a sophisticated metric based on bandwidth and delay.
- It uses triggered updates to speed-up convergence.
- It supports unequal-cost load balancing to a single destination. IGRP uses a composite metric, which includes bandwidth, delay, reliability, load, and MTU, when choosing paths to a destination. By default, the algorithm uses only bandwidth and delay

Configuring IP IGRP

Router(config)# router igrp autonomous_system_#
Router(config-router)# network IP_network_#

Unlike RIP, IGRP understands the concept of an autonomous system and requires you to configure the autonomous system number in the routing process. For routers to share routing information, they must be in the same AS.

Load Balancing

With RIP, you don't need to configure anything to enable equal-cost load balancing; and RIP doesn't support unequal-cost load balancing. IGRP supports both equal- and unequal-cost paths for load balancing to a single destination. Equal-cost paths are enabled by default, where IGRP supports up to six equal-cost paths (four by default) to a single destination in the IP routing table. IGRP, however, also supports unequal-cost paths, but this feature is disabled by default.

The variance feature allows you to include equal- and unequal-cost IGRP routes in the routing table.

Router(config-router)# variance multiplier
Troubleshooting IP IGRP
show ip protocols
show ip route
debug ip igrp events
debug ip igrp transactions

OSPF(Open Shortest Path First)

The Open Shortest Path First (OSPF) protocol is a link state protocol that handles routing for IP traffic. Because it is based on an open standard, OSPF is very popular in many corporate networks today and has many advantages, including these:

- It will run on most routers, since it is based on an open standard.
- It uses the SPF algorithm, developed by Dijkstra, to provide a loop-free topology.
- It provides fast convergence with triggered, incremental updates via Link State Advertisements (LSAs).
- It is a classless protocol and allows for a hierarchical design with VLSM and route summarization.

Given its advantages, OSPF does have its share of disadvantages:

- OSPF maintains three types of tables. So it requires more memory to hold the adjacency (list of OSPF neighbors), topology (a link state database containing all of the routers and their routes), and routing tables.
- It requires extra CPU processing to run the SPF algorithm, which is especially true when you first turn on your routers and they are initially building the adjacency and topology tables.
- For large networks, it requires careful design to break up the network into an appropriate hierarchical design by separating routers into different areas.
- It is complex to configure and more difficult to troubleshoot.

Hierarchical Design: Areas

Areas are used to provide hierarchical routing. Basically, areas are used to control when and how much routing information is shared across your network. In flat network designs, such as IP RIP, if a change occurs on one router, perhaps a flapping route problem, it affects every router in the entire network. With a correctly designed hierarchical network, these changes can be contained within a single area. OSPF implements a two-layer hierarchy: the backbone (area 0) and areas off of the backbone. Through a correct IP addressing design, you should be able to summarize routing information between areas. By performing this summarization, the routers have a smaller topology database (they know only about links in their own area and the summarized routes) and their routing tables are smaller (they know only about their own area's routes and the summarized routes).

Metric Structure

Unlike RIP, which uses hop count as a metric, OSPF uses cost. Cost is actually the inverse of the bandwidth of a link: the faster the speed of the connection, the lower the cost. The most preferred path is the one with the lowest cost. By using cost as a metric, OSPF will choose more intelligent paths than RIP. OSPF supports load balancing of up to six equal-cost paths to a single destination. The default measurement that Cisco uses in calculating the cost metric is: cost = 10 ^8/(interface bandwidth).

Router Identities

Each router in an OSPF network needs a unique ID. The ID is used to provide a unique identity to the OSPF router. This is included in any OSPF messages the router generates. The router ID is chosen according to one of the two following criteria:

- The highest IP address on its loop back interfaces (this is a logical interface on a router)
- The highest IP address on its active interfaces

It is highly recommended that you use a loop back interface because it is always up and thus the router can obtain a router ID.

Finding Neighbors

OSPF learns about its neighbors and builds its adjacency and topology tables by sharing LSAs. There are different types of LSAs. When learning about the neighbors that a router is connected to, as well as keeping tabs on known neighbors, OSPF routers will generate hello LSAs every 10 seconds. When a neighbor is discovered and an adjacency is formed with the neighbor, a router expects to see hello messages from the neighbor. If a neighbor is not seen within the dead interval time, which defaults to 40 seconds, the neighbor is declared dead. When this occurs, the router will advertise this information, via an LSA message, to other neighboring OSPF routers. In order for two routers to become neighbors, the following must match on each router:

- The area number and its type
- The hello and dead interval timers

Let's assume that you turned on all your routers simultaneously on a segment. In this case, the OSPF routers will go through three states called the exchange process:

1. Down state the new router has not exchanged any OSPF information with any other router.
2. Init state A destination router has received a new router's hello and adds it to its neighbor list (assuming that certain values match). Note that communication is only unidirectional at this point.
3. Two-Way state the new router receives a unidirectional reply to its initial hello packet and adds the destination router to its neighbor database. Once the routers have entered a two-way state, they are considered neighbors. At this point, an election process takes place to elect the designated router (DR) and the backup designated router (BDR).

