Topic6-Part II Module7.pdf

Part II Routing Protocols and Concepts 1 Part II Module 7: Distance Vector Routing Protocol 7.1 Distance Vector Routing 3 4 7.1.1 Distance vector routing updates o Routing table updates occur periodically or when the topology in a distance vector protocol network changes. o It is important for a rou...

Part II Routing Protocols and Concepts 1 Part II Module 7: Distance Vector Routing Protocol 7.1 Distance Vector Routing 3 4 7.1.1 Distance vector routing updates o Routing table updates occur periodically or when the topology in a distance vector protocol network changes. o It is important for a routing protocol to update the routing tables efficiently. o Distance vector algorithms call for each router to send its entire routing table to each of its adjacent neighbors. 5 RIP example Initial After exchange 6 7.1.2 Distance vector routing loop issues o The network is said to have converged if All routers have consistent knowledge and correct routing tables. o Routing loops can occur when inconsistent routing tables are not updated due to slow convergence in a changing network. Step 1 For Router C, the preferred path to Network 1 is by way of Router B, and the distance from Router C to Network 1 is 3. 7 When 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 Step 3 When Router A sends out its update, Routers B and D stop routing to Network 1. Step 2 However, Router C has not received an update. For Router C, Network 1 can still be reached through Router B. 8 Now Router C sends a periodic update to Router D, which indicates a path to Network 1 by way of Router B. Step 4 (Via B) Router D changes its routing table to reflect this incorrect information, and sends the information to 9 Router A. Router A sends the information to Routers B and E, and the process continues. Step 5 (Via B) (Via C, B) Any packet destined for Network 1 will now loop from Router C to B to A to D and back to again to C. 10 7.1.3 Defining a maximum count With this approach, the routing protocol permits the routing loop to continue until the metric exceeds its maximum allowed value. 11 7.1.4 Elimination routing loops through split-horizon 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. 12 13 7.1.6 Avoiding routing loops with triggered updates o New routing tables are sent to neighbor routers on a regular basis. For example, RIP updates occur every 30 seconds. o However a triggered update is sent immediately in response to some change in the routing table. o Triggered updates, used in conjunction with route poisoning (metric=16), ensure that all routers know of failed routes before any holddown timers can expire 14 7.1.7 Preventing routing loops with holddown timers o o holddown timers can be used to avoid a count to infinity problem: When a router receives an update from a neighbor, which indicates that a previously accessible network is now inaccessible, it marks the route as inaccessible and starts a holddown timer. Before the holddown timer expires, if the router an update is received from the same neighbor (indicates that the network is accessible) marks the network as accessible and removes the holddown timer an update arrives from a different neighbor router (with a better metric for the network) marks the network as accessible and removes the holddown timer an update is received from a different router with a higher (worse) metric the update is ignored 15 7.1.8 Advantages and disadvantages of distance vector routing protocols 16 https://www.computernetworkingnotes.com/ccna-studyguide/routing-loops-explained-with-examples.html https://www.computernetworkingnotes.com/ccna-studyguide/split-horizon-explained-with-examples.html https://www.computernetworkingnotes.com/ccna-studyguide/infinity-metric-and-route-poisoningexplained.html 17 7.2 RIP - Routing Information Protocol - 18 7.2.1 IP routing process RIP includes a number of features that are common in other routing protocols. For example, RIP implements split horizon and holddown mechanisms to prevent the propagation of incorrect routing information 19 7.2.2 Configuring RIP The router rip command enables RIP as the routing protocol. The network command is then used to tell the router on which interfaces to run RIP. 20 10.0.0.0 , 1 10.0.0.0 , 2 o RIP sends routing-update messages at regular intervals. o When a router receives a routing update that includes changes to an entry, it updates its routing table to reflect the new route. o The received metric value for the path is increased by 1, and the source interface of the update is indicated as the next hop in the routing table. o RIP routers maintain only the best route to a destination. o A configurable item that affects convergence time is the update interval. The default RIP update interval in Cisco IOS is 30 seconds. 21 7.2.6 Troubleshooting RIP update issues 22 7.2.3 Using the ip classless command Sometimes a router receives packets destined for an unknown subnet of a network that has directly connected subnets. 10.10.0.0/16 Use the ip classless global configuration command to instruct the Cisco IOS software to forward these packets to the best supernet route. s0 5.5.0.0/16 s1 To s0 or discard? For example, if an enterprise uses the entire subnet 10.10.0.0 /16, then a supernet route for 10.10.10.0 /24 would be 10.10.0.0 /16. 