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egypt_air_777

Null 0 Interface

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Dear All,

 

I have a Question concerning the Null 0 Interface on Cisco Routers, what if i made a route for a network on my Cisco Router pointing a Null Interface, will the Traffic Destined for that Network be dropped or not ?

I need more Clarification on the Null 0 Interface, Everytime i read it from Cisco Books, I don't Understand it Easily

 

Thanks,

Mohamed B.Elkhodary

Vodafone Egypt Telecommunications

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Hii dude..

 

Well the Null interface is a logical interface that silently absorbs packets without forwarding them...

So your packets will not be forwarded....

 

Hope that helps..

 

Ciao

Cheerzz

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Null 0 interface is used to avoid routing loops, whenever you create a summary address lets say 172.16.1.0 255.255.255.240, a null 0 interface pointing to the 172.16.1.0/28 is also created automatically, this means that the router is summarizing a set of routes 172.16.1.0/28 and if any route comes back which is the part of this summary address i.e. 172.16.1.0/28, the router will send it to Null 0, where it will be discarded

 

thats why we disable the auto summary in EIGRP, becoz bydefault the eigrp creates a summary address of 172.16.0.0/16 that points to Null0, and hence capturing entire network 172.16.0.0 class range. no other network can use 172.16.0.0 class

 

suppose you have to use a 172.16.1.0 in one part of network and 172.16.2.0 in other part of network and there is a discontiguous network inbetween, With EIGRP autosummay turned on the routers, the Null 0 interface for network address 172.16.0.0/16 will be created inside routers, and since both routers are running auto summary and summarizing 172.16.0.0 /16 so router 1 will not install 172.16.2.0 routes from router2 into its routing table, becoz router thinks that learned route (172.16.0.0 is a part of summarized address 172.16.0.0/16) and sends this to Null 0 where it is discarded

 

to prevent from this, always use No auto summary on all eigrp running routers, this will remove the null 0 for the 172.16.0.0/16 and hence routes between 172.16.1.0 and 172.16.2.0 can be exchanged in between networks

Edited by talent pk

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if any route comes back which is the part of this summary address i.e. 172.16.1.0/28, the router will send it to Null 0, where it will be discarded

 

Above is confusing. I don't think its true.

 

say you have a route 172.16.1.0/28 pointing to Null0 and a more specific route comes from somewhere else say 172.16.1.4/30, new route would get added into routing table and would be used for data forwarding. summary route would remain though. If there is a data packet which matches no other route but the summary route, then it would get dropped because outgoing interface would be Null0.

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you are speaking of more specific route, im speaking of the match made on the summary address. if you have route coming that is more specific that will be installed in the routing table, however if the route coming in matches the summary address it will be disarded.

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