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Distance Vector Routing Protocols

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1 Distance Vector Routing Protocols
CCNA 2 v3 – Module 7 NESCOT CATC CCNA 2 V3 Module 07 DC

2 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. 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 A sends update to B and D C still unaware of failure, advertises route via B D updates its routing table to include route from C, and forwards this incorrect information to A. X Network fails NESCOT CATC

3 Distance Vector Routing Protocols
Maximum Hop Count Distance Vector Routing Protocols 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. CCNA 2 V3 Module 07 DC

4 Split Horizon Route Poisoning
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. B A Router A advertises route to Network A 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 Triggered Updates and Holddown Timers
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. 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. 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

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

7 I know some subnets, but not
IP Classless 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. I know some /24 subnets, but not /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 RIP Configuration Issues
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. RIPs performance can be tuned to improve convergence time: To disable split horizon: GAD(config-if)# no ip split-horizon 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)# timers basic Holdtime Flush Update Invalid The default RIP update interval is 30 secs. Longer intervals can conserve bandwidth, shorter intervals may decrease convergence time: GAD(config-router)# update-timer 40 To disable sending routing updates on specified interfaces: GAD(config-router)# passive-interface f0/0 To configure the router to send and receive packets from only version 2: GAD(config-router)# version 2 To control how packets received from an interface are processed: GAD(config-if)# ip rip receive version 1 2

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

10 Verifying RIP Configuration
Dublin# show ip route Route received from RIP neighbor is in the routing table Additional commands to check RIP configuration: Command Definition show interface Interface's IP information and status show running-config Statistics for all interfaces configured on router show ip interface Current configuration in RAM NESCOT CATC

11 Troubleshooting RIP Typical RIP configuration errors:
incorrect network statement discontiguous subnets split horizons Pretoria# To analyse RIP update issues: debug ip rip Other commands to troubleshoot RIP: Summary of interface status and parameters debug ip rip {events} View the routing table show ip protocols {summary} Summary of entries in RIP routing database Definition Command 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 RIP Load Balancing 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 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 Redistributing Static Routes into RIP
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: 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 IGRP Features 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 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 Default metric components – the cumulative interface delay along the path – of the link based on exchange of keepalives These parameters are considered only if enabled via configuration – 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: Type
Interior Routes to networks outside AS. Used to identify default gateway. Different routers may choose different routes as the gateway of last resort. 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 within the AS. Derived from directly connected interfaces and information from other IGRP-speaking devices. Do not include subnet information. NESCOT CATC

16 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 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) NESCOT CATC

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

18 Verifying and Troubleshooting IGRP
Command Related Use show ip route To check if IGRP is enabled show running-config Filter the tail of the output from full command show interface Filter interface specific information from full command debug ip igrp events IGRP activity including route update details ping Hop by hop path determination utility 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

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