Distance Vector Routing Protocols

PJC CCNA Semester 2 Ver. 3.0by

William Kelly

Distance Vector Routing Protocols Intro to Distance Vector Protocols Load Balancing Routing Loops and their solution Examining Routing Tables Administrative Distance Gateway of Last Resort Integrating Static Routes with RIP Basics of RIP and IGRP Troubleshooting Routing Protocols

Distance Vector Protocols Intro. The two important Distance

vector routing protocols are RIP and IGRP

The ENTIRE routing table is sent periodically to neighboring routers

A topology change or a periodic update sends routing tables to neighbors

Load Balancing The maximum paths range from 1 to 6

By default most IP routing protocols install 4 routes in parallel

Static Routes always install 6 routes Rip can only load balance paths that

have same number of hops IGRP can load balance up to 6 unequal

paths The maximum-paths maximum

command allows the number of parallel paths used to load balance to be changed in configuration mode

Load Balancing (switching) process switching (packets)

The router alternates paths on a per packet basis

fast switching (per destination) All packets in the packet stream

bound for a specific host take the same path

Packets bound for a second host on the same destination network would all take an alternate path

What is a Routing Loop?

Network 1

Router B

Router D

Router CRouter A Router E

RoutingLoop

Edge of UpdateSpreads

Router C hasold Route toNetwork 1

that it sendsto D

How to Solve Routing Loops

Count to Infinity Split Horizon Route Poisoning Triggered Updates Hold Down Timers

Count to Infinity Distance Vector Protocols define Infinity as

a specific number Looping continues until Infinity (16 for RIP)

is reached When infinity is reached the Network is

considered unreachable In our example the loop would continue

until the county reached 16 and then network 1 would be marked unreachable

Split Horizon A routing loop occurs when incorrect

information is sent to a router that just sent out correct information

The solution is to avoid sending information back in the direction it came

In our example Routers A and E would never have received C’s bad info and thus no routing loop would have occurred.

Route Poisoning Route poisoning is used to overcome loops

in large networks by setting the hop count to one more than the maximum

The poison reverse rule states “Once you learn of a route through an interface, advertise it as unreachable back through that same interface”.

Route poisoning is essentially the same as poison reverse + split horizon

Triggered Updates By sending updates faster than the

default update convergence occurs must faster

A topology change quickly propagates through a network

Triggered updates used with route poisoning marks a “down” network as unreachable quickly through the network

Hold Down Timers1. When a router receives information that a

previously accessible route is inaccessible it starts a hold down timer and marks the route as inaccessible

2. If an update is received indicating the route is back up before the hold down timer expires then the route is marked accessible again

3. If an update is received from a different neighbor with a better metric then the route is marked accessible and the hold down time is removed

4. If an update is received from a different neighbor with a poorer metric before the hold down timer expires then the update is IGNORED

Examining Routing Tables

Show ip route command How did I receive a route? What is the metric? What is the administrative distance? Is the route directly connected? What is the output interface to get to a

route?

Examining Routing TablesVista#sh ip routeCodes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E – EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area

* - candidate default, U - per-user static route, o – ODR

P - periodic downloaded static route

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 3 subnetsC 172.16.8.0 is directly connected, FastEthernet0/0R 172.16.5.0 [120/1] via 172.16.7.1, 00:00:02, Serial0/0C 172.16.7.0 is directly connected, Serial0/0

Administrative Distance Administrative distance is a default value

assigned to each routing protocol that will favor the best route from one protocol over another one

Distances for common protocols are:Directly Connected 0

Static Routes 1

IGRP 100

OSPF 110

RIP 120

Gateway of Last Resort Routers try to keep routing tables as small as possible A router may not be able to match a destination

network with an entry in it’s routing table Default routes can be entered statically or learned

dynamically ip default network x.x.x.x establishes a default route

in networks using dynamic routing Any router set with default network x.x.x.x that has a

route to x.x.x.x flags x.x.x.x as a candidate default route

A default route can be statically set by saying: ip route 0.0.0.0 0.0.0.0 {next hop ip or exit interface} If no path to the destination network is found

in the routing table then the quad zero default is used

Integrating Static Routes with RIP If your static route was not defined with a

network command then it is not distributed unless you use the redistribute static command

ip route dest. mask ip/interface admin_distance Packets bound for specific destination

networks can be forced to follow a certain path

Using a higher administrative distance can provide a backup path in case of main link failure

RIP Basics To configure use route rip, then network

x.x.x.x The metric is hop count A hop count of 16 is infinity Period updates are sent every 30 seconds It is a distance vector protocol The entire routing table is sent during

updates The administrative distance is 120 The hold down timer default is 180 seconds

IGRP Basics To configure use route igrp as-number, then

network x.x.x.x The metrics are bandwidth, delay, load, and

reliability Period updates are sent every 90 seconds The hold down time is 3x the update timer or

90 secs. x 3 + 10 seconds = 280 seconds. It is a distance vector protocol The entire routing table is sent during updates The administrative distance is 100 Scalable for very large networks

Troubleshooting Routing Protocols

Is the routing protocol set? show ip protocols

Is the route in the routing table? show ip route

Are the interfaces configured? show running-config

Am I using the same version of RIP throughout my network?

ping, traceroute, debug