1. Chabot College
ELEC 99.08
Routing Loops

2. Routing Loops Topics Convergence Distance Vector Routing Loops RIP v2
Discontiguous Networks

3. Convergence
Convergence means that routers agree about the network’s available routes - the routing tables are consistent.
Inconsistent routing tables is the main cause of routing loops.

4. Routing Loops - the Problem
Problem: Using distance-vector routing, routers can learn “bad” routes and then propagate these to other routers.
Result: inconsistent routing tables
Result: looping packets - counting to infinity.

5. Routing Loops Example
Example: is directly connected to Oak’s E0.
Oak’s routing table shows is 0 hops away. E0 S1 S1
fre S0
hay
oak S0 E0

6. Routing Loops Example
RIP, running on Oak, tells Hay that is reachable through Oak with a distance of 0.
Hay adds 1 hop to the distance and enters the route to into its routing table. 1 S1 E0 S1
RIP S1
fre S0
hay
oak S0 E0

7. Routing Loops Example
RIP, running on Hay, tells Fre that is 1 hop away.
Fre adds 1 hop to the distance and enters the route to into its routing table. 2 S1 1 S1 E0
RIP S1 S1
fre S0
hay
oak S0 E0

8. Routing Loops Example
The network is now “converged”. All routers have a consistent picture of the network. 2 S1 1 S1 E0 S1 S1
fre S0
hay
oak S0 E0

9. Routing Loops Example Now network 192.168.3.0 fails...
Oak stops routing packets to it and marks it as “unreachable" with a distance of 16 hops. 2 S1 1 S1 16 E0 S1 S1
fre S0
hay
oak S0 E0

10. Routing Loops Example So far, Hay and Fre don’t know of the failure.
Before Oak can tell Hay, Hay sends out its regular 30-second update to Fre and Oak. That update tells Oak that there is a route to through Hay with a distance of 1. 2 S1 1 S1 16 E0
RIP S1
RIP S1
fre S0
hay
oak S0 E0

11. Routing Loops Example
Oak accepts the update, adds 1 to the distance and changes the vector (interface) to the dest.
Now Oak wrongly believes that there is a route to through Hay with a distance of 2. 2 S1 1 S1 2 S0 S1
RIP S1
fre S0
hay
oak S0 E0

12. Routing Loops Example
Next Oak sends its regular update to Hay, which adds 1 to the distance to 2 S1 3 S1 2 S0 S1
RIP S1
fre S0
hay
oak S0 E0

13. Routing Loops Example
Hay again sends regular updates to Fre and Oak, and each router adds 1 to the distance to
See the problem? 4 S1 3 S1 4 S0
RIP S1
RIP S1
fre S0
hay
oak S0 E0

14. Routing Loops Example The routing tables are not converged.
If Hay receives a packet bound for , where will it send it?
What will happen next? 4 S1 3 S1 4 S0 S1 S1
fre S0
hay
oak S0 E0

15. Routing Loop Management
Hop count limit
stops looping packets after limit
default is 15 hops for RIP (16 is considered “unreachable” or down)
can be set:
Commands
default-metric NN
hay(config)#router rip
hay(config-router)#default-metric 10
hay(config-router)#^Z

16. Routing Loop Management
Split horizon
prevents sending information about a route back to the source from which an update originated.
reduces the spread of bad routes & speeds convergence.
is enabled by default on each interface

17. Routing Loops Management
Split horizon on S0 prevents Oak from accepting incorrect update from Hay about 1 S1 16 E0
split-horizon S1
RIP S1
fre S0
hay
oak S0 E0

18. Routing Loop Management
Holdowns
when changes occur, holdowns force routers to “hold” the changes for a certain period of time.
The router will not accept an updated route with a poorer metric until the holdown timer expires.
this prevents regular updates from wrongly reinstating a route that has gone bad.
Commands
timers basic update invalid hold flush
hay(config)#router rip
hay(config-router)#timers basic
hay(config-router)#^Z
See:

19. RIP Timers

20. Rapid Convergence - Sample Config
10 hops max
oak#conf t oak(config)#router rip oak(config-router)#default-metric 10 oak(config-router)#timers basic oak(config-router)#exit oak(config-if)#^Z
Fast timers

21. Routing Loop Management
Triggered Updates
When changes occur, updates are sent immediately.
Speeds convergence.
Supported only by RIP version 2
Works together with holdowns.
Consider how triggered updates & holdowns could prevent the example problem.

22. RIP Version 2 Key New Features:
understands subnet masks, including Variable Length Subnet Masks
performs “triggered updates” - sending new routing information to neighbor routers immediately after a change in the network
provides an authentication mechanism (passwords)
supports IP multicasting
supports EGP route tags
For our Lab, the key advantage is the support for subnet masks...

23. RIP Version 2 - Commands Configuring RIP version 2 - same as RIP but
adds version 2 statement
version should be the same for all routers
Enables RIP routing
Sets RIP version 2
oak(config)#router rip
oak(config-router)#version 2
oak(config-router)#network
oak(config-router)#network
oak(config-router)#^Z

24. Discontiguous Networks - Problem
RIP and IGRP do not support routing between subnets that are separated by other networks.
RIP won’t tell router B how to reach the /24 subnet:
/16 net A B
/ Net
/ 24 Net
See

25. Discontiguous Networks - Solution
RIP version 2 understands subnets and
can be configured not to summarize subnets
will not have a problem with the discontiguous subnets shown below with this config:
rtrB(config)#router rip
rtrB(config-router)#version 2
rtrB(config-router)#no auto-summary
rtrB(config-router)#network
rtrB(config-router)#network
/16 net A B
/ subnet
/ 24 subnet

26. Command Summary default-metric 10 timers basic 30 180 180 240
version 2
no auto-summary