Overview of Dynamic Routing Protocols

CIT 340

2

Overview of Routing Protocols

• Note: IGRP and EIGRP are Cisco proprietary protocols. They are meant as an alternative between the limited RIP routing protocol and the more complicated and resource intensive OSPF and IS-IS routing protocols. IGRP was discontinued with IOS 12.2 in 2005.

• The dates shown are when the RFC or other document was finalized. The protocol may have been implemented earlier than this date.

Interior Gateway Protocols

Exterior Gateway Protocols

Distance Vector Routing Protocols

Link State Routing Protocols

Path Vector

Classful RIPv1 (1982/1988)

IGRP (1985)

EGP (1982)

Classless RIPv2 (1994)

EIGRP (1992)

OSPFv2 (1991)

IS-IS (1990)

BGPv4 (1995)

IPv6 RIPng (1997)

EIGRP for IPv6

(not yet released)

OSPFv3 (1999)

IS-IS for IPv6 (2000)

BGPv4 for IPv6

(1999)

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The Routing Protocols we will focus on

• Note: RIPv2 will not be discussed in detail but will be used as an example of transitioning from a classful to a classless routing protocol.

Interior Gateway Protocols

Exterior Gateway Protocols

Distance Vector Routing Protocols

Link State Routing Protocols

Path Vector

Classful RIPv1 (1982/1988)

IGRP (1985)

EGP (1982)

Classless RIPv2 (1994)

EIGRP (1992)

OSPFv2 (1991)

IS-IS (1990)

BGPv4 (1995)

IPv6 RIPng (1997)

EIGRP for IPv6

(not yet released)

OSPFv3 (1999)

IS-IS for IPv6 (2000)

BGPv4 for IPv6

(1999)

4

Dynamic Routing Protocol Basics

• Routing Protocol = A language a router speaks with other routers to share information about the reachability and status of networks.

• Routing protocols perform:– Best-path determination– Route-table-update functions– Next-best path should the best-path become unusable

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Algorithms

• All dynamic routing protocols are built around an algorithm.

• Algorithm = Step-by-step procedure for solving a problem.

• At a minimum the algorithm must specify:– A procedure for passing reachability information

about networks to other routers.– A procedure for receiving reachability information

from other routers.– A procedure for determining optimal routes based

on the reachability information it has and for recording this information in a route table.

– A procedure for reacting to, compensating for, and advertising topology changes in a network.

A tribute to al-Khwarizmi, 9th century Persian mathematician, the originator and namesake of algorithms.

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Algorithms

Distance Vector Routing Protocols • RIP, IGRP:

– Variant of Bellman-Ford (or Ford-Fulkerson)• EIGRP:

– Diffusing Update Algorithm (DUAL) first proposed by E.W. Dykstra and C.S. Scholten

– Latest and most prominent work done by J.J. Garcia-Luna Aceves (UC Santa Cruz)

Link State Routing Protocols • OSPF, IS-IS:

– Dijkstra’s SPF (Shortest Path First) algorithm, E.W. Dijkstra

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Path Determination

• Router interfaces must be members of different networks.• Router interfaces participate in the network like other hosts on that

network.• Ethernet interfaces:

– Have MAC Addresses– ARP Tables– Participate in the ARP Request and ARP Reply process like other

hosts on that network.

Serial 0192.168.10.1 255.255.255.252

Ethernet 1

172.16.2.1 255.255.255.0 MAC: 0abbccddeeff

Ethernet 0

172.16.1.1 255.255.255.0 MAC: 0cddeeffaabb

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Path Determination

Simplistic questions illustrating the complexity of routing protocols:

• What should Router A do with the routing updates from B and C?

• What mechanism is used to ensure that all routers receive all routing information?

• If Router A has hears about 192.168.4.0/24 from B and C which router should be the next hop used to reach that network? Should both be used?

• What metric is used to determine best path?

Routing Updates

?

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Metrics

• Metric = A variable assigned to routes as a meaning of ranking them from best to worst or from most preferred to least preferred.– Hop count– Bandwidth– Delay– Reliability– Load– Cost

• RIP– Hop count

• IGRP and EIGRP– Bandwidth– Delay– Reliability– Load

• OSPF– Cost (Cisco defines cost as

Bandwidth)• IS-IS

– Default (Cisco supported)– Delay– Expense– Error

• BGP– Policies and Attributes

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Convergence

• Convergence = The process of bringing all route tables to a state of consistency.

Convergence Vegetarian Catering

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Convergence

• During the time it takes for convergence to occur, routers will continue to route packets using their current routing tables.

