CCNA 2 v3.1 Module 7 Distance Vector Routing Protocols

Objectives

Distance Vector Updates

Problem: Routing Loops Routing loops can occur when inconsistent routing tables are not updated due to slow convergence in a changing network.

Problem: Counting to Infinity A sends update to B and D E sends update to A C still unaware of failure, advertises route via B X Network fails D updates its routing table to include route from C, and forwards this incorrect information to A.

Solution: Defining a Maximum for Infinity The routing protocol permits the routing update to propagate (loop) until the metric exceeds its maximum allowed value.  In RIP, if the hop count exceeds the maximum of 15 hops the packet is discarded and the network is considered unreachable.

Solution: Split Horizon Split Horizon is another mechanism used to avoid routing loops. Information about routes is prevented from being advertised out the router interface through which the information was received.

Solution: Route Poisoning Poison Reverse updates are used to overcome large routing loops by sending explicit information when a subnet or network is not accessible. Sets the hop count for that network to infinity (one more than the maximum). Advertise it with

Solution: Triggered Updates Router detects topology change, immediately sends update to adjacent routers – doesn’t wait for the update timer to expire. Wave of updates (single route not routing tables) propagates throughout the network. Ensure all routers know of failed routes before holddown timers expire.

RIP Triggered Updates Triggered updates can be configured for RIP int s0 ip rip triggered Periodic broadcasts are suppressed on the interface where triggered updates are enabled

Solution: Holddown Timers The count to infinity problem can be avoided by using holddown timers. When the router marks a route inaccessible it starts a holddown timer. if a route with a worse metric is received for a route that is marked as inaccessible ignore it until holddown timer expires (see next slide)

Wait until holddown timer expires Holddown Timers Is update from same neighbor? Update received: network inaccessible, start holddown timer Update received: network accessible again Yes Network accessible, remove holddown timer No Different neighbor, better metric? No Ignore update, Wait until holddown timer expires Yes

Key Characteristics of RIP

RIP Two versions of RIP: RIP v1: Classful routing protocol Does not include subnet masks in updates RIP v2: Classless routing protocol Carry additional packet routing information. Authentication mechanism to secure table updates. Supports variable length subnet masking (VLSM). RIP updates occur every ________ The maximum number of hops in a path is ____ RIP implements split horizon and holddown mechanisms. 30 secs. 15.

RIP Commands We will now cover the following in more detail Using router rip to enable and start RIP Use the network commands to seed the routing table Monitoring IP packet flow using the show ip protocol command Displaying the routing table using the show ip route command

Configuring RIP

Using the ip classless Command I know some 10.0.0.0/24 subnets, but not 10.2.2.0/24 So packet is dropped

IP Classless A router by default assumes that all subnets of a directly connected network should be present in the routing table. If a packet is received with an unknown destination address within an unknown subnet of a directly attached network, the router assumes that the subnet does not exist. So the router will drop the packet even if there is a default route. Configuring ip classless on the router resolves this problem by allowing the router to ignore the classful boundaries of the networks in its routing table and simply route to the default route.

RIP Configuration Issues To reduce routing loops and counting to infinity, RIP uses the following: Defining infinity Split horizon Route poisoning & poison reverse Triggered updates Holddown timers

Preventing Routing Updates through an Interface

The show ip protocols Command

The show ip route Command

Typical RIP configuration errors: incorrect network statement discontiguous subnets (bad planning) can result in wrong route being advertised split horizons

Troubleshooting RIP Update Issues debug ip rip show ip rip database Displays summary address entries in the RIP routing database show ip protocols {summary} show ip route shows routing table debug ip rip {events} send and receive info show ip interface brief summarized information on protocols per interface

Debug Router RIP

Load Balancing with RIP Here each path is considered equal by RIP metric (2 hops)

RIP Load Balancing Router(config-router)# maximum-paths 5 Load balancing allows a router to simultaneously use multiple paths to a destination. RIP can load balance over 6 equal-cost paths, (default 4 paths). Router(config-router)# maximum-paths 5 RIP performs what is referred to as “round robin” load balancing: If process switching is enabled, paths alternate on a ___________ basis. If fast switching is enabled, paths alternate on a _______________ basis. per-packet per-destination

