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Distance-Vector Routing Protocols

Explore distance-vector routing protocols like RIP, routing information protocols, convergence issues, routing loops, solutions like maximum hop count and split horizon to optimize network routing efficiency.

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Distance-Vector Routing Protocols

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  1. Distance-Vector Routing Protocols • The distance-vector routing algorithm passes complete routing table contents to neighboring routers • It’s possible to have a network that has multiple links to the same remote network • If that’s the case, the administrative distance is checked first • If the AD is the same, the protocol will have to use other metrics to determine the best path to use to that remote network

  2. Routing Information Protocol (RIP) • RIP uses only hop count to determine the best path to a network • If RIP finds more than one link to the same remote network with the same hop count, it will automatically perform load balancing • RIP can perform load balancing for up to six equal-cost links

  3. Routing Information Protocol (RIP) • In following Figure, the four routers start off with only their directly connected networks in the routing table • After a distance-vector routing protocol is started on each router, the routing tables are updated with all route information gathered from neighbor routers • Each router sends its complete routing table out to each active interface

  4. Routing Information Protocol (RIP) • The routing table of each router includes • Network number, Exit interface, Hop count to the network

  5. Routing Information Protocol (RIP)

  6. Routing Information Protocol (RIP) • In Figure above, the routing tables are complete because they include information about all the networks in the internetwork • They are considered converged • When the routers are converging, it is possible that no data will be passed • That’s why fast convergence time is a serious issue • RIP has slow convergence time • The routing table in each router keeps information regarding • the remote network number • the interface to which the router will send packets to reach that network • the hop count or metric to the network

  7. Routing Loops • Distance-vector routing protocols keep track of any changes to the internetwork by broadcasting periodic routing updates out all active interfaces • This broadcast includes the complete routing table • This works just fine, but it’s expensive in terms of CPU process and link bandwidth • And if a network outage happens, real problems can occur • Plus, the slow convergence of distance-vector routing protocols can result in inconsistent routing tables and routing loops

  8. Routing Loops • Routing loops can occur because every router isn’t updated simultaneously, or even close to it • Here’s an example—let’s say that the interface to Network 5 in above Figure fails • All routers know about Network 5 from Router E • Router A, in its tables, has a path to Network 5 through Router B

  9. Routing Loops • When Network 5 fails, Router E tells Router C • This causes Router C to stop routing to Network 5 through Router E • But Routers A, B, and D don’t know about Network 5 yet, so they keep sending out update information • Router C will eventually send out its update and cause B to stop routing to Network 5, but Routers A and D are still not updated • To them, it appears that Network 5 is still available through Router B with a metric of 3

  10. Routing Loops • The problem occurs when Router A sends out its regular 30-second “Hello, I’m still here— these are the links I know about” message, which includes the ability to reach Network 5 • Now Routers B and D receive the news that Network 5 can be reached from Router A • So Routers B and D then send out the information that Network 5 is available • Any packet destined for Network 5 will go to Router A, to Router B, and then back to Router A • This is a routing loop—how do you stop it?

  11. Maximum Hop Count • The routing loop problem just described is called counting to infinity, and it’s caused by gossip (broadcasts) and wrong information being communicated and propagated throughout the internetwork • Without some form of intervention, the hop count increases indefinitely each time a packet passes through a router • One way of solving this problem is to define a maximum hop count.

  12. Maximum Hop Count • RIP permits a hop count of up to 15, so anything that requires 16 hops is deemed unreachable • After a loop of 15 hops, Network 5 will be considered down in previous example • Thus, the maximum hop count will control how long it takes for a routing table entry to become invalid or questionable • Though this is a workable solution, it won’t remove the routing loop itself • Packets will still go into the loop, but instead of traveling on unchecked, they’ll just whirl around for 16 bounces and die

  13. Split Horizon • Another solution to the routing loop problem is called split horizon • This reduces incorrect routing information/overhead by enforcing the rule that routing information cannot be sent back in the direction from which it was received • The routing protocol differentiates which interface a network route was learned from, and once this is determined, it won’t advertise the route back out that same interface • This would have prevented Router A from sending the updated information it received from Router B back to Router B

  14. Routing Loops Example

  15. Route Poisoning • Another way to avoid network loops is route poisoning • For example, when Network 5 goes down, Router E initiates route poisoning by advertising Network 5 as 16, or unreachable (sometimes referred to as infinite) • This poisoning of the route to Network 5 keeps Router C from being susceptible to incorrect updates about the route to Network 5 • When Router C receives a route poisoning from Router E, it sends an update, called a poison reverse, back to Router E • This ensures all routes on the segment have received the poisoned route information

  16. RIP Limitation • Network 172.16.30.0 is a T1 link with a bandwidth of 1.544Mbps, Network 172.16.20.0 is a 56K link • You’d want the router to choose the T1 over the 56K link • But hop count is the only metric used with RIP routing • So two links would be seen as being of equal cost • This little problem is called pinhole congestion.

  17. Routing Information Protocol (RIP)

  18. RIP Timers

  19. RIP Summary

  20. RIP Summary

  21. RIP Summary • A true distance-vector routing protocol • It sends the complete routing table out to all active interfaces every 30 seconds • RIP only uses hop count to determine the best way to a remote network • Maximum allowable hop count of 15 by default, meaning that 16 is deemed unreachable • RIP works well in small networks • Inefficient on large networks with slow WAN links or on networks with a large number of routers installed

  22. RIP Summary • RIP version 1 uses only classful routing, which means that all devices in the network must use the same subnet mask • This is because RIP version 1 doesn’t send updates with subnet mask information • RIP version 2 provides something called prefix routing, and does send subnet mask information with the route updates • This is called classless routing

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