Pohang University of Science and Technology (POSTECH) Copyright © 2007 SE Lab. Dept. of CSE...

Pohang University of Science and Technology (POSTECH)

Copyright © 2007 SE Lab. Dept. of CSEPOSTECH, R.O. Korea

Interior Gateway Routing ProtocolInterior Gateway Routing Protocol

(IGRP)(IGRP)

Tung Dao Manh.

Future Internet Class,

2007.03.27

1

POSTECHCopyright © 2007

SE Lab. Dept. of CSEPOSTECH, R.O. Korea

AgendaAgenda

Introduction IGRP Characteristics

Distance vector routing IGRP Metrics IGRP Timers IGRP Route Types Unequal-cost load balancing IGRP packet format

IGRP Stability Features Count to infinity Hold-down Split horizon with poison reverse Triggered update with route poisoning

Comparison between IRGP and RIP Conclusion

2



Introduction

Developed by Cisco in the mid-1980’s as a better alternative solution to RIP (Routing Information Protocol).

• A distance vector protocol

• Main goal was to be a robust protocol for routing within an autonomous system

• IGRP was initially decided to run in any network environments, but ported to run in OSI Connectionless-Network Protocol (CLNP).

Many organizations with large internetworks replaced RIP with IGRP, because it is superior than RIP in:

• More scalability with supported hop-count up to 255

• More flexibility with sophisticated metric, a composite metric of 5 variables; and Multi-path support across unequal path.

Background

3



Introduction

Routing Protocols

Interior Exterior

Dist-Vector Link-State Hybrid

RIP IGRP OSPF EIGRP

EGPBGP

Routing Protocol Classification

4



Introduction

Interior Routing protocols operated within an Autonomous System.

Exterior Routing protocols operated between Autonomous Systems.

Interior Routing Vs Exterior Routing

5



Introduction

Definition: • Collection of routers and networks under the same administration or

routing policy.

• Usually under single ownership, trust and administrative control.

When using AS:• Multi-home networks to different providers.

• Routing policy is different to external peers.

Autonomous System (AS)

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Introduction

Characteristics:• Internal connectivity: All parts of an AS must remain connected,

meaning that its all routers must exchange routing information in order to maintain the connectivity.

• A single routing protocol required to run in an AS, between all routers.• In 1982, terminology, routers inside an AS were called “interior

gateways” and the protocol was an “Interior Gateway Protocol” (IGP).• Each AS is identified by a 16-bit “AS number”.

Autonomous System (AS)

EGP is used to exchange information among Ass.

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IGRP Characteristics

Mathematically compare routes using some measurement of distance (distance vector).

Routers send all or a portion of their routing table in a routing-update message at regular intervals to each of their neighboring routers.

As routing information proliferates, routers:• Identify new destinations

• Learn failure through the network

• Calculate distance to all known destinations.

Distance Vector Routing (1/3)

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IGRP Characteristics Distance Vector Routing (2/3)

Routing table E

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IGRP Characteristics Distance Vector Routing (3/3)

Destination Outgoing Link Distance

A Local 0

B

C

12

A

CB

Routing table A

Routing table B


A

B Local 0


A

C Local 0

Routing table C

• Initially, each router initializing its local knowledge: remember its own address and be able to identify the links attached to it.

• It then broadcasts its routing table to all its neighbors.

• It updates its routing table when noticing a change.

• The process continues until the network converges


A Local 0

B 1 1

C

Update

broadcast

Update


A 2 1

B 2 2

C Local 0

Broadcast


A Local 0

B 1 1

C 2 1

Update


A 1 1

B Local 0

C 1 2

Broadcast

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Stability Features Count To Infinity

Problem: Counting to infinity


A Local 0

B 1 1

C 2 11 2A

CB


A 1 1

B Local 0

C 1 2

Routing table A

Routing table B

A captures the change


A Local 0

B 1 1

C 2 Inf


A Local 0

B 1 1

C 2 3

Update routing table

Up

da

ting


A 1 1

B Local 0

C 1 4

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Stability Features Split Horizon

Solution: Split Horizon• Split horizon is based on very simple precaution: it is never useful to send

information about a route back in the direction from which it came.


A Local 0

B 1 1

C 2 inf

1 2A

CB


A 1 1

B Local 0

C 1 2

Routing table A

Routing table B

Up

da

te


A 1 1

B Local 0

C 1 inf

The Network converges !

