CCNA 2 v3.1 Module 7

Module 7 Distance Vector Routing Protocols RIP and IGRP CCNA 2 Module 7 Distance Vector Routing Protocols RIP and IGRP

Distance vector routing updates Routing table updates occur periodically or when topology change Routers send their entire routing table to all adjacent neighbors Routing table contains info about routing metrics Hop count Bandwidth Cost Load Reliability Delay

Distance vector routing loop Loops occur when inconsistent routing tables are not updated due to slow convergence in a changing network

Count To Infinity Count to infinity To overcome count to infinity Packets loop continuously around the network Packet never reach their destination May occur because Network is down Interface not configured correctly To overcome count to infinity Specify a maximum hop count Split Horizon Route Poisoning Hold Down timers

Maximum hop count Used to overcome count to infinity Hop count The number of routers a packet goes through It increases each time a packet goes through a router RIP maximum hop count is 15 Packet is discarded if the hop count of 16 is reached This is because the network 1 is unreachable

How does the Maximum Hop Count Work?

Split Horizon Solution to count to infinity Do not send routing updates to the router you learned the information about Reduces incorrect routing information Reduces routing overhead

Route poisoning

Poison reverse When route poisoning is used with triggered updates it will speed up convergence time because neighboring routers do not have to wait 30 seconds before advertising the poisoned route. Failed routes are advertised with infinite metrics.

Avoiding routing loops with triggered updates Sent immediately when topology changes Used in conjunction with route poisoning Ensure all routers know of failed routes before any holddown timers can expire i.e., Don’t wait for the time limit to expire

Preventing routing loops with holddown timers When a router receives an update saying a route is down, it marks the route as inaccessible and starts a hold-time timer If at any time before the holddown timer expires an update is received from: the same neighbour indicating the network is accessible again Router marks network as accessible + removes holddown timer A different neighbour with a better metric A different neighbour with a poorer metric The update is ignored

RIP Routing Protocol Open standard RFC 1058 and Internet Standard (STD) 56 Documented by: Request for Comments (RFC) 1058 Internet Standard (STD) 56. RIP Version 1 (RIP v1) Classful Routing Protocol RIP Version 2 (RIP v2) Classless Routing Protocol Ability to carry additional packet routing information. Authentication mechanism to secure table updates. Supports variable length subnet masking (VLSM)

RIP has slow convergence which results in Routing loops Count to infinity To reduce these RIP uses Count-to-infinity Split horizon Poison reverse Holddown counters Triggered updates

Configuring RIP Enter global configuration mode Router#config t Specify RIP as the routing protocol Router(config)#router rip Specify directly connected networks Router(config-router)#network 10.10.10.0 A router running RIP can be configured to Send triggered updates independently the scheduled updates (every 30 seconds) Router(config-if)#ip rip triggered

RIP Configuration Issues Count-to-infinity Does not have to be configured with RIP To disable Split Horizon Router(config-if)#no ip split-horizon Default holddown for RIP is 180 seconds The default RIP update 30 seconds Can be configured for longer intervals To conserve bandwidth Can be configured for for shorter intervals To decrease convergence time Command Router(config)#router rip Router(config-router)#update-timer seconds

To prevent RIP updates leaving an interface Router(config-router)#passive-interface Fa0/0 To configure RIP to exchange routing information in a non-broadcast network (Frame Relay) RIP needs to be told of other neighboring RIP routers Router(config-router)#neighbor <ip address>

Configure Router to Send/Receive Packets To configure software to send and receive Rip V1 and V2 packets Router(config-router)#version{1|2} To configure an interface to send RIP V1 packets Router(config-if)#ip rip send version 1 To configure an interface to send RIP V2 packets Router(config-if)#ip rip send version 2 To configure an interface to send RIP V1 and V2 packets Router(config-if)#ip rip send version 1 2 To configure and interface to receive RIP V1 packets Router(config-if)#ip rip receive version 1 To configure and interface to receive RIP V2 packets Router(config-if)#ip rip receive version 2 To configure and interface to receive RIP V1 and V2 packets Router(config-if)#ip rip receive version 1 2

Verifying RIP configuration show ip protocols Shows which routing protocols are carrying IP traffic on the router Confirms RIP routing is configured The correct interfaces are sending and receiving RIP updates The router is advertising the correct networks show ip route Displays the routing table Verify that routes received by RIP neighbours are installed in the routing table Additional commands to check RIP configuration show interface interface show ip interface interface show running-config

Troubleshooting RIP update issues debug ip rip Displays RIP routing updates as they are sent and received Problems such as discontiguous subnets or duplicate networks can be diagnosed with this command

Discontiguous subnet

Duplicate Subnets Duplicate Subnets

Other commands to troubleshoot RIP: show ip rip database show ip protocols {summary} show ip route debug ip rip {events} show ip interface brief

