Null0

skwira001

I am having a difficult time understanding this Null0 interface for summary routes. I understand that it is a logical directly connected interface for summary routes. I understand it prevents routing loops. I just don't understand how the router passes the the legitimate traffic and tosses the extraneous addresses into the bit bucket if the entire summary route goes to the Null0 interface.

skwira001

Oh, and I'm talking about EIGRP by the way.

kryolla

its a bit bucket for entries in the routing table that you do not have. If you are getting 5 routes and you want to summarize it with 1 but the summary address consists of 8 routes it will send the remaing 3 routes to null 0 and the remaining 5 since they have a longer match will be forwarded. This could be the desirable behavior or not so you have a choice of getting it installed or not in the case of a default route now the longer match is the summary going to null instead of the default route.

Forsaken_GA

skwira001 wrote: »

I am having a difficult time understanding this Null0 interface for summary routes. I understand that it is a logical directly connected interface for summary routes. I understand it prevents routing loops. I just don't understand how the router passes the the legitimate traffic and tosses the extraneous addresses into the bit bucket if the entire summary route goes to the Null0 interface.

I think the part your getting confused on is probably prefix matching. I'm willing to bet you're under the impression that a shorter prefix trumps a longer prefix, and that's not the cause. To expand on what kyrolla was trying to explain -

Let's say I have 50 /24 subnets in the range of 192.168.0.0 to 192.168.63.0. Rather than advertise 50 /24 routes, I want to advertise a single /18, as this keeps down overhead. So let's say someone sends traffic to 192.168.40.231, and this is one of the /24's I have defined.

Two routes in the routing table would match this address. The /18 Summary route to Null0, and the /24 route. Since the /24 route has the longest prefix, it wins, and the traffic gets forwarded out the proper interface.

Now let's say someone tries to end to 192.168.63.147, and that /24 is *not* one of the ones I defined. Since the /18 is the only route that would match, it has the longest prefix, and traffic is therefore forwarded to the Null0 interface

Hope that helps!