Designated and Backup Designated Routers

An OSPF router will not form adjacencies to just any router. Instead, a client/server design is implemented in OSPF. For each network multi-access segment, there is a DR and a BDR as well as other routers. When an OSPF router comes up, it forms adjacencies with the DR and the BDR on each multi-access segment that it is connected to. Any exchange of routing information is between these DR/BDR routers and the other OSPF neighbors on a segment (and vice versa). An OSPF router talks to a DR using the IP multicast address of 224.0.0.6. The DR and the BDR talk to all routers using the 224.0.0.5 multicast IP address.

The OSPF router with the highest priority becomes the DR for the segment. If there is a tie, the router with the highest router ID will become the DR. By default, all routers have a priority of 1 (priorities can range 0-255). If the DR fails, the BDR is promoted to DR and another router is elected as the BDR. OSPF uses incremental updates. This means that whenever changes take place, only the change is shared with the DR, which will then share this information with other routers on the segment. OSPF routers share information about their connected routes with the DR, which includes the link-state type, the ID of the advertising router, the cost of the advertised link, and the sequence number of the link.

Configuring OSPF

Router(config)# router ospf process_ID
Router(config-router)# network IP_address wildcard_mask area area_#

Note that process ID do not need to match between different routers and that they have nothing to do with autonomous system numbers.

A wildcard mask is 32 bits in length. A 0 in a bit position means there must be a match, and a 1 in a bit position means the router doesn't care. Actually, a wildcard mask is an inverted subnet mask, with the 1's and 0's switched.

Class A: 0.255.255.255.255
Class B: 0.0.255.255
Class C: 0.0.0.255

Loopback Interfaces

A loopback interface is a logical, virtual interface on a router. By default, the router doesn't have any loopback interfaces, but they can easily be created. Here are some reasons you might want to create a loopback interface:

- To assign a router ID to an OSPF router
- To use for testing purposes, since this interface is always up

To create a loopback interface, use the following command:

Router(config)# interface loopback port_#
Router(config-if)# ip address IP_address subnet_mask

Troubleshooting OSPF

• show ip protocols
• show ip route
• show ip ospf interface
• show ip ospf neighbor
• debug ip ospf adj
• debug ip ospf events
• debug ip ospf packet

EIGRP (Enhanced Interior Gateway Routing Protocol)

The Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco-proprietary routing protocol for IP. It's actually based on IGRP, with many enhancements built into it. These characteristics include:

- Fast convergence
- Loop-free topology
- VLSM and route summarization
- Multicast and incremental updates
- Routes for multiple routed protocols (Supports IP, IPX, APPLE TALK)

Characteristics of EIGRP

Both offer load balancing across six paths (equal or unequal).

- They have similar metric structures.
- EIGRP has faster convergence (triggered updates and saving a neighbor's
routing table locally).
- EIGRP has less network overhead, since it uses incremental updates.

EIGRP and IGRP use the same metric structure. Both can use bandwidth, delay, reliability, and MTU when computing a best metric path to a destination. By default, only bandwidth and delay are used in the metric computation. One interesting point about these protocols is that if you have some routers in your network running IGRP and others running EIGRP, and both sets have the same autonomous system number, routing information will automatically be shared between the two.

EIGRP uses the Diffusing Update Algorithm (DUAL) to update the routing table. This algorithm can enable very fast convergence by storing a neighbor's routing information in a local topology table. If a primary route in the routing table fails, DUAL can take a backup route from the topology table and place this into the routing table without necessarily having to talk to other EIGRP neighboring routers to find an alternative path to the destination.

Interaction with Other EIGRP Routers

EIGRP uses hello packets to discover and maintain neighbor relationships, much as OSPF does. EIGRP generates hello packets every 5 seconds on LAN, point-to-point, and multipoint connections with speeds of at least T1/E1 speeds. Otherwise, hellos are generated every 60 seconds. The dead interval period is three times the hello interval. EIGRP uses the multicast address of 224.0.0.10 for the destination in the hello packets.

