what does this mean ??

[gauravm]

hi ,

could anyone tell me what does this route means??

ip route 172.19.80.0 255.255.240.0 null0

also , would appreciate if anyone could brief me what is null0 relevance and what are its best usage parameters.

[rais]

This route entry says that all traffic destined to IP addresses space 172.19.80-95.X should be dropped.

Null0 is often quoted as a blackhole...you can keep putting things in it and it never overflows. Things sent in never come out.

HTH.

[gauravm]

but mine these IP's(80-95) correspond to the ethernet and Serial addresses to the other branches locations .

this route is defined at the hub location.

how come that working then ??

would appreciate if you can put some light on this .

[jsayer]

Null0 routes are often used as placeholders or summary routes also. If there are more specific subnets in the routing table on a particular router, the /20 route to null0 can be used to pull traffic to this router which then knows about the more specific subnets.

Another example is used with BGP advertisements. Since BGP will only advertise a route that is either directly connected, redistributed, or known by a static, this can be an easy way to advertise summarized routes to an Internet provider. At the internet edge use a summary route to Null0: with a network statement under BGP. As long as the edge router learns about more specific routes using the igp, the Internet will direct traffic to this router which can then forward due to longest-match.

A third way the null0 is used is if you want to black-hole traffic to subnets that are not in use within the summary route. This can avoid routing loops and help protect against some virus types.

Hope this helps,

John

[gauravm]

Thanks John.

i am now convinced.

just a little query" i have seen that sometimes in my branches location i need to define default static route to its serial address while its running the eigrp/rip also.but i guess same doesnt happen when this null0 route entry is defined at the hub/central location".

does this uses the same logic.

Thanks a ton!!!

[jsayer]

Very often remote branch locations only have one link to the rest of the network, in which case a static default route is all that is necessary to route traffic outbound. This saves on bandwidth overhead on the link between the branch and the hub location also. In this case, the dynamic routing protocol should not be extended to the branch offices. Instead a corresponding static route for the branch's LAN subnet should be configured at the hub location to allow the hub router to know how to route the traffic back to the branch. The summary route to Null0 can then be redistributed into the dynamic routing protocol to pull traffic destined for any of the remote sites to this router.

If you are having to do both a dynamic routing protocol and a static default route, it probably means that the default route is not being passed by the dynamic routing protocol.

Not sure if that helps or raises more questions, but I don't think the need for a default static along with dynamic routing at the branches is related to the null0 route.

John