RIP, IGRP, & EIGRPCharacteristics and Design

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Chapter Topics

RIPv1 RIPv2 IGRP EIGRP

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RIPv1

RFC 1058 distance-vector routing protocol uses router hop count as the metric classful routing protocol

does not support VLSM or CIDR no method for authenticating route

updates

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RIPv1

Sends a copy of its routing table to its neighbors every 30 seconds

Uses split horizon with poison reverse

Based on the popular routed program used in UNIX systems since the 1980’s

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RIPv1

Cisco implementation of RIP adds support for load balancing will load-balance traffic if there are

several paths with the same metric Cisco RIP sends triggered updates Administrative distance of 120

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RIPv1

Summarizes to IP network values at network boundaries Network boundary occurs at a router

that has one or more interfaces that do not participate in the specified IP network

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RIPv1 Forwarding Information Base RIPv1 protocol keeps the following

information about each destination: IP address

IP address of the destination host or network Gateway

The first gateway along the path to the destination

Interface The physical network that must be used to

reach the destination Metric

A number indicating the number of hops to the destination

Timer The amount of time since the entry was last

updated

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RIPv1 Forwarding Information Base

Database is updated with the route updates received from neighboring routers

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RIPv1 Message Format

RIP messages are encapsulated using UDP port 520

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RIPv1 Message Format Command

Describes the purpose of the packet. The RFC describes five commands, two of which are obsolete and one of which is reserved. The two used commands are

request Requests all or part of the responding

router's routing table. response

Contains all or part of the sender's routing table. This message might be a response to a request, or it might be an update message generated by the sender

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RIPv1 Message Format

Version Set to the value of 1 for RIPv1.

Address Family Identifier (AFI) Set to a value of 2 for IP.

IP address The destination route. It might be a

network address, subnet, or host route. Special route 0.0.0.0 is used for the default route.

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RIPv1 Message Format

Metric A field that is 32 bits in length. It

contains a value between 1 and 15 inclusive, specifying the current metric for the destination. The metric is set to 16 to indicate that a destination is not reachable.

Because RIP has a maximum hop count, it implements counting to infinity

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RIPv1 Message Format In the RIP message that there are

no subnet masks accompanying each route

Five 32-bit words are repeated for each route entry Five 32-bit words equals 20 bytes for

each route entry Up to 25 routes are allowed in each

RIP message The maximum datagram size is

limited to 512 bytes

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RIPv1 Timers Cisco implementation of RIP uses

four timers Update Invalid Flush Holddown

IP sends its full routing table out all configured interfaces

Table is sent periodically as a broadcast (255.255.255.255) to all hosts

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Ripv1 Timers

Update Timer frequency of the periodic broadcasts default is 30 seconds

Invalid Timer The length of time that must elapse

before a router determines that a route has become invalid

Default is 180 seconds

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Ripv1 Timers Flush Timer

Sets the time between a route becoming invalid and its removal from the routing table

Default is 240 seconds Holddown Timer

Cisco implementation Sets the amount of time during which routing

information is suppressed After the metric for a route entry changes, the

router accepts no updates for the route until the holddown timer expires.

Default is 180 seconds

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Ripv1 Timers

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RIPv1 Design

Things to remember does not support VLSM and CIDR RIPv1 requires the same subnet mask for the

entire IP network RIPv1 is limited to 15 hops broadcasts its routing table every 30 seconds usually limited to accessing networks where it

can interoperate with servers running routed or with non-Cisco routers

also appears at the edge of larger networks

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RIPv2

First described in RFC 1388 and RFC 1723 (1994); the current RFC is 2453, (1998)

Need to use VLSM and other requirements prompted the definition of RIPv2

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RIPv2

RIPv2 improves upon RIPv1 with ability to use VLSM support for route authentication, multicasting of route updates

uses the IP address 224.0.0.9 Support of CIDR

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RIPv2

Some features remain the same Updates every 30 seconds Retains the 15-hop limit Uses triggered updates Uses UDP port 520 Retains the loop-prevention strategies

of poison reverse and counting to infinity

Administrative distance of 120 Summarize IP networks at network

boundaries

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RIPv2 Authentication

Prevent communication with any RIP routers that are not intended to be part of the network UNIX stations running routed

RFC 1723 defines simple plain-text authentication

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MD5 Authentication

Cisco implementation provides the ability to use Message Digest 5 (MD5) authentication RFC 1321

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RIPv2 Forwarding Information Base

Maintains the same routing table database as in Version 1 Difference is that it also keeps the

subnet mask information IP Address Gateway Interface Metric Timer

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RIPv2 Message Format

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RIPv2 Message Format

Takes advantage of the unused fields in the RIPv1 message format Adding subnet masks and other

information

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RIPv2 Message Format Description of each field

Command Indicates whether the packet is a request or a

response message Version

Specifies the RIP version used AFI

Specifies the address family used Set to a value of 2 for IP

Route tag Provides a method for distinguishing between

internal routes (learned by RIP) and external routes (learned from other protocols)

Optional Tag

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RIPv2 Message Format

IP address IP address (network) of the destination

Subnet mask Subnet mask for the destination

Next hop Indicates the IP address of the next hop

where packets are sent to reach the destination.