• It is during this time that routing errors may occur.• Therefore, convergence time is an important factor in any routing protocol.• The faster a network can reconverge after a topology change, the better.

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Distance Vector Routing Protocols

• Most routing protocols fall into one of two classes:

1. Distance Vector

2. Link State

• Distance Vector = Derived from the fact that routes are advertised as vectors of (distance, direction), where distance is defined in terms of a metric and direction is defined in terms of the next-hop router.

• Built around Bellman-Ford algorithm.

Serial 0

Ethernet 1

Ethernet 0

Distance

Distance

Distance

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Distance Vector Routing Protocols

• Distance Vector Routing Protocols include:– RIP for IP– XNS (Xerox Networking System’s) RIP– Novell’s IPX RIP– Cisco’s IGRP– Cisco EIGRP– DEC’s DNA Phase IV– AppleTalks Routing Table Maintenance Protocol

(RTMP)

• Only RIP for IP and EIGRP are current routing protocols.

14

Distance Vector Routing Protocols

Common Characteristics (generalization)• Periodic Updates

– RIP’s 30 seconds to IGRP’s 90 seconds. (Notable exception is EIGRP)

• Neighbors – Sharing a common data link or higher-level adjacency.

• Broadcast updates– Sends updates to a broadcast IP address (Some protocols use

multicast addresses)• Full Routing Table Updates

– Tell their neighbors everything they know by sending their entire routing table.

Periodic updates

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Routing by Rumor

16

Distance Vector Techniques Review

• Route Invalidation Timers– “If I don’t hear from you within a certain time period I will invalidate

you and your routes.”

• Split Horizon and Split Horizon with Poison Reverse– “I don’t need to tell you since you are the one who told me.”

• Counting to Infinity and Setting a Maximum– “In case there is a routing loop being created, I will set a maximum

metric and then declare the route unreachable.”

• Triggered Updates or Flash Updates– “As soon as I hear a change, I will let my other neighbors know.”

• Holddown Timers – “I’m skeptical of this new “poorer” information and will wait to see if

convergence is in the works.”

• Asynchronous Updates and Timing Jitter– “So we all don’t talk at once, we’ll each wait a certain random time.”

Event-driven or triggered updates

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Routing by Rumor

• Distance vector routing protocols provide road signs to networks.

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As opposed to Link State Routing Protocols

• Link state routing protocols provide road maps to networks (next).

19

Link State Routing Protocols

• Link state routing protocols sometimes called:– Shortest Path First (SPF) protocol– Distributed Database protocol

• Built around algorithm from graph theory, E.W. Dijkstra’s short path algorithm.

• Link state routing protocols include:– OSPF– IS-IS– DEC’s DNA Phase V (legacy)– Novell’s Netware Link Services

Protocol (NLSP) (legacy)

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Link State Routing Protocols

These topics will be discussed in detail during OSPF• Neighbor and Neighbor Discovery

– Process of discovering neighbors on common links

• Link State Flooding– Process of flooding link state information within

a topology– Link state protocols converge much faster than

distance vector routing protocols when the topology changes

• Link State Database– Major part of a link state routing protocol– Important that routers have common link state

databases• SPF Algorithm

– Using the link state database, this algorithm is used to calculate the shortest paths to networks.

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Link State Routing Protocols

1 – Flooding of link-state information

2 – Building a Topological Database

3 – SPF Algorithm

4 – SPF Tree

5 – Routing Table

22

Interior and Exterior Gateway Protocols

• Note: IGRP and EIGRP are Cisco proprietary protocols. They are meant as an alternative between the limited RIP routing protocol and the more complicated and resource intensive OSPF and IS-IS routing protocols. IGRP was discontinued with IOS 12.2 in 2005.

• The dates shown are when the RFC or other document was finalized. The protocol may have been implemented earlier than this date.

Interior Gateway Protocols

Exterior Gateway Protocols

Distance Vector Routing Protocols

Link State Routing Protocols

Path Vector

Classful RIPv1 (1982/1988)

IGRP (1985)

EGP (1982)

Classless RIPv2 (1994)

EIGRP (1992)

OSPFv2 (1991)

IS-IS (1990)

BGPv4 (1995)

IPv6 RIPng (1997)

EIGRP for IPv6

(not yet released)

OSPFv3 (1999)

IS-IS for IPv6 (2000)

BGPv4 for IPv6

(1999)

23

Interior and Exterior Gateway Protocols

• Border Gateway Protocol (BGP) for IP

• Exterior Gateway Protocol (EGP) for IP (yes, an EGP named EGP)

• ISO’s InterDomain Routing Protocol (IDRP)

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Interior and Exterior Gateway Protocols

• Interior Gateway Protocols = Routing protocols within an autonomous system.

