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Community Member

Null 0 Eigrp Redistribution

In the EIGRP redistribution process, why exactly is the route to null 0 created?

I rate all responses.

6 REPLIES
Community Member

Re: Null 0 Eigrp Redistribution

I think this is normal behaviour, when you redistribute other protocol into EIGRP , it dynamically creates the route to null0.

Regards,

Cisco Employee

Re: Null 0 Eigrp Redistribution

Hi Friend,

Its is basically to avoid any routing loops which may occur with summarised routes.

Have a look at this link

http://216.239.59.104/search?q=cache:VIPLVU3T-coJ:www.cisco.com/warp/public/471/route_to_null_interface.pdf+null+0+interface+in+EIGRP&hl=en&ct=clnk&cd=1&gl=us#2

And just read the Introduction which can be of help

HTH

Ankur

*Pls rate helpfull post

Re: Null 0 Eigrp Redistribution

Hi there,

Most probably you are using "ip summary-address" eigrp summarization command. This causes the creation of an EIGRP summary default route to the null 0 interface with an administrative distance of 5, most commonly to prevent routing loops.

HTH, please rate if it does help,

Mohammed Mahmoud.

Community Member

Re: Null 0 Eigrp Redistribution

..but if you receive a BGP summary route from your ISP, for example; the AD will be 20 and the EIGRP Summ route willbe 5, thus routing all LAN traffic to the null0 interface.

having said this, if you decide to use EIGRP Summary Route, make sure you add an AD higher than 20.

Community Member

Re: Null 0 Eigrp Redistribution

One basic working principle of eigrp is that eigrp will advertise a route only when it sees that that ip is one of its active interfaces. For ex. if you advertise a network in eigrp using "network 10.10.10.0 0.0.0.255" command, eigrp will look into the interfaces on the router and check if any of the interfaces are enabled with ip address of 10.10.10.x. Only then it advertises that route. Now, when you advertise a summarized network in eigrp, that summarized network ip will mostly not be resembling any of your active interface ip. Hence EIGRP creates a logical null interface and points that summarized route to the null interface.

- Manoj

Gold

Re: Null 0 Eigrp Redistribution

The earlier post on this thread was correct--it's to prevent routing loops. We don't create the discard route (the route to null 0) to advertise the summary route. EIGRP will always advertise a summary route, as long as there's some internal component within the summary range, no matter what local interfaces are connected.

:-)

Russ

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