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Stability Features Poison-reverse update

Solution: Poison-reverse update• Is a way in which a router tells its neighboring router that one of the routers is

no longer connected; or disqualifies a route back a long the interface on which it learned the route by setting the unconnected router to a defined number.


A Local 0

B 1 1

C 2 1

1 2A

CB


A 1 1

B Local 0

C 1 2

Routing table A

Routing table B


A 1 1

B Local 0

C 1 inf


A captures and

updates


A Local 0

B 1 1

C 2 inf

B modifies routing table

before sending

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Stability Features Triggered Update

Solution: Triggered Update• Is an attempt to increase the responsiveness of the protocol by requesting

nodes to send message as soon as they notice a change in their routing table with out having to wait for the end of the period.


A Local 0

B 1 1

C 2 1

1 2A

CB


A 1 1

B Local 0

C 1 2

Routing table A

Routing table B

Up

da

te


A 1 1

B Local 0

C 1 inf


A captures the change

and updates


A Local 0

B 1 1

C 2 inf

A send update

immediately to B

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IGRP Characteristics IGRP Timers (1/2)

IGRP maintains a numbers of timers and variables containing time

intervals, including:• Update timer

• Invalid timer

• Hold-time period

• Flush timer

Update timer: updates are broadcast by all routers on all connected interfaces. The default is once every 90 seconds.

Invalid timer: specify how long a router should wait in the absence of routing-update messages about a specific route before declaring that route is invalid.

Hold-time variable: specify the “hold-down” period, indicating how long the state “hold-down” of the destination should last.

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IGRP Characteristics IGRP Timers (2/2)

90 secs – Update and Invalid timers are then reset.Update

270 secs - Invalid timer expires, route now “unreachable” but still in routing table until flush timer expires. Holddown timer of 280 sec begins.

Invalid

280 secs – Holddown timer expires, will now accept a poorer route to same network. Still in routing table

Holddown

630 secs – Route will now be removed from the routing table.

FlushThe flush timer starts after the first 90 second update is missed.

Update timer: how frequently routing update messages should be sent Invalid timer: how long a router should wait in the absence of routing-update messages

about a specific route before declaring that route invalid (unreachable), but still in the routing table

Holddown timer: specifies the amount of time for which information about poorer routes are ignored.

Flush timer: how much time should pass before a route is flushed from the routing table

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IGRP Characteristics IRGP Routes (1/4)

IGRP flags candidates for a default route. Candidates are identified as routes to the edge of the network.

The candidate with the lowest metric is selected as the default route

Handling of default routes

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Interior routes:

Interior routes are routes between subnets in the network attached to a router interface. If the network attached to a router is not subnetted, IGRP does not advertise interior routes.

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System routes:

“System routes are routes to networks within an autonomous system. The Cisco IOS software derives system routes from directly connected network interfaces and system route information provided by other IGRP-speaking routers or access servers. System routes do not include subnet information.”

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routes to summarized networks within an AS.




Exterior routes: routes to networks outside the AS and are called default networks.

“Exterior routes are routes to networks outside the autonomous system that are considered when identifying a gateway of last resort. The Cisco IOS software chooses a gateway of last resort from the list of exterior routes that IGRP provides. The software uses the gateway (router) of last resort if a better route is not found and the destination is not a connected network. If the autonomous system has more than one connection to an external network, different routers can choose different exterior routers as the gateway of last resort.”

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IGRP IGRP Stability Features

Count to infinity

Split horizons with poison-reverse

Triggered update with route poisoning

Hold-downs: when a route is removed, no new route would be accepted for the same destination for some period of time, the triggered update has time to propagate throughout the network.

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IGRP Characteristics IGRP Metrics (1/3)

Composite metric•Bandwidth•Delay•Load (opt)•Reliability (opt)•MTU (opt): MTU has never been used by IGRP or EIGRP as a routing metric.

•By default, only Bandwidth and Delay are usedAdditionally, Hop-Count is tracked

•Only used to limit network diameter•0..255, default maximum hop-count = 100

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IGRP Characteristics IGRP Metrics (2/3)

IGRP uses a 24-bit metric field•Bandwidth: Minimum bandwidth in the route, in kilobits per second (kps).•Delay: The cumulative interface delay along the path in ten of microseconds.•Reliability: Likelihood of successful transmission, express as an integer from 0..255.•Loading: The load on a link towards the destination based on bits per second •MTU: Maximum Transmission Unit, is the size of the largest datagram that can be sent over a network. minimum MTU in path, expressed in bytes.