Metric Indicates how many router hops to reach

the destination

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RIPv2 Timers

Same as RIPv1 Update – 30 s Invalid – 180 s Holddown – 180 s Flush – 240 s

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RIPv2 Design Things to remember

Supports VLSM within networks and CIDR for network summarization

Summarization of routes in a hierarchical network

Limited to 16 hops Multicasts its routing table every 30 seconds Usually limited to accessing networks where it

can interoperate with servers running routed or with non-Cisco router

Appears at the edge of larger internetworks

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IGRP

Developed by Cisco to overcome the limitations of RIPv1

Distance-vector routing protocol Composite metric

Uses bandwidth and delay as parameters instead of hop count

Can also use reliability and load Not limited to the 15-hop limit of RIP

Default is 100 hops – configurable up to 255

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IGRP

Routers usually select paths with a larger minimum-link bandwidth over paths with a smaller hop count Links do not have a hop count They are exactly one hop

Classful protocol cannot implement VLSM or CIDR

Summarizes at network boundaries

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IGRP

Loop avoidance Split horizon with poison reverse Triggered updates Holddown timers

Can load balance over unequal-cost links

Cisco Proprietary

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IGRP Timers

Update Timer 90 seconds

Invalid Timer 270 seconds

Holddown Timer 280 seconds

Flush Timer 630 seconds

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IGRP Metrics

IGRP uses a composite metric based on bandwidth, delay, load, and reliability Default is bandwidth and delay

Formula for calculation

IGRPmetric = {k1 x BW + [(k2 x BW)/(256 – load)] + k3 x delay} x {k5/(reliability + k4)}

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IGRP Metrics

By default, k1 and k3 are set to 1, and k2, k4, and k5 are set to 0

Change the default metrics using the metric weight tos k1 k2 k3 k4 k5 subcommand under router igrp tos is always 0 to use all metrics:

Router(config)#router igrp 10

Router(config-router)#metric weight 0 1 1 1 1 1

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IGRP Design

Things to remember does not support VLSM does not support CIDR and network

summarization within the major network boundary

network diameter can be larger than that of networks using RIP

broadcasts its routing table every 90 seconds

limited to Cisco-only networks

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IGRP Design

Starting with IOS version 12.3, IGRP is NO LONGER SUPPORTED Must migrate to EIGRP or to another

protocol

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EIGRP

Released in the early 1990s as an evolution of IGRP

classless protocol Permits the use of VLSM Supports CIDR

does not send routing updates periodically

authentication with simple passwords or with MD5

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EIGRP

Autosummarizes networks at network borders

Can load-balance over unequal–cost paths

Uses IP protocol 88 Cisco Proprietary

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EIGRP An “advanced distance-vector”

protocol Advertises its routing table to its

neighbors Uses hellos and forms neighbor

relationships Sends partial updates when a metric or

the topology changes does not send full routing-table updates in

periodic fashion Uses DUAL to determine loop-free

paths

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EIGRP

Administrative Distances Internal routes have an AD of 90 Summary routes have an AD of 5 External routes (from redistribution)

have an AD of 170

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EIGRP Components

EIGRP has four components that characterize it: Protocol-dependent modules Neighbor discovery and recovery Reliable Transport Protocol (RTP) DUAL

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Protocol-Dependent Modules

Different modules that independently support IP, Internetwork Packet Exchange (IPX), and AppleTalk

The logical interface between DUAL and routing protocols

Module sends and receives packets but passes received information to DUAL, which makes routing decisions.

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Neighbor Discovery and Recovery

multicasts hello packets (224.0.0.10) every 5 seconds for most networks

router builds a table with EIGRP neighbor information

holdtime to maintain a neighbor is three times the hello time: 15 seconds

multicasts hellos every 60 seconds on multipoint WAN interfaces (X.25, Frame Relay, ATM) with speeds less than 1544 Mbps Holdtime is now 180 seconds

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RTP

ensures the reliable delivery of route updates

uses sequence numbers to ensure ordered delivery

sends update packets using multicast address 224.0.0.10

acknowledges updates using unicast hello packets with no data

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DUAL

Implemented to select paths and guarantee freedom from routing loops Mathematically proven to result in a

loop-free topology No need for periodic updates or route-

holddown mechanisms

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DUAL

Selects a best path and a second best path to reach a destination Best path is the successor Second best path (if available) is the

feasible successor Feasible distance is the lowest

calculated metric of a path to reach the destination

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DUAL

Passive state when the router is not performing any

recomputations for an entry

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DUAL

Active State successor goes down and there are no

feasible successors routers send query packets to

neighboring routers Neighbor can send a reply packet

It has a feasible successor Neighbor can send a query packet

Does not have a feasible successor Route does not return to passive state

until it has received a reply packet from each neighboring router

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EIGRP Timers Sends updates only when necessary

Sends them only to neighboring routers There is no periodic update timer On high-speed networks, the default hello

packet interval is 5 seconds multipoint networks with link speeds of T1 and

slower, hello packets are unicast every 60 seconds

Holdtime to maintain a neighbor adjacency is three times the hello time 15 seconds or 180 seconds

Updates are sent to the multicast address 224.0.0.10 (all EIGRP routers)

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EIGRP Metrics

Uses the same composite metric as IGRP BW term is multiplied by 256 for finer

granularity Metric is based on bandwidth, delay,

load, and reliability MTU is not an attribute

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EIGRP Packet Types

EIGRP uses five packet types: Hello

Discovery of neighbors. They are multicast to 224.0.0.10

Acknowledgment Acknowledges the reception of an update packet It is a hello packet with no data. Sent to the unicast address of the sender of the

update packet Update

Update packets contain routing information for destinations.

Unicasts update packets to newly discovered neighbors

Multicasts update packets to 224.0.0.10 when a link or metric changes. Update packets are acknowledged to ensure reliable transmission.

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EIGRP Packet Types

Query sent to find feasible successors to a

destination Always multicast.

Reply Sent to respond to query packets. Provide a feasible successor to the sender

of the query Packets are unicast to the sender of the

query packet.

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EIGRP Design

Things to remember Supports VLSM, CIDR, and network

summarization Not limited to 16 hops as RIP is Does not broadcasts its routing table

periodically No large network overhead

Use EIGRP for large networks Potential routing protocol for the core

of a large network Provides for route authentication

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Summary