Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

You may experience some slow load times, errors, and slight inconsistencies. We ask for your patience as we finalize the launch. Thank you.

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started.

New Member

null0

hi all

1)ip summary-address eigrp 1 10.1.0.0 255.255.0.0

2)ip route 10.1.0.0 255.255.0.0 null0

q?how we sumarized the route.than we send the route to null0.how come.ps clear me

10xs

ali

  • Other Network Infrastructure Subjects
7 REPLIES

Re: null0

command ip summary-address will send the summary of the network to the other router connected with that perticular interface...

and in second statement you are mathcing all the packet with the destination network id 10.1.0.0 and pinting it towards the null interface it means discarding the packet ...

so what is the logic here

regards

Devang

New Member

Re: null0

HI DEVANG

U DIDN'T GET ME VERY WELL,I MEAN I SENT THE SUMARY ROUTE(10.1.0.0) THEN ALSO I SENT THIS SUMMARY(10.1.0.0) TO THE NULL 0.HOPE IF I CLARIFY THIS

10XS

Ali

Re: null0

ali,

here you are not sending summery to the null0

when you use command ip route 10.1.0.0 255.255.0.0 null0

then it is the static route which will apear in your routing table...and when the packet whith destination 10.1.0.0 will receive on router then it will send that packet to the null0 interface it means it is going to discard the packet...so this is used to avoid some time loops by discarding the packet ...here you are not sending summary to the null0 interface ...but you are making routing disision using the static route to null0 interface...

hope i am able to clear your idea...

regards

Devang

Re: null0

Hi Ali,

The idea behind this is simple and let me try to put it here again...

You summarise the routes and advertise it WHY so that the routing table of the peer routers will have less entries in routin table and need not be populated with all routes which are in that summarised route.

Now WHAT happens if any of the nodes behind the peer routers try to reach any of the specific route in that summarised route it will be fwded to your distribution router on which you have summarised teh routes and from there it will look for most specific route from that summarised route NOW lets assume that specific route is down then WHAT will happen it will not find any entry in routing table and either will be dropped or incase you have a default route it will be fwded to that default router and from there again it will check the summarised route and will be fwded to the distribution router and this will go on and on and on.....so to stop this if you have a static route for summarised route pointing to null0 interface and if most specific route within that summarised route is down so any request for that destination will be fwded to null0 static route because that is the next best match and will be dropped.

So that is the intention behind this configuration to avoid possible routing loop.

HTH, if yes please rate the post.

Ankur

Re: null0

hi ankur,

first of all thanks for you email id.

it means when you send summery to access layer and when access layer will send any packet for the destination include in summary to the distribution layer...if the destination for the packet is down, at that time if there is default route ...then packet will send to that interface even the destination is down which is not a good right that is why you are using the null0 am i right???

thanks

Devang

Re: null0

Hey Devang,

You are always welcome.

You got the point!! So if there is a default route packet will be fwded to the interface where default route is pointing as most specific match is not available because it is down. This is indeed not preferrable so that is the reason null0 is used to avoid that behavior.

Regards,

Ankur

New Member

Re: null0

Hi guys!

i appreciated all ur reply & ur spent to help me.

10xs

Ali

194
Views
13
Helpful
7
Replies