Null0

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.

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Comments

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!