Preventing routing updates through an interface network command If it is issued for a given network using RIP or IGRP router immediately begin sending advertisements out all interfaces within the specified network address range Router(config)#router RIP Router(config-router)#network 192.101.21.0 passive interface command Stops the router from sending updates through a particular interface to a neighbor Router continues to listen and use routing updates from that neighbor Prevent other systems from learning about routes dynamically Router(config-router)#passive-interface fa0/0

Load balancing with RIP Allows a router to take advantage of multiple best paths to a given destination Calculated by dynamic routing protocol Entered statically by administrator RIP performs “round robin” load balancing over 6 equal-cost paths, with 4 paths being default takes turns forwarding packets over the parallel paths

Interface pointer cycles through the interfaces and routes in a deterministic fashion such as 1-2-3-4-1-2-3-4-1 RIP metric is hop count no regard is given to the speed of the links 56 Kbps path will be given the same preference as the 155 Mbps path

Equal cost routes can be found by using the “show ip route “

When a router learns multiple routes to a specific network, the route with the lowest administrative distance is installed in the routing table

When a router has multiple routes to a network route with lowest admin distance is installed in routing table If the router has many routes, learned via the same routing process with the same admin distance router chooses the path with the lowest cost or metric If the router has multiple paths to a destination with the same admin distance and cost load-balancing can occur Cisco IOS allows up to 6 equal cost routes in a routing table EIGRP allows up to 4 equal cost routes Static routes always install 6 routes BGP by default allows only 1 route IGRP can load balance up to 6 unequal links

Router(config-router)#maximum-paths [no.] Cisco IOS offers two methods of load balancing: per-packet:If process switching is enabled, Router will alternate paths on a per-packet basis per-destination: If fast switching is enabled Only one route will be used for that destination address All packets bound for a specific host take the same path Packets bound for a different host on the same network may use an alternate route

Integrating static routes with RIP User-defined routes Force packets to take a specific path Useful for specifying a “gateway of last resort”; a default route Administrative Distance (AD) Each dynamic routing protocol has a default AD A static route can be defined as less desirable than a dynamically learned route, as long as the AD of the static route is higher than that of the dynamic route floating static route A route configured on a router to take the place of the RIP route in the event that the RIP routing process fails RIP can advertise static routes using the redistribute static command

IGRP features Distance vector routing protocol Developed by Cisco Sends routing updates every 90 second Sends entire table Advertising networks for an autonomous system Metrics used are normally bandwidth and delay Bandwidth Delay – amount of time to get to the destination Reliability – reliability of link determined by exchange of keepalives Load MTU – maximum transmission unit of the path

IGRP converges faster than RIP thereby avoiding the routing loops IGRP does not share the hop count limitation of RIP. Useful Commands show interface interface show running-config show running-config interface interface show running-config | begin interface interface show running-config | begin igrp show ip protocols

Metric K1 represents bandwidth by default is set to 1 Metric K3 represents delay and by default is set to 1 Metric K2, K4 and K5 are set to 0.

show ip route shows the IGRP metric values in brackets. A link with a higher bandwidth will have a lower metric A route with a lower cumulative delay will have a lower metric

IGRP Routes GRP advertises three types of routes: Interior Routes between subnets of a network attached to a router interface If the network attached to a router is not subnetted, IGRP does not advertise interior routes System Routes to networks within an autonomous system number The IOS derives system routes from directly connected network interfaces System routes information provided by other IGRP routers. System routes do not include subnet information Exterior Routes to networks outside the autonomous system that are considered when identifying a gateway of last resort. Gateway of last resort is used if a better route is not found The destination is not a connected network Exterior routes are also referred to as "candidate default"

IGRP Routes

IGRP stability features Holddowns Prevent regular update messages from inappropriately reinstating a route that may not be up Split horizons It is usually not useful to send information about a route back in the direction from which it came Prevent routing loops Poison reverse Poison reverse updates then are sent to remove the route and place it in holddown. With IGRP, poison reverse updates are sent only if a route metric has increased by a factor of 1.1 or greater.

IGRP Timers update timer specifies invalid timer specifies how frequently routing update messages should be sent Default is 90 sec invalid timer specifies how long a router should wait in the absence of routing-update messages about a route before declaring that route invalid Default is 3 times the update period (270 sec) holddown timer specifies the amount of time for which information about poorer routes is ignored Default is 3 times the update timer period + 10 sec (280 sec) flush timer indicates how much time should pass before a route is flushed from the routing table Default is 7 times the routing update timer (after 630 sec)

Configuring IGRP To set IGRP routing on To turn IGRP routing off Router# config t Router(config)# router igrp 101 Router(config-router)#network 192.101.2.0 To turn IGRP routing off Router(config)# no router igrp 101

Troubleshooting IGRP Problems to look out for Useful commands mistyped network statement discontiguous subnets Incorrect Autonomous System Number Useful commands show ip protocols show ip route debug ip igrp events debug ip igrp transactions ping traceroute