Administrative Distance

Integrating Static Route with RIP Static routes are user-defined routes A router running RIP can receive a default route via an update from another router running RIP. Another option is for the router to generate the default route itself. The administrator can override a static route with dynamic routing information by adjusting the administrative distance values. The default administrative distance for RIP is 120 By default the administrative distance of a static route is 1 and that of a directly connected link is 0 (most desirable). Static routes are not normally distributed but distribution can be enabled Router(config) router rip Router(config-router)# redistribute static

Floating Static Route A backup route that takes over if the original route fails A route that has an AD of 130 will not be used until the RIP route with an AD of 120 is not available.

IGRP Interior Gateway Routing Protocol is a DV protocol proprietary to Cisco. IGRP sends routing updates at 90 second intervals, advertising networks for a particular AS. Key design characteristics of IGRP are a follows: The versatility to automatically handle indefinite, complex topologies The flexibility as it has multiple metric which can handle different bandwidth and delay characteristics Scalable to large networks (max TTL 255)

IGRP Features

IGRP Commands Configure using the router igrp and network commands The show ip protocol command For monitoring IP packet flow – Displays parameters including metric values K1 to K5. K1= bandwidth, K3= delay. The show ip interfaces command The show ip route command Shows the actual metric values for each specific route The debug ip igrp command

IGRP Metrics Bandwidth – The lowest bandwidth value in the path Delay – The cumulative interface delay along the path Reliability – The reliability on the link towards the destination as determined by the exchange of keepalives Load – The load on a link towards the destination based on bits per second MTU – Maximum Transmission Unit for the path

IGRP Routes: Interior, System, & Exterior

Interior Exterior System Routes between subnets of a network attached to a router interface. If the network is not subnetted, IGRP does not advertise them. Exterior Routes to networks outside AS. Used to identify default gateway. Different routers may choose different routes as the gateway of last resort. System Routes to networks within the AS. Derived from directly connected interfaces and information from other IGRP-speaking devices. Do not include subnet information.

IGRP Stability Features Holddowns, Split horizons, & Poison-reverse updates With IGRP, poison reverse updates are sent only if a route metric has increased by a factor of 1.1 or greater.

Show IP Protocols

Show IP Protocols IGRP default timer values Router# show ip protocols Routing protocol is “IGRP 101” Sending updates every 90 seconds, next due in 51 seconds Invalid after 270 seconds, holddown 280 seconds, flushed after 630 seconds < output omitted > How frequently routing update messages should be sent Time before a route is flushed from the routing table (Default: 7 x U) How long to wait in the absence of specific updates before declaring a route invalid (Default: 3 x U) Amount of time for which information about poorer routes is ignored (Default: 3 x U + 10)

Configuring IGRP

Migrating RIP to IGRP

Migrating RIP to IGRP Consider this network on which RIP is already running: 192.168.3.0/24 192.168.1.0/24 A B 192.168.2.0/24 Next update A# show ip route < output omitted > C 192.168.1.0/24 is directly connected, FastEthernet0/0 C 192.168.2.0/24 is directly connected, Serial0/0 R 192.168.3.0/24 [120/1] via 192.168.2.2, 00:00:29, Serial0/0 IGRP is then configured on both routers, example: A(config)# router igrp 101 A(config-router)# network 192.168.1.0 A(config-router)# network 192.168.2.0 Once A has received an IGRP update from B: A# show ip route < output omitted > C 192.168.1.0/24 is directly connected, FastEthernet0/0 C 192.168.2.0/24 is directly connected, Serial0/0 I 192.168.3.0/24 [100/80135] via 192.168.2.2, 00:00:69, Serial0/0 AD and metric

Verifying IGRP Configuration Some commands for checking IGRP configuration are as follows: 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

Troubleshooting IGRP The following commands are useful when troubleshooting IGRP: show ip protocols {summary} show ip route debug ip igrp events IGRP protocol events debug ip igrp transactions IGRP protocol transactions ping traceroute

Summary