10.10.10.99/24 The ip classless command is enabled by default in Cisco IOS Software Release 11.3 and later. To disable this feature, use the no form of this command. When this feature is disabled any packets received that are destined for a subnet that falls within the subnetwork addressing scheme of the router will be discarded. IP classless only affects the operation of the forwarding processes in IOS. IP classless does not affect the way the routing table is built. How about using default route? o The router only uses the default route if the major network destination does not exist in the routing table. o A router by default assumes that all subnets of a directly connected network should be present in the routing table. o If a packet is received with an unknown subnet of a directly attached network, the router assumes that the subnet does not exist. o So the router will drop the packet even if there is a default route. o To resolve this problem, configure ip classless on the router. 24 25 7.2.4 Common RIP configuration issues o Holddown timers help prevent counting to infinity but also increase convergence time. o The default holddown for RIP is 180 seconds. o This will prevent any inferior route from being updated but may also prevent a valid alternative route from being installed. o The holddown timer can be decreased to speed up convergence but should be done with caution. o Ideally, the timer should be set just longer than the longest possible update time for the internetwork. o In the example in Figure, the loop consists of four routers. If each router has an update time of 30 seconds, the longest loop would be 120 seconds. Therefore, the holddown timer should be set to slightly more than 120 seconds. 26 RIP version 1 -supports only classful routing and doesn’t send subnet masks in routing updates.Uses broadcasts for updates. RIP version 2– supports classless routing and sends subnet masks in routing updates. This version uses the multicast address of 224.0.0.9 to send routing updates. There is also a version of RIP developed for IPv6 networks called RIPng. 27 28 7.2.5 Verifying RIP configuration 29 30 7.2.7 Preventing routing updates through an interface the passive-interface command prevents the transmission of routing updates through a router interface, but the router continues to listen and use routing updates from that neighbor. 31 7.2.8 Load balancing with RIP RIP is capable of load balancing over as many as six equal-cost paths. The default is four paths. RIP performs what is referred to as “round robin” load balancing. Since the metric for RIP is hop count, the speed of the links is not considered. So, the 56-Kbps path will be given the same preference as the 155-Mbps path. 32 7.2.9 Load balancing across multiple paths When a router learns multiple routes to a specific network, the route with the lowest administrative distance is installed in the routing table. 33 o If same AD, the path with lowest cost will be chosen. o E.g., Router E chooses the second path, since 20 < 30 and 45. o If there’re paths with the same AD and cost, load-balancing can occur. o (The costs may need to be manually configured in order to achieve load balancing.) o Cisco IOS imposes a limit of up to six equal cost routes in a routing table, but some IGPs have their own limitations. o EIGRP allows up to four equal cost routes. o Router(config-router)# maximum-paths [number ] 34 o Two methods of load balancing for IP packets. o per-packet: – the router will alternate paths on a per-packet basis. o per-destination: – All packets that are bound for a specific host will take the same path. – Packets bound for a different host on the same network may use an alternate route. o By default the router uses per-destination load balancing, also called fast switching. – To disable fast switching, use the no ip route-cache command. – Using this command will cause traffic to be load balanced on a per-packet basis. 35 RIP timers: https://www.computernetworkingnotes.com/ccna-studyguide/rip-tutorial-basic-operation-of-rip-protocol.html 7.2.9 e-Lab Activity: Load Balancing Across Multiple Paths 36 7.2.10 Integrating static routes with RIP The static route is assigned a higher AD on purpose.37 S 172.16.0.0/16 [130/1] 38 7.3 IGRP 39 7.3.1 IGRP features By default, the IGRP routing protocol uses bandwidth and delay as metrics. Additionally, IGRP can be configured to use a combination of variables to determine a composite metric. These variables are as follows: Bandwidth Delay Load Reliability 40 When Cisco created IGRP in the early 1980s, it was the first company to solve the problems associated with the use of RIP to route datagrams between interior routers. IGRP examines the bandwidth and delay of the networks between routers to determine the best path through an internetwork. IGRP converges faster than RIP. This prevents routing loops that are caused by disagreement over the next routing hop. Further, IGRP does not share the hop count limitation of RIP. As a result of this and other improvements over RIP, IGRP enabled many large, complex, topologically diverse internetworks to be deployed. 41 42 7.3.2 IGRP metrics 𝐾2 × 𝑏𝑎𝑛𝑑𝑤𝑖𝑑𝑡ℎ 𝐾5 𝑀𝑒𝑡𝑟𝑖𝑐 = 𝐾1 × 𝑏𝑎𝑛𝑑𝑤𝑖𝑑𝑡ℎ + + 𝐾3 × 𝑑𝑒𝑙𝑎𝑦 256 − 𝑙𝑜𝑎𝑑 𝑟𝑒𝑙𝑖𝑎𝑏𝑖𝑙𝑖𝑡𝑦 + 𝐾4 The default constant values are K1 = K3 = 1 and K2 = K4 = K5 = 0. If K5 = 0, the [K5/(reliability + K4)] term is not used. So, given the default values for K1 through K5, the composite metric calculation used by IGRP reduces to Metric = Bandwidth + Delay. 43 the IGRP metric values 44 7.3.4 IGRP stability features Holddowns Split horizons Poison reverse updates 45 7.3.5 Configuring IGRP Enable & disa All routers in the same AS AS correction 46 7.3.7 Verifying IGRP configuration 47 7.3.8 Troubleshooting IGRP 48