Tài liệu học CCNA kỳ 2 mod7 distancevectorrouting

1 Module 7 – Distance Vector Routing Protocols NIIT-ICT Hanoi info@niithanoi.vn 2 Overview • Describe how routing loops can occur in distance vector routing • Describe several methods used by distance vector routing protocols to ensure that routing information is accurate • Configure RIP • Use the ip classless command • Troubleshoot RIP • Configure RIP for load balancing • Configure static routes for RIP • Verify RIP • Configure IGRP • Verify IGRP operation • Troubleshoot IGRP 2 NIIT-ICT Hanoi info@niithanoi.vn 3 Distance Vector Routing Updates NIIT-ICT Hanoi info@niithanoi.vn 4 Distance Vector Routing Updates • RIP – Hop Count • IGRP and EIGRP – Bandwidth, Delay, Reliability, Load • Cisco’s OSPF – Bandwidth • IS-IS –Cost • BGP – Number of AS or policy 3 NIIT-ICT Hanoi info@niithanoi.vn 5 Routing Loop Issues Routing Loops • Distance vector routing protocols are simple in their implementation and configuration, but this comes at a price. • Pure distance vector routing protocols suffer from possible routing loops. • Routing loops can cause major network problems, from packets getting lost (blackholed) in your network, to bringing down your entire network. • Several remedies to have been added to distance-vector algorithms to help prevent routing loops including: – Split horizon – Hold-down timers – Defining a maximum metric NIIT-ICT Hanoi info@niithanoi.vn 6 Routing Loop Issues What can cause routing loops? • Routing loops can occur when there are: – Incorrect or inconsistent routing updates due to slow convergence after a topology change. (Example coming up next.) – Incorrect or incomplete routing information (see presentation on Discard Routes) – Static routes incorrectly configured with an intermediate address which does not become resolved in the routing table. (see presentation on Static Routes – Additional Information) 4 NIIT-ICT Hanoi info@niithanoi.vn 7 Routing Loop Issues Routing Loop Example • Assume for the remainder of this example that Router C’s preferred path to network 1 is by way of Router B. • Router C’s routing table has a distance of 3 to network 1 via Router B. NIIT-ICT Hanoi info@niithanoi.vn 8 Routing Loop Issues Network 1 Fails • Router E sends an update to Router A. • Router A stops routing packets to network 1. • But Routers B, C, and D continue to do so because they have not yet been informed about the failure. • Router A sends out its update. • Routers B and D stop routing to network1, (via Router A). • However, Router C is still not updated. • To router C, network 1 is still reachable via router B. 5 NIIT-ICT Hanoi info@niithanoi.vn 9 Routing Loop Issues Router C sends a periodic update to Router D • Router C sends a periodic update to Router D indicating a path to network 1 (by way) of via Router B. (4 hops). Router D’s Routing Table information for Network 1 • Current path to Network 1 = Unreachable (down) • Information from Router C: Network 1 : 4 hops by way of Router C • Normally, RouterD ignores this routing information because it usually has a better route, 2 hops, via Router A, but this route is now down. • Router D changes its routing table to reflect this (good) better, but incorrect information, Network 1 by way of Router C (4 hops) • Router D propagates the information to Router A. NIIT-ICT Hanoi info@niithanoi.vn 10 Routing Loop Issues Routers A changes its routing table • Router A adds new route to its routing table, get to Network 1 by way of Router D (5 hops). • Propagates the information to Routers B and E. Router B (and Router E) change their routing tables • Router B now believes it can get to Network 1 by way of Router A (6 hops).  Wow! I was about to tell Router C that Network 1 was down via Router B, but now I have new information! • Propagates the incorrect information to Router C. 6 NIIT-ICT Hanoi info@niithanoi.vn 11 Routing Loop Issues Router C changes its routing table • Router C still believes it can get to Network 1 by way of Router B (7 hops).  