eigrp auto-summary and a null 0 route

From: Steven (adream@xxxxxxx)
Date: Thu Jan 10 2002 - 03:51:04 GMT-3


   
On one router which runs eigrp only ,the config and route table is as follows,

sh run
...
router eigrp 1
 network 20.0.0.0
 network 168.71.7.0 0.0.0.255
 network 168.71.9.0 0.0.0.255
 no auto-summary
 no eigrp log-neighbor-changes

 ...

   168.71.0.0/24 is subnetted, 4 subnets
C 168.71.9.0 is directly connected, Serial1
C 168.71.7.0 is directly connected, Serial0
D 168.71.6.0 [90/20768000] via 168.71.9.1, 00:00:17, Serial1
D 168.71.5.0 [90/20384000] via 168.71.9.1, 00:00:17, Serial1
     20.0.0.0/24 is subnetted, 1 subnets
C 20.1.1.0 is directly connected, Loopback0

after i add "auto-summary "command under router eigrp 1

the route table change to the following one ,

     168.71.0.0/24 is subnetted, 4 subnets
C 168.71.9.0 is directly connected, Serial1
C 168.71.7.0 is directly connected, Serial0
D 168.71.6.0 [90/20768000] via 168.71.9.1, 00:00:03, Serial1
D 168.71.5.0 [90/20384000] via 168.71.9.1, 00:00:03, Serial1
     20.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D 20.0.0.0/8 is a summary, 00:00:01, Null0
C 20.1.1.0/24 is directly connected, Loopback0

i just want to know what "D 20.0.0.0/8 is a summary, 00:00:01, Null0"
mean.Why is it added?

Thanks.



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