For EIGRP routers to become neighbors, the following information must match:

- The AS number
- The K-values (these enable/disable the different metric components)

When two routers determine whether they will become neighbors, they go through the following process:

1. The first router generates a Hello with configuration information.
2. If the configuration information matches, the second router responds with an Update message with topology information.
3. The first router responds with an ACK message, acknowledging the receipt of the second's ACK.
4. The first router sends its topology to the second router via an Update message.
5. The second router responds back with an ACK.

At this point, the two routers have converged.

Here are the message types for which an EIGRP router expects an ACK back:

- Update Contains a routing update
- Query Asks a neighboring router to validate routing information
- Reply Responds to a query message

If an EIGRP router doesn't receive an ACK from these three packet types, the router will try a total of 16 times to resend the information. After this, the router declares the neighbor dead. When a router sends a hello packet, no corresponding ACK is expected.

Configuring EIGRP

Setting up EIGRP is almost as simple as configuring IGRP:

Router(config)# router eigrp autonomous_system_#
Router(config-router)# network IP_network_#

You must specify the AS number when configure EIGRP. Even though EIGRP is classless, you must
configure it as a classful protocol when specifying your network numbers with the network command.

Troubleshooting EIGRP

- show ip protocols
- show ip route
- show ip eigrp neighbors
- show ip eigrp topology
- show ip eigrp traffic
- debug ip eigrp

Routing Methods Described

Preface:

Within this paper I'll explain a variety of routing methods and provide some
fundamental particulars featuring about the subject. This paper by no means should be thought about
a resource of full detail about the listed methods. For detailed
details about routing methods I recommend considering the Request
Comments (RFC) for your protocol, goggling them, or possibly going to different
vendor's websites for example 'cisco', Juniper, or Bay systems. Most vendors' web
sites have detailed explanations about all of the Methods there tools
support features. Before you understand routing protocol you'll have to
possess a fundamental understanding or IP systems, Variable Length Subnet Masks (VLSM),
Network Topologies (bus, star, Hub, etc..), and also the OSI model. If you're not
acquainted with any of these subjects you might want to educate yourself and find out about
them before getting into routing methods. Routing methods are why is the
internet work by moving traffic from network to network. Without having routing
methods the web couldn't work because systems would certainly be separate
LANs without any link with others LANs. The web in fundamental terms is simply a
large amount of LANs wired together to create a huge WAN.

Protocol types:

Distance Vector: distance vector routing is a kind of
routing protocol that finds out routes on interconnected systems. The Length
Vector routing formula may be the in line with the Bellman-Ford formula. Good examples of
distance-vector routing methods include RIP (Routing Information Protocol),
Cisco's IGRP (Internet Gateway Routing Protocol) try Google to determine all of the
different methods within the Distance Vector family. Distance Vector methods are
appropriate for more compact systems as most of the methods within this family aren't
scalable in lager complex systems because they are limited. The primary limit to
Distance Vector Methods can there be manner in which requires each router simply
inform its neighbors of their routing table. This routing table update with other
people is bandwidth intensive to lager systems.

RIP (Routing Information Protocol):

Routing Information Protocol (RIP) is among the first methods for use in
networking and it is considered a distance vector routing protocol. RIP uses
broadcast User Datagram Protocol (UDP) data packets to switch routing
information. You will find two versions RIP V1 &lifier V2. RIP Version 1 may be the original
version and it has many restrictions. The metric that RIP uses to rate the need for
different routes is hop count. The hop count metric functions by setting static
routes having a worth of and all sorts of other hubs values are positioned by the amount of
hops (as much as 15) the data must travel though to get at an finish point. RIP
Version 5 supports plain text and MD5 authentication, route summarization,
classless inter-domain routing (CIDR), variable-length subnet masks (VLSMs),
Multicast support. Some suppliers support other non-standard features for RIP but
be cautious as numerous vendor centric features aren't compatible inside a mixed vendor
network.

IGRP (Inter Gateway Routing Protocol):

Interior Gateway Routing Protocol (IGRP) is really a distance vector routing protocol
that is a proprietary and introduced by 'cisco'. It's utilized by hubs to switch
routing data inside an autonomous system (AS). IGRP supports multiple metrics
for routes, including bandwidth, load, delay and MTU. This enhances reliability
over RIP because IGRP uses advanced metrics to check two routes right into a
combined route. The 2 routes together are combined right into a single metric, using
an equation which may be modified via command line. The utmost hop count of IGRP
is 255 that is a noticable difference over RIPs 15 hop max. Bear in mind that IGRP is really a
'cisco' proprietary protocol and cannot be utilized in an assorted vendor network.