• Exterior Gateway Protocols = Routing protocols between an autonomous system.

• Autonomous System– Older definition: Group of routers under a common administrative

domain running a common routing protocol. (Nowadays it is common the some companies may run multiple routing protocols.)

– Newer definition: A network under a common administration.– Note: AS might also sometimes refer to a process domain.

25

Redistribution

• Redistribution is used to route between different IGP routing protocols.

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Classful vs. Classless Routing Protocols

Classless Routing Protocols

“The true characteristic of a classless routing protocol is the ability to carry subnet masks in their route advertisements.” Jeff Doyle, Routing TCP/IP

Benefits:

• All-zeros and all-ones subnets

– - Although some vendors, like Cisco, can also handle this with classful routing protocols.

• VLSM

– Can have discontiguous subnets

– Better IP addressing allocation

• CIDR

– More control over route summarization

Classless Routing Protocols

Classless Routing Protocols:

• RIPv2

• EIGRP

• OSPF

• IS-IS

• BGPv4

Remember classful/classless routing protocols is different than classful/classless routing behavior.

Classlful/classless routing protocols (RIPv1, RIPv2, IGRP, EIGRP, OSPF, etc.) has to do with how routes get into the routing table; how the routing table gets built.

Classful/classless routing behavior (no ip classless or ip classless) has to do with the lookup process of routes in the routing table (after the routing table has been built).

It is possible to have a classful routing protocol and classless routing behavior or visa versa.

It is also possible to have both a classful routing protocol and classful routing behavior; or both a classless routing protocol and classless routing behavior.

SantaCruz1

router rip

network 172.30.0.0

network 192.168.4.0

version 2

no auto-summary 

 

SantaCruz2

router rip

network 172.30.0.0

network 192.168.4.0

version 2

no auto-summary 

 

ISP

router rip

redistribute static

network 10.0.0.0

network 192.168.4.0

version 2

no auto-summary

ip route 207.0.0.0 255.0.0.0 null0

e0

ISP

SantaCruz2SantaCruz1

192.168.4.20/30

172.30.1.0/24

Internet

s0

s0 s0

s1

172.30.100.0/24

e0

192.168.4.24/30

.21

.22

.25

.26

10.0.0.0/8

e0.1

.1.1

Lo0Lo0

172.30.110.0/24172.30.2.0/24 .1

.1

.1

static route to207.0.0.0/8

207.0.0.0/16207.1.0.0/16207.2.0.0/16207.3.0.0/16

etc.

` 172.30.200.16/28

172.30.200.32/28

Lo1

Lo2

RIPv2 Example

e0

ISP

SantaCruz2SantaCruz1

192.168.4.20/30

172.30.1.0/24

Internet

s0

s0 s0

s1

172.30.100.0/24

e0

192.168.4.24/30

.21

.22

.25

.26

10.0.0.0/8

e0.1

.1.1

Lo0Lo0

172.30.110.0/24172.30.2.0/24 .1

.1

.1

static route to207.0.0.0/8

207.0.0.0/16207.1.0.0/16207.2.0.0/16207.3.0.0/16

etc.

` 172.30.200.16/28

172.30.200.32/28

Lo1

Lo2

SantaCruz2#show ip route 

172.30.0.0/16 is variably subnetted, 6 subnets, 2 masks

C 172.30.200.32/28 is directly connected, Loopback2

C 172.30.200.16/28 is directly connected, Loopback1

R 172.30.2.0/24 [120/2] via 192.168.4.21, 00:00:21, Serial0

R 172.30.1.0/24 [120/2] via 192.168.4.21, 00:00:21, Serial0

C 172.30.100.0/24 is directly connected, Ethernet0

C 172.30.110.0/24 is directly connected, Loopback0

192.168.4.0/30 is subnetted, 2 subnets

R 192.168.4.24 [120/1] via 192.168.4.21, 00:00:21, Serial0

C 192.168.4.20 is directly connected, Serial0

R 10.0.0.0/8 [120/1] via 192.168.4.21, 00:00:21, Serial0

R 207.0.0.0/8 [120/1] via 192.168.4.21, 00:00:21, Serial0

VLSM and the Routing Table

Supernet, classless routing protcols will route supernets (CIDR)