Distance Vector Routing Protocols

1. Distance Vector Routing Protocols CCNA 2 v3 – Module 7 NESCOT CATC

2. Distance Vector Routing – Loops Routing loops can occur when inconsistent routing tables are not updated due to slow convergence in a changing network. • E sends update to A • C still unaware of failure, advertises route via B X • Network fails • A sends update to B and D • D updates its routing table to include route from C, and forwards this incorrect information to A. Distance Vector Routing – Updates Updates occur periodically or when the topology changes. Updates proceed systematically from router to router. Router sends its entire routing table to each neighbor. NESCOT CATC

3. Invalid updates can continue to loop until some other process stops them. Each router the packet passes through increases the hop count by 1. Packets continuously looping around the network count to infinity.     The routing protocol permits the routing loop to continue 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. Maximum Hop Count

4. Router A advertises route to Network A A B 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. • Router B updates its routing table • Router B does not include Network A in update to A 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 to one more than the maximum. When used with triggered updates it will speed up convergence time. NESCOT CATC

5. Is update from same neighbor? Update received: network accessible again Yes Network accessible, remove holddown timer No Ignore update, Wait until holddown timer expires No Different neighbor, better metric? Yes RIP updates occur every 30 secs, but a triggered update is sent immediately. Router detects topology change, immediately sends update to adjacent routers – doesn’t wait for the update timer to expire. Wave of updates propagates throughout the network. Ensure all routers know of failed routes before holddown timers expire. Triggered Updates and Holddown Timers Route down! Pass it on… Route down! Send triggered update Interface down! Send triggered update now x The count to infinity problem can be avoided by using holddown timers. When the router marks a route inaccessible it starts a holddown timer. Update received: network inaccessible, start holddown timer

6. Configuration Example: • 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). Two versions ofRIP: RIP Routing Process RIP updates occur every ________ The maximum number of hops in a path is ____ RIP implements split horizon and holddown mechanisms. 30 secs. 15. GAD(config)# router rip GAD(config-router)# network 192.168.13.0 GAD(config-router)# network 192.168.14.0 NESCOT CATC

7. If one part of a major network is known, but the subnet toward which the packet is destined within that major network is unknown, the packet is dropped. IP Classless I know some 10.0.0.0/24 subnets, but not 10.2.2.0/24 Default route The router only uses the default route if the major network destination does not exist in the routing table. To forward these packets to the best supernet route possible: Router(config)# ip classless Configuring ip classless on the router allows it to ignore classful boundaries of the networks in its routing table and route to the default route. NESCOT CATC

8. To disable split horizon: GAD(config-if)# The default holddown is 180 secs. Decrease it to speed up convergence. Set the timer just longer than the longest possible update time for the network: GAD(config-router)# Update Invalid Holdtime Flush The default RIP update interval is 30 secs. Longer intervals can conserve bandwidth, shorter intervals may decrease convergence time: GAD(config-router)# To disable sending routing updates on specified interfaces: GAD(config-router)# To configure the router to send and receive packets from only version 2: GAD(config-router)# To control how packets received from an interface are processed: GAD(config-if)# RIP routers rely on neighbors for network information - Routing By Rumour. Convergence is when all routers in the Internetwork have the same routing information. Slow convergence of DV protocols results in inconsistencies. RIP Configuration Issues RIPs performance can be tuned to improve convergence time: no ip split-horizon timers basic 30 180 180 240 update-timer 40 passive-interface f0/0 version 2 ip rip receive version 1 2

9. RIP routing is configured Interfaces sending and receiving RIP updates Router is advertising the correct networks Verifying RIP Configuration Dublin# show ip protocols NESCOT CATC

10. Route received from RIP neighbor is in the routing table Verifying RIP Configuration Dublin# show ip route Additional commands to check RIP configuration: Statistics for all interfaces configured on router show ip interface Current configuration in RAM NESCOT CATC

11. To analyse RIP update issues: Pretoria# Other commands to troubleshoot RIP: Command Definition Summary of entries in RIP routing database show ip protocols {summary} View the routing table debug ip rip {events} Summary of interface status and parameters • Typical RIP configuration errors: • incorrect network statement • discontiguous subnets • split horizons Troubleshooting RIP debug ip rip show ip rip database • Data for each routing protocol active on router show ip route Check routing updates are being sent show ip interface brief NESCOT CATC

12. Load balancing allows a router to simultaneously use multiple paths to a destination. RIP can load balance over 6 equal-cost paths, (default4 paths). RIP Load Balancing 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 Here each path from GAD to BHM is considered equal by RIP metric (2 hops) Equal cost routes can be found by using #show ip route. Each route is represented by a routing descriptor block. An asterisk (*) next to one of the entries corresponds to the active route. NESCOT CATC

13. Static routes are important for destinations not included in dynamic routing processes. They are also useful for specifying a ____________. • Each dynamic routing protocol has a default ________________________ • A static route can be defined as less desirable than a dynamically learned route if its ____ is higher than the dynamic route’s.     • If a static route points to an interface that is not part of the RIP process (as defined with a network command) RIP will not advertise the route unless configured to: Redistributing Static Routes into RIP default route administrative distance (AD). AD Router(config)# router rip Router(config-router)# redistribute static • _____________________ are routes with an AD set greater than the AD of the dynamic routing protocol in use. • Static routes are removed from the routing table when their corresponding interface ___________ or when the next hop is ________________. • Static routes can be removed using the ______________ global configuration command. Floating Static routes goes down no longer valid no ip route NESCOT CATC

14. Default metric components These parameters are considered only if enabled via configuration 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. IGRP Features • Key design characteristics of IGRP are a follows: • The versatility to automatically handle indefinite, complex topologies • The flexibility to handle different bandwidth and delay characteristics • Scalable to large networks • IGRP can be configured to use a combination of variables for its metric: • Bandwidth • Delay • Reliability • Load • MTU – based on lowest bandwidth value in the path – the cumulative interface delay along the path – of the link based on exchange of keepalives – amount of traffic on a link based on bits per second – Maximum Transmission Unit value of the path show ip protocols – displays parameters including metric values K1 to K5. K1= bandwidth, K3= delay. show ip route – displays metric values in brackets for each route. NESCOT CATC

15. IGRP Routes Match the IGRP route type to its definition: NESCOT CATC

16. How long to wait in the absence of specific updates before declaring a route invalid (Default: 3 x U) How frequently routing update messages should be sent Time before a route is flushed from the routing table (Default: 7 x U) Amount of time for which information about poorer routes is ignored (Default: 3 x U + 10) IGRP Stability Features Like RIP, IGRP has a number of features designed to enhance its stability: • 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. IGRP default timer values show ip protocols Router# 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 > NESCOT CATC

17. 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 Consider this network on which RIP is already running: Configuring IGRP 192.168.3.0/24 192.168.1.0/24 A B 192.168.2.0/24 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 AD and metric NESCOT CATC

18. Verifying and Troubleshooting IGRP Check routing table for any ‘I’ routes show ip protocols Verify the router is configured for IGRP networks show running-config begin … Verify an interface is properly configured show running-config int … Overview of IGRP activity, updates sent and received debug ip igrp transactions End to end connectivity test at layer 3 traceroute NESCOT CATC