Distance Vector Routing Protocols

1. Distance Vector Routing Protocols PJC CCNA Semester 2 Ver. 3.0 by William Kelly

2. Distance Vector Routing Protocols • Intro to Distance Vector Protocols • Load Balancing • Routing Loops and their solution • Examining Routing Tables • Administrative Distance • Gateway of Last Resort • Integrating Static Routes with RIP • Basics of RIP and IGRP • Troubleshooting Routing Protocols

3. Distance Vector Protocols Intro. • The two important Distance vector routing protocols are RIP and IGRP • The ENTIRE routing table is sent periodically to neighboring routers • A topology change or a periodic update sends routing tables to neighbors

4. Load Balancing • The maximum paths range from 1 to 6 • By default most IP routing protocols install 4 routes in parallel • Static Routes always install 6 routes • Rip can only load balance paths that have same number of hops • IGRP can load balance up to 6 unequal paths • The maximum-paths maximum command allows the number of parallel paths used to load balance to be changed in configuration mode

5. Load Balancing (switching) • process switching (packets) • The router alternates paths on a per packet basis • fast switching (per destination) • All packets in the packet stream bound for a specific host take the same path • Packets bound for a second host on the same destination network would all take an alternate path

6. What is a Routing Loop?

7. How to Solve Routing Loops • Count to Infinity • Split Horizon • Route Poisoning • Triggered Updates • Hold Down Timers

8. Count to Infinity • Distance Vector Protocols define Infinity as a specific number • Looping continues until Infinity (16 for RIP) is reached • When infinity is reached the Network is considered unreachable • In our example the loop would continue until the county reached 16 and then network 1 would be marked unreachable

9. Split Horizon • A routing loop occurs when incorrect information is sent to a router that just sent out correct information • The solution is to avoid sending information back in the direction it came • In our example Routers A and E would never have received C’s bad info and thus no routing loop would have occurred.

10. Route Poisoning • Route poisoning is used to overcome loops in large networks by setting the hop count to one more than the maximum • The poison reverse rule states “Once you learn of a route through an interface, advertise it as unreachable back through that same interface”. • Route poisoning is essentially the same as poison reverse + split horizon

11. Triggered Updates • By sending updates faster than the default update convergence occurs must faster • A topology change quickly propagates through a network • Triggered updates used with route poisoning marks a “down” network as unreachable quickly through the network

12. Hold Down Timers • When a router receives information that a previously accessible route is inaccessible it starts a hold down timer and marks the route as inaccessible • If an update is received indicating the route is back up before the hold down timer expires then the route is marked accessible again • If an update is received from a different neighbor with a better metric then the route is marked accessible and the hold down time is removed • If an update is received from a different neighbor with a poorer metric before the hold down timer expires then the update is IGNORED

13. Examining Routing Tables • Show ip route command • How did I receive a route? • What is the metric? • What is the administrative distance? • Is the route directly connected? • What is the output interface to get to a route?

14. Examining Routing Tables Vista#sh ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E – EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o – ODR P - periodic downloaded static route Gateway of last resort is not set 172.16.0.0/24 is subnetted, 3 subnets C 172.16.8.0 is directly connected, FastEthernet0/0 R 172.16.5.0 [120/1] via 172.16.7.1, 00:00:02, Serial0/0 C 172.16.7.0 is directly connected, Serial0/0

15. Administrative Distance • Administrative distance is a default value assigned to each routing protocol that will favor the best route from one protocol over another one • Distances for common protocols are:

16. Gateway of Last Resort • Routers try to keep routing tables as small as possible • A router may not be able to match a destination network with an entry in it’s routing table • Default routes can be entered statically or learned dynamically • ip default network x.x.x.x establishes a default route in networks using dynamic routing • Any router set with default network x.x.x.x that has a route to x.x.x.x flags x.x.x.x as a candidate default route • A default route can be statically set by saying: ip route 0.0.0.0 0.0.0.0 {next hop ip or exit interface} • If no path to the destination network is found in the routing table then the quad zero default is used

17. Integrating Static Routes with RIP • If your static route was not defined with a network command then it is not distributed unless you use the redistribute static command ip route dest. mask ip/interface admin_distance • Packets bound for specific destination networks can be forced to follow a certain path • Using a higher administrative distance can provide a backup path in case of main link failure

18. RIP Basics • To configure use route rip, then network x.x.x.x • The metric is hop count • A hop count of 16 is infinity • Period updates are sent every 30 seconds • It is a distance vector protocol • The entire routing table is sent during updates • The administrative distance is 120 • The hold down timer default is 180 seconds

19. IGRP Basics • To configure use route igrp as-number, then network x.x.x.x • The metrics are bandwidth, delay, load, and reliability • Period updates are sent every 90 seconds • The hold down time is 3x the update timer or 90 secs. x 3 + 10 seconds = 280 seconds. • It is a distance vector protocol • The entire routing table is sent during updates • The administrative distance is 100 • Scalable for very large networks

20. Troubleshooting Routing Protocols • Is the routing protocol set? • show ip protocols • Is the route in the routing table? • show ip route • Are the interfaces configured? • show running-config • Am I using the same version of RIP throughout my network? • ping, traceroute, debug