Distance Vector Routing

Distance Vector Routing CCNA Exploration Semester 2 Chapter 4

Topics • Characteristics of distance vector routing protocols • Distance vector routing protocols in use today • How they discover routes • How they maintain routing tables • Routing loops

Routing protocols Interior Exterior Distance vector Link state RIP v1RIP v2IGRPEIGRP OSPFIS-IS EGPBGP

Distance vector knowledge • A distance vector protocol learns: • The distance to a network, measured in hops or in some other way • The direction of the network: which port should be used to reach it • It puts the routes in the routing table • It does not know any more details of the route or the other routers along the way

Distance vector Network 192.168.48.0 is 3 hops away using port fa0/0 Network 192.168.22.0 is 2 hops away using port fa0/1

Link state knowledge • A link state routing protocol finds out about all the routers in the system and the networks they link to. • It builds up a complete picture of the topology • It can then work out the best path to any network • It puts these best paths in the routing table

Link state I know all the routers and paths in this system of networks.

Metrics • RIP v1 and 2 hop count, maximum 15 • IGRP and EIGRP bandwidth, delay, load, reliability

Distance vector • Exchange complete routing tables with immediate neighbours • Do this at regular intervals • Adjust the metric, e.g. add 1 to the hop count, or add number based on bandwidth and delay of link.

Send update Adjust the metric 192.168.13.0 is 2 hops away 192.168.13.0 is 3 hops away

Sending updates • RIP v1 Whole routing table Broadcast every 30 sec • RIP v2 Whole routing table Multicast every 30 sec • IGRP Whole routing table Broadcast every 90 sec • EIGRP Initial learning process then small updates when topology changes

Link state Slow to convergeEasy to configureSmall networksLittle use of resources Fast to convergeHarder to configureLarge networksMuch use of resources Distance vector EIGRP RIP v1RIP v2IGRP OSPFIS-IS Routing protocols

Distance vector updates 10.4.0.0 10.1.0.0 10.2.0.0 10.3.0.0 Routers start up. R1 adds directly connected networks to table.

Distance vector updates 10.4.0.0 10.1.0.0 10.2.0.0 10.3.0.0 Exchange of routing table information.

Distance vector updates 10.4.0.0 10.1.0.0 10.2.0.0 10.3.0.0 R1 has learned about 10.3.0.0 from R2. It does not know about 10.4.0.0

Distance vector updates 10.4.0.0 10.1.0.0 10.2.0.0 10.3.0.0 Exchange of routing table information.

Distance vector updates 10.4.0.0 10.1.0.0 10.2.0.0 10.3.0.0 R1 has learned about 10.4.0.0 from R2. R2 previously learned about it from R3.

Update timer R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:04, Serial0/0 • Show ip route gives number of seconds since last update. Routing Protocol is “rip”Sending updates every 30 seconds, next due in 3 seconds • Show ip protocols says when next update is due. • Update timer default is 30 seconds

RIP timers Routing Protocol is “rip”Sending updates every 30 seconds, next due in 26 secondsInvalid after 180 seconds, hold down 180, flushed after 240 Routing table contains two RIP routes R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:04, Serial0/0 R 10.4.0.0 [120/2] via 10.2.0.2, 00:00:04, Serial0/0

RIP timers Routing Protocol is “rip”Sending updates every 30 seconds, next due in 30 secondsInvalid after 180 seconds, hold down 180, flushed after 240 30 seconds – updateRoute to 10.3.0.0 refreshedRoute to 10.4.0.0 not included R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0 R 10.4.0.0 [120/2] via 10.2.0.2, 00:00:30, Serial0/0

RIP timers Routing Protocol is “rip”Sending updates every 30 seconds, next due in 30 secondsInvalid after 180 seconds, hold down 180, flushed after 240 240 seconds – updateRoute to 10.3.0.0 refreshedRoute to 10.4.0.0 not included R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0 Route has been removed.

RIP_JITTER • RIP updates can become synchronised • This is a problem if routers are linked by hubs because the updates will collide • RIP_JITTER is a random variable that makes updates vary a little from the default 30 seconds

Triggered updates • These are to speed up convergence • Interface goes up/down, route added/removed • Router detects change, sends update to neighbour at once without waiting for timer • Neighbour passes on update at once.

EIGRP • Does not send regular updates • Does not send its whole routing table • Sends only information about changes • Sends only to routers that need the information • Non-periodic, partial, bounded.

Routing loop • A packet is sent from router to router in a loop until it is eventually dropped when its TTL field drops to 0 • Caused by incorrect or out of date information in routing tables • Very bad for network – uses up bandwidth and processing power in routers

Avoiding routing loops • Defining a maximum metric to prevent count to infinity • Holddown timers • Split horizon • Route poisoning or poison reverse • Triggered updates

Maximum metric • Routers exchanging wrong information can report higher and higher values of the metric. • RIP sets a maximum metric. • The hop count can go up to 15. • If it reaches 16 then the route is regarded as unreachable.

Holddown timers • Router receives update saying that a network is down. • Router marks the network as possibly down and starts holddown timer. • Update with a better metric for that network arrives: network is reinstated and holddown timer removed. • Update with the same or worse metric for that network arrives: update is ignored. • Timer runs out : network removed from table. • Packets still forwarded to network while timer runs.

Split horizon Route to 10.1.1.0 in 4 hops Route to 10.1.1.0 in 3 hops • Router receives information about a route through an interface. • It will not send out information about the same route through that interface.

Route poisoning • A router detects that a route has gone down. • It marks that route as unreachable in its routing table. (16 hops for RIP) • It sends out updates that show the route as unreachable. • Neighbour routers pass on these “poison” updates.

Poison reverse • This is an exception to split horizon. • If a router receives an update marking a route as unreachable then it will send this information back to the router that sent it.

RIP v1 Classful, does not send subnet mask in updates so does not support VLSM Sends updates as broadcasts No authentication No manual route summarisation RIP v2 Classless, includes the subnet mask in routing updates, so supports VLSM. Sends updates as multicasts Authentication for security Supports manual route summarization. RIP v1 and RIP v2

RIP v2 or EIGRP? • RIP runs on any make of router, EIGRP only on Cisco routers. • EIGRP is suitable for large networks • EIGRP uses a more efficient metric and may choose faster routes. • EIGRP converges faster than RIP • EIGRP uses less bandwidth but it needs more processing power and RAM • RIP is simpler to configure

The End

Distance Vector Routing