Can any one explain why ( EIGRP's automatic inclusion of Null0 summary routes)

EIGRP automatically includes a null0 summary route as a child route whenever both of following conditions exist:

There is at least one subnet that was learned via EIGRP.

Automatic summarization is enabled.

the question is why ? and why we cann't find this feature in other routing protocols

Loop prevention.

Say your network has a Router A and a Router B. Imagine Router A is advertising a default route to Router B, and Router B is advertising a summary 10.0.0.0/8 to router A. But what if Router B only had a route to 10.0.1.0/24 and no route 10.0.0.0/8 to Null0.

The problem would be when a packet was sent from Router A to Router B with the destination address of 10.0.2.100 or any other IP within the 10net but not in the 10.0.1.0/24 network. If Router B didn't have the Null0 route it would send the packet back to Router A because of the default route from Router A. Then Router A would send it back to Router B because of the 10.0.0.0/8 summary route from Router B. This would probably happen about 127 times before the TTL expired.

Further to what sacox31s said, even if it didn't cause a loop, it would certainly generate unnecessary traffic. Even if the default route pointed somewhere other than Router A, Router B would still route traffic for the 10.0.2.0/24 network to wherever the default gateway is.

Essentially Router B would be routing packets through the network only to be (eventually) dropped. The idea with the Null0 route is that if you are advertising a summary route then you must be able to reach all of the subnets in that summary or else they don't exist.

Obviously this isn't always the case, and it is possible that the subnets exist somewhere else. That's why you would be better served to turn off auto-summary and manually summarize those subnets you actually have routes for. This still generates a Null0 route, but only for the manually summarized route.

joshlowe sacox31s

thankx for all it is clear now

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http://rapidshare.com/files/367418224/ccnp-eigrp.zip