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IGRP Characteristics

Composite Metric:• Metric = (k1*IGRP-BW + (k2 *IGRP-BW)/(256-Load) + k3*IGRP-

DLY) * k5/(Reliability+k4)o Minimum IGRP-BW of all outgoing interfaces along the route

to the destination. IGRP-BW = 10^7/BW.o IGRP-DLY: Sum of all delays of the route. Default: K1=K3=1,

K2=k4=k5=0.• Default metric = (IGRP-BW) + (IGRP-DLY.)• Unreachable routes are marked with IGRP-DLY=0xFFFFFF

• (k1, k2, k3, k4, k5) is metric weights, having great impact on network performance The load on a link towards the destination based on bits per second.

IGRP Metrics (3/3)

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IGRP Unequal-cost Load Balancing

Load balancing is the way in which a router loads share across multi-paths to the same destination, helping decrease load over a single path.

Unlike RIP, IGRP not only support equal-cost balancing but also unequal cost balancing, allowing us can load share unequal-cost paths.

This process is controlled by the parameter, the Variance. The variance is defined with a multiplier that represents the difference between the metrics of the paths. In the above example The variance should be higher than 25.4

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Metric = 7100 Metric = 180671 = 25.4 * 7100



IGRP Characteristics IGRP Packet Format(1/2)

•Version: is always set to 1

•OPCode: 1 for request packet; 2 for update packet. A request packet consists header with no entry.

• Edition: This number is incremented by the sender so that the receiving router does not use an old update

•Autonomous system number: the ID number of IGRP process, this tag allows multi IGRP processes to exchange information over a common link.

General Header fields:

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IGRP Characteristics IGRP Packet Format (2/2)

•Number of interior routes: indicates how many of the routing entries in this update are subnets of a directly connected network.

•Number of system routes: indicates how many of the routing entries in this update are not from a directly connected network.

•Number of exterior routes: indicates how many of the routing entries in this update are default networks.

•Checksum: calculated on the header and the entries. With its field set to 0, the 16-bit one's complement sum is calculated and then inserted into this field. At the other end, the 16-bit one's complement is calculated again by the other router but this time including the already calculated value in the Checksum field. The result on a good packet will be 0xFFFF.

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IGRP Characteristics IGRP Packet Format (2/2)

•Bandwidth: IGRP bandwidth.

•MTU: The smallest MTU encountered along the route to this particular destination network

•Reliability - A number between 0x01 and 0xFF to indicate the error rates totalled along the route. 0xFF is reliable.

•Load - A number between 0x01 and 0xFF expressing the total load along a route where 0xFF is totally loaded.

•Hop Count - A number between 0x00 (directly connected network) and 0xFF.

•Note that the Delay and Bandwidth values are 24-bit with IGRP.

•Up to 104 fourteen-byte entries. Maximum packet size = 1468 bytes (including header).

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•Destination - Destination network, just containing the last three octets for interior routes (e.g. 24.5.0 for the network 10.24.5.0) since the first octet will be known. For System and External routes, the routes would have been summarized so the last octet will always be zero (e.g. 10.24.5.0 will be entered as 10.24.5).

•Delay: The number of 10 microsecond chunks which is the sum of delays.



Comparison between IGRP and RIP

Characteristic RIPv1 RIPv2 IGRPCount to infinity X X XSplit horizon X X XHold-down timer X X XTriggered updates with route poisoning X X X Load balancing—Equal paths X X X Load balancing—Unequal paths X VLSM support X Routing algorithm B-F B-F B-FMetric Hops Hops CompHop count limit 16 16 100Scalability Med Med Large

Characteristic RIPv1 RIPv2 IGRPCount to infinity X X XSplit horizon X X XHold-down timer X X XTriggered updates with route poisoning X X X Load balancing—Equal paths X X X Load balancing—Unequal paths X VLSM support X Routing algorithm B-F B-F B-FMetric Hops Hops CompHop count limit 16 16 100Scalability Med Med Large

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Conclusion

IGRP has proven to be one of the most successful routing protocols.

• A highly successful and widely deployed routing protocol.

• Preserving many the effective features of RIP, while greatly expanding its capabilities.

Disadvantage of IGRP is the lack of supporting for variable-length subnet masks (VLSM). EIGRP comes to solve that problem.

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