Redistribute static Route with Null0

Minary

I have searched around and can't find a clear and authortative answer to why Null0 is used here so a default route is generated to downstream devices:

router eigrp 200
redistribute static
!
ip route 0.0.0.0 0.0.0.0 Null0

I have seen some explainations but I don't want to cloud the thread if someone has a better answer.

thanks !

EdTheLad

Null0 is the discard path. It's normally used for summary routes, to prevent loops. Lets say RA had a default route pointing to RB, RA had 10 networks, 10.0.1.0 -> 10.0.10.0 locally. You create a summary 10.0.0.0 255.0.0.0 which gets advertised to RB, lets say network 10.0.2.0 fails, the summary is still advertised to B. Traffic coming form B to 10.0.2.0 will arrive on RA, since 10.0.2.0 is down, traffic will take the default route to RB and you have a loop. So when you config a summary a null route is placed in the global rib.

Now say i want to advertise a default route, i first need to have it in my routing table, so as per above you need a static route, which will then be redistributed. If i point the static at an interface, that means all unmatched traffic will exit that interface, maybe i don't want that to happen. I want unknown traffic to get dropped, that's when i use a default to Null0.

Minary

That is a great explaination. Thanks.