Of course now it believes it is 7 hops instead of 3. • Propagates the newer but still incorrect information to Router D. Here we go again! • Data packets destined for Network 1 get caught in a routing loop, from Routers A to D to C to B to A to D etc. • As routing updates continue between the routers, the hop count gets greater – to infinity? (Not quite – we will see in a moment.) NIIT-ICT Hanoi info@niithanoi.vn 12 Defining a Maximum Problem: Count to infinity Solution: Defining a Maximum • Distance vector routing algorithms are self-correcting, but a routing loop problem can require a count to infinity. • To avoid this prolonged problem, distance vector protocols define infinity as a specific maximum number. • This number refers to a routing metric which may simply be the hop count. • When the metric value exceeds the maximum value, and as each router receives this maximum metric, the network is then considered unreachable. 7 NIIT-ICT Hanoi info@niithanoi.vn 13 Split Horizon • “The effect of split horizon is that a router will send out different routing messages on different interfaces. In effect a router never sends out information on an interface that it learned from that interface.” (Lewis, Cisco TCP/IP Routing) NIIT-ICT Hanoi info@niithanoi.vn 14 Split Horizon • This example from the curriculum is not an example of split hoizon, but using hold-down timers. • “Split-horizon attempts to avoid this situation. If a routing update about Network 1 arrives from Router A, Router B or Router D cannot send information about Network 1 back to Router A. Split-horizon thus reduces incorrect routing information and reduces routing overhead.” • Initially, this is true, but the loop is a result of Router C sending out the updates, because it has not converged. 8 NIIT-ICT Hanoi info@niithanoi.vn 15 Split Horizon Rule – Avoiding Routing Loops • Routers RTA and RTB have their initial routing tables and are ready to exchange routing information via a distance-vector routing protocol like RIP. Split Horizon disabled • If split horizon were disabled the routing updates would include all of the networks in their routing tables including their directly connected networks and any networks learned from any interface. RTA RTB 10.1.1.0/24 Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.2.0/24 10.1.3.0/24 s0 s0e0 e0 .1 .1.1 .2 Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0 Simple Split Horizon Initial routing tables NIIT-ICT Hanoi info@niithanoi.vn 16 Split Horizon Disabled • After the initial exchange of updates everything in the routing tables look fine. • Because split horizon disabled, the 10.1.2.0/24 network is sent by both routers, but neither router includes the other’s route to 10.1.2.0/24 (1 hop) in the routing table, because it has a current route with a better metric of 0. Initial routing tables New routing tables 10.1.2.0/24 network is included because split horizon has been disabled RTA RTB 10.1.1.0/24 Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.2.0/24 10.1.3.0/24 s0 s0e0 e0 .1 .1.1 .2 Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0 Routing Update Next-hop Net. Hops Address 10.1.1.0/24 1 10.1.1.1 10.1.2.0/24 1 10.1.1.1 Routing Update Next-hop Net. Hops Address 10.1.2.0/24 1 10.1.2.2 10.1.3.0/24 1 10.1.2.2 Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2 Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0 10.1.1.0/24 1 10.1.2.1 9 NIIT-ICT Hanoi info@niithanoi.vn 17 Split Horizon Disabled • After the next exchange of updates everything in the routing tables look fine and the routing tables are converged. • Because split horizon disabled, the besides the 10.1.2.0/24 network, the networks learned from the other router in the previous update is also sent by both routers. • However, neither router includes the those networks, because it has a current route with a better metric of 0. RTA RTB 10.1.1.0/24 10.1.2.0/24 10.1.3.0/24 s0 s0e0 e0 .1 .1.1 .2 Routing Update Next-hop Net. Hops Address 