Question Of The Day: 28 March, 2008

Topic: Route Summarization

You have the following subnets in your network:

132.16.32.0/24
132.16.121.0/24
132.16.34.0/24
132.16.33.0/24
132.16.5.0/24
132.16.181.0/24
132.16.27.0/24
132.16.2.0/24

Write a single summary route that will encompass all of these routes while being as specific as possible.

See you Monday.

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Yesterday’s Question

 Question Of The Day: 27 March, 2008 

Topic: Route Redistribution 

r1 is running EIGRP and RIP:

r1#sh ip proto sum
Index Process Name
0     connected
1     static
2     eigrp 100
3     rip

r1 has the following EIGRP routes:

r1#sh ip route eigrp
     2.0.0.0/32 is subnetted, 1 subnets
D       2.2.2.2 [90/2297856] via 10.1.12.2, 00:01:06, Serial1/0.12
     22.0.0.0/32 is subnetted, 1 subnets
D EX    22.2.2.2 [170/2560512256] via 10.1.12.2, 00:00:23, Serial1/0.12

r1’s network admin wants to redistribute the EIGRP routes and pass them on to r3 in the RIP domain (directly connected to r1).  Here is his configuration:

r1(config)#route-map EIGRP->RIP perm 10
r1(config-route-map)#match route-type external
r1(config-route-map)#set tag 1170
r1(config-route-map)#route-map EIGRP->RIP perm 20
r1(config-route-map)#set tag 190
r1(config-route-map)#router rip
r1(config-router)#redistribute eigrp 100 route-map EIGRP->RIP

Which EIGRP routes will appear in r3’s (RIP) routing table?

Answer: The redistributed routes appear in r3’s routing table with a metric of 6 (set by the route-map) and r1’s locally generated RIP routes appear with a metric of 1.

r3#sh ip route rip
     2.0.0.0/32 is subnetted, 1 subnets
R       2.2.2.2 [120/6] via 10.1.13.1, 00:00:03, Serial1/0
     22.0.0.0/32 is subnetted, 1 subnets
R       22.2.2.2 [120/6] via 10.1.13.1, 00:00:03, Serial1/0
     10.0.0.0/24 is subnetted, 2 subnets
R       10.1.12.0 [120/1] via 10.1.13.1, 00:00:03, Serial1/0
     12.0.0.0/32 is subnetted, 1 subnets
R       12.12.12.12 [120/6] via 10.1.13.1, 00:00:03, Serial1/0
     13.0.0.0/32 is subnetted, 1 subnets
R       13.13.13.13 [120/1] via 10.1.13.1, 00:00:03, Serial1/0

The order of preference for defining the metric when redistributing routes into a protocol (values in parentheses are the values used in our example):

1) If a route-map is used, then use the metric specified in the route-map (6).
2) If no route-map is used or the route-map does not specify a metric, then use the metric specified in the ‘redistribute’ command (2).
3) If a metric is not specified in a route-map nor is it specified in the ‘redistribute’ command, then use the ‘default-metric’(4).
4) If no metric is specified by any of the above methods, then the routes will not be redistributed (see this QOD).

route-map > redistibution metric > default metric
redistribute (IP)

The metric value specified in the redistribute command supersedes the metric value specified using the default-metric command.

set metric (BGP, OSPF, RIP)

default-metric (RIP)

Usage Guidelines
The default-metric command is used in conjunction with the redistribute router configuration command to cause the current routing protocol to use the same metric value for all redistributed routes. A default metric helps solve the problem of redistributing routes with incompatible metrics. Whenever metrics do not convert, using a default metric provides a reasonable substitute and enables the redistribution to proceed.

The default-metric is only in conjunction with redistribution.  Don’t fall into the trap of thinking that the default metric will affect routes that are not redistibuted.