Link Condition: Link Condition routing protocol requires each
router (peer) to keep a minimum of an incomplete map from the network. Whenever a network
link changes condition (as much as lower, or the other way around), a notification, known as a hyperlink
condition advertisement (LSA) is flooded through the network. All of the hubs
note the modification, and recompute their routes accordingly. This process is much more
reliable, simpler to debug and fewer bandwidth-intensive than Distance-Vector. It
can also be more complicated and much more compute- and memory-intensive. Link condition routing
methods are located in lots of lager systems and supply Scalable solutions for
more complicated systems.

OSPF (Open Least Path First):

Open least path first (OSPF) is really a link-condition routing protocol that requires
the delivering of link-condition ads (LSAs) to any or all other hubs inside the
same hierarchical area or autonomous system (AS). An As possible split into a
quantity of areas, that are categories of contiguous systems and attached hosts.
Info on attached connects, metrics used, along with other variables are
incorporated in OSPF LSAs. As OSPF hubs accumulate link-condition information, they
make use of the SPF formula to calculate the least road to each node.

(IS-IS) Intermediate System-to-Intermediate System:

Intermediate System-to-Intermediate System (IS-IS) is really a routing protocol
produced by the ISO and it is natively an ISO Connectionless Network Service or
CLNS protocol so it doesn't use IP to hold routing information messages. It
uses OSI methods to provide its packets and establish its adjacencies. IS-IS
continues to be enhanced to hold IP (Ip Address) which is known as Integrated
IS-IS. Integrated IS-IS supports VLSM and converges quickly. It's also scalable
to aid large systems and is paramount protocol in lots of bigger ISP's.

Hybrid: Hybrid routing Methods really are a combination or
both Distance Vector and Link condition methods and just one protocol suits
this area. EIGRP is 'cisco' Systems Proprietary protocol according to their original
IGRP. For additional info on Hybrid Methods look below towards the section on "EIGRP".

EIGRP (Enhanced Inter Gateway Routing Protocol):

Enhanced Inter Gateway Routing Protocol (EIGRP) is 'cisco' Systems Proprietary
protocol according to their original IGRP. EIGRP is really a balanced hybrid IP routing
protocol, with optimizations to reduce both routing instability incurred
after topology changes, along with the utilization of bandwidth and processing energy in
the router. EIGRP has Protocol-Dependent Modules that may cope with AppleTalk
and IPX in addition to IP. The benefit with this particular is the fact that just one routing process
need run rather than a routing process for each one of the methods. EIGRP provides
loop-free operation and almost instant synchronised synchronization of hubs.

Inter-Autonomous System: Inter-Autonomous System routing methods are made
for connecting lager systems or Autonomous Systems (AS) together and permit
muitiple Autonomous Systems to network. An example for the necessity of an
inter-autonomous system protocol would be to connect several Online Sites
Companies (Web service provider) together there customer can connect with one another. Without
getting into to an excessive amount of detail both "Link Condition &lifier Distance Vector" methods are
considered intra-autonomous system methods because they are made to just route
traffic inside a singal AS. Inter-Autonomous System methods primary goal would be to
propergate the intra-autonomous system information between different autonomous
systems.

BGP4 (Border Gateway Protocol Version 4):

Border Gateway Protocol may be the backbone routing protocol for the majority of the
internet and enables for peering and company systems for connecting. BGP is
described like a path vector protocol. With BGP the insurance policy or characteristics for
making the particular route choices one of the interconnected autonomous systems
is dependant on Weight ,Local preference, Multi-exit discriminator, Origin, AS path,
Next hop, &lifier Community. BGP details are propagated with the network by
trades of BGP messages (4 types: Open, Update, Notification, &lifier Keep Alive)
between peers. Another key feature to BGP is that's supports Classless Inter
Domain Routing (CIDR) using the support of CIDR BGP can help to eliminate how big the
Internet routing tables. BGP neighbors exchange full routing information when
the TCP (port 179) link between neighbors is first established. When
changes towards the routing table accrue, the BGP hubs send for their neighbors
only individuals routes which have transformed. BGP hubs don't send periodic routing
updates and advertise just the optimal pathways to some destination.