10.1.1.0/24 1 10.1.1.1 10.1.2.0/24 1 10.1.1.1 10.1.3.0/24 2 10.1.1.1 Routing Update Next-hop Net. Hops Address 10.1.2.0/24 1 10.1.2.2 10.1.3.0/24 1 10.1.2.2 10.1.1.0/24 2 10.1.2.2 Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2 Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0 10.1.1.0/24 1 10.1.2.1 Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2 Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0 10.1.1.0/24 1 10.1.2.1 Previous routing tables Networks in red were included because split horizon has been disabled New routing tables NIIT-ICT Hanoi info@niithanoi.vn 18 Split Horizon Disabled – 10.1.3.0/24 down • Note: Routing tables are not sent at the exactly same time. We will learn about this in Ch. 12 Routing Protocols, that this is done on purpose to avoid collisions on broadcast networks like Ethernet. • Here, the 10.1.3.0/24 network fails, and before RTB sends out its routing update, RTB receives a routing update from RTA. RTA RTB 10.1.1.0/24 10.1.2.0/24 10.1.3.0/24 s0 s0e0 e0 .1 .1.1 .2 Routing Update Next-hop Net. Hops Address 10.1.1.0/24 1 10.1.1.1 10.1.2.0/24 1 10.1.1.1 10.1.3.0/24 2 10.1.1.1 Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2 Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0 10.1.1.0/24 1 10.1.2.1 Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2 Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 2 10.1.2.1 10.1.1.0/24 1 10.1.2.1 X Previous routing tables Networks in red were included because split horizon has been disabled New routing tables 10 NIIT-ICT Hanoi info@niithanoi.vn 19 Split Horizon Disabled – 10.1.3.0/24 down • RTB notices that it has a route to 10.1.3.0/24 via RTA. Even though it is 2 hops it is certainly better than its current situation of “unreachable” so it accepts this better, but incorrect information from RTA. • RTB now forwards all packets destined for 10.1.3.0/24 to RTA at 10.1.2.1. • RTA receives these packets and forwards them to RTB at 10.1.2.2. • RTB forwards them back to RTA at 10.1.2.1. • And so on! The packets get blackholed in this routing loop. RTA RTB 10.1.1.0/24 10.1.2.0/24 10.1.3.0/24 s0 s0e0 e0 .1 .1.1 .2 Routing Update Next-hop Net. Hops Address 10.1.1.0/24 1 10.1.1.1 10.1.2.0/24 1 10.1.1.1 10.1.3.0/24 2 10.1.1.1 Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2 Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 0 e0 10.1.1.0/24 1 10.1.2.1 Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2 Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 2 10.1.2.1 10.1.1.0/24 1 10.1.2.1 X Previous routing tables Networks in red were included because split horizon has been disabled New routing tables NIIT-ICT Hanoi info@niithanoi.vn 20 Split Horizon Disabled – 10.1.3.0/24 down • Meanwhile, its RTB’s turn to send its routing update. • RTB increments the hop count to 10.1.3.0/24 to 3 hops and sends it to RTA. • When RTA sends out its next routing table it will increment the hop count to 10.1.3.0/24 to 4 hops and sends it to RTB. • And on and on, until “infinity” which in RIP is 16 hops. Previous routing tables Networks in red were included because split horizon has been disabled New routing tables RTA RTB 10.1.1.0/24 10.1.2.0/24 10.1.3.0/24 s0 s0e0 e0 .1 .1.1 .2 Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 3 10.1.2.2 Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 2 10.1.2.1 10.1.1.0/24 1 10.1.2.1 X Routing Table Net. Hops Ex-Int 10.1.1.0/24 0 e0 10.1.2.0/24 0 s0 10.1.3.0/24 1 10.1.2.2 Routing Table Net. Hops Ex-Int 10.1.2.0/24 0 s0 10.1.3.0/24 2 10.1.2.1 10.1.1.0/24 1 10.1.2.1 Routing Update Next-hop Net. Hops Address 10.1.2.0/24 1 10.1.2.2 10.1.3.0/24 3 10.1.2.2 10.1.1.0/24 2 10.1.2.2 [...]... 10.1 .2. 0 /24 0 s0 10.1.3.0 /24 1 10.1 .2. 2 Routing Update Next-hop Net Hops Address 10.1.1.0 /24 1 10.1.1.1 10.1 .2. 0 /24 16 10.1 .2. 1 10.1.3.0 /24 16 10.1 .2. 1 Split Horizon with Poison Reverse • • • RTB 10.1.3.0 /24 1 e0 Routing Table Net Hops Ex-Int 10.1 .2. 0 /24 0 s0 10.1.3.0 /24 0 e0 10.1.1.0 /24 1 10.1 .2. 1 Routing Update Next-hop Net Hops Address 10.1.3.0 /24 1 10.1 .2. 2 10.1 .2. 0 /24 16 10.1 .2. 2 10.1.1.0 /24 16... 10.1.3.0 /24 1 10.1 .2. 2 Routing Table Net Hops Ex-Int 10.1 .2. 0 /24 0 s0 10.1.3.0 /24 0 e0 10.1.1.0 /24 1 10.1 .2. 1 New routing tables Routing Update Next-hop Net Hops Address 10.1.3.0 /24 1 10.1 .2. 2 22 11 10.1.1.0 /24 1 10.1 .2. 0 /24 RTA 1 2 s0 RTB s0 e0 10.1.3.0 /24 1 e0 Routing Table Previous routing tables Routing Table Net Hops Ex-Int 10.1.1.0 /24 0 e0 10.1 .2. 0 /24 0 s0 Net Hops Ex-Int 10.1 .2. 0 /24 0 s0 10.1.3.0 /24 ... info@niithanoi.vn 26 13 Split Horizon with Poison Reverse 10.1.1.0 /24 1 RTA e0 10.1 .2. 0 /24 1 2 s0 s0 Routing Table Net Hops Ex-Int 10.1.1.0 /24 0 e0 10.1 .2. 0 /24 0 s0 10.1.3.0 /24 1 10.1 .2. 2 Routing Update Next-hop Net Hops Address 10.1.1.0 /24 1 10.1.1.1 10.1 .2. 0 /24 16 10.1 .2. 1 10.1.3.0 /24 16 10.1 .2. 1 RTB 10.1.3.0 /24 1 e0 Routing Table Net Hops Ex-Int 10.1 .2. 0 /24 0 s0 10.1.3.0 /24 0 e0 10.1.1.0 /24 1 10.1 .2. 1 Routing... RTB 2 s0 Routing Table 10.1.3.0 /24 1 X e0 Routing Table Net Hops Ex-Int 10.1.1.0 /24 0 e0 10.1 .2. 0 /24 0 s0 10.1.3.0 /24 1 10.1 .2. 2 Net Hops Ex-Int 10.1 .2. 0 /24 0 s0 10.1.3.0 /24 (down) e0 10.1.1.0 /24 1 10.1 .2. 1 Previous routing tables Routing Update Next-hop Net Hops Address 10.1.3.0 /24 16 10.1 .2. 2 Routing Table Routing Table Net Hops Ex-Int 10.1.1.0 /24 0 e0 10.1 .2. 0 /24 0 s0 10.1.3.0 /24 (down) 10.1 .2. 2 Net... Ex-Int 10.1 .2. 0 /24 0 s0 10.1.3.0 /24 (down) e0 10.1.1.0 /24 1 10.1 .2. 1 Previous routing tables Routing Update Next-hop Net Hops Address 10.1.3.0 /24 16 10.1 .2. 2 Routing Table Net Hops Ex-Int 10.1.1.0 /24 0 e0 10.1 .2. 0 /24 0 s0 10.1.3.0 /24 (down) 10.1 .2. 2 Routing Table Net Hops Ex-Int 10.1 .2. 0 /24 0 s0 10.1.3.0 /24 (down) e0 10.1.1.0 /24 1 10.1 .2. 1 New routing tables Split Horizon Enabled – 10.1.3.0 /24 down •... Enabled 21 10.1.1.0 /24 1 10.1 .2. 0 /24 RTA 1 2 s0 s0 e0 RTB 10.1.3.0 /24 1 e0 Routing Table Routing Table Net Hops Ex-Int 10.1.1.0 /24 0 e0 10.1 .2. 0 /24 0 s0 Net Hops Ex-Int 10.1 .2. 0 /24 0 s0 10.1.3.0 /24 0 e0 Routing Update Next-hop Net Hops Address 10.1.1.0 /24 1 10.1.1.1 Routing Table Net Hops Ex-Int 10.1.1.0 /24 0 e0 10.1 .2. 0 /24 0 s0 10.1.3.0 /24 1 10.1 .2. 2 Routing Update Next-hop Net Hops Address 10.1.1.0 /24 ... 10.1.1.0 /24 1 10.1.1.1 Routing Update Next-hop Net Hops Address 10.1.3.0 /24 1 10.1 .2. 2 Routing Table Routing Table Net Hops Ex-Int 10.1.1.0 /24 0 e0 10.1 .2. 0 /24 0 s0 10.1.3.0 /24 1 10.1 .2. 2 Net Hops Ex-Int 10.1 .2. 0 /24 0 s0 10.1.3.0 /24 0 e0 10.1.1.0 /24 1 10.1 .2. 1 Routing Update Next-hop Net Hops Address 10.1.1.0 /24 1 10.1.1.1 New routing tables Routing Update Next-hop Net Hops Address 10.1.3.0 /24 1 10.1 .2. 2... RIPv2 interface fastethernet0/1 ip address 1 72. 25.150.193 25 5 .25 5 .25 5 .24 0 ip rip send version 1 2 • • • Interface FastEthernet0/0 is configured to send and receive RIP v1 updates FastEthernet0/1 is configured to send both version 1 and 2 updates FastEthernet0 /2 has no special configuration and therefore sends and receives version 2 by default interface fastethernet0 /2 ip address 1 72. 25.150 .22 5 22 5 .25 5 .25 5 .24 0...Simple Split Horizon 10.1.1.0 /24 1 RTA 10.1 .2. 0 /24 2 s0 e0 10.1.3.0 /24 RTB 1 s0 Routing Table 1 e0 X Routing Table Net Hops Ex-Int 10.1.1.0 /24 0 e0 10.1 .2. 0 /24 0 s0 10.1.3.0 /24 16 10.1 .2. 2 Net Hops Ex-Int 10.1 .2. 0 /24 0 s0 10.1.3.0 /24 16 10.1 .2. 1 10.1.1.0 /24 1 10.1 .2. 1 Split Horizon Disabled • Once both routers have 16 hops for 10.1.3.0 /24 , they will both mark this network as unreachable... Using debug ip packet with Fast Switching and Process Switching Router# debug ip packet IP: s=1 92. 168.3 .2 (FastEthernet0), g=1 92. 168.1 .2, forward IP: s=1 92. 168.3 .2 (FastEthernet0), g=1 92. 168 .2. 2, forward IP: s=1 92. 168.3 .2 (FastEthernet0), g=1 92. 168.1 .2, forward IP: s=1 92. 168.3 .2 (FastEthernet0), g=1 92. 168 .2. 2, forward • • d=10.0.0.1 (Serial0/0), d=10.0.0.1 (Serial0/1), d=10.0.0.1 (Serial0/0), d=10.0.0.1