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Community Member

Route maps

What happens when all sequences in a route-map are checked without a match? In BSCN by Catherine Paquet and Diane Teare, on page 633, it states "the route will not be accepted nor forwarded (this is the implicit deny any at the end of the route map.)"

But in another book by the same title, authors, and ISBN number, it says, "if no match is found in the route map, then the packet is forwarded through the normal routing channels. If it is desired not to revert to normal forwarding and to drop a packet that does not match the specified criteria, then a set statement to route the packets to interace null 0 should be specified as the last entry in the route map."

Which statement is correct?

Jeff

4 REPLIES
Gold

Re: Route maps

Hi Jeff -

From my understanding, route-maps are defined by access-lists. Route-maps allow routing decisions to be made on criteria other than the destination address of the routed packet. Other criteria can be source address of the packet or source autonomous-system, route-maps are also used to manupilate routing table update information, therefore route-maps can be used to manupilate IGP metrics during the route redistibution process and also for BGP-4 updates. With route-maps, if you meet a difined 'match' criteria, you are manupilated by the specified 'set' command and if you are not then you are dropped or if you have set null interface then this will be used for the dicarded packet (bit-bucket).

So, to your question, will the packet be forwarded or dropped, the packet will be dropped if it does not meet the specified access-list 'match' and 'set' command.

Hope this helps --

Bronze

Re: Route maps

I believe it depends on whether you're using route maps for policy routing or for route filtering. In the former case, packets not matching a route-map are forwarded via normal, destination-based routing. When using route-maps to filter inbound route advertisements, however, my understanding is that there is indeed an implicit 'deny' at the end.

Community Member

Re: Route maps

Does the set statement determine if the route-map is being used for policy routing or for route-filtering?

Jeff

Re: Route maps

Jeff,

the set statement is used to set (modify) a value, such as next hop, ip precedence and many more...

The application of route-map varies in policy routing / route filtering. In policy routing, the route-map is configured on an interface,

int e0

ip policy route-map Test

where as in case of route filtering, its applied for a routing process as follows

router rip

redistribute static metric 10 route-map Test

The parameters that you set inside a route-map may differ in case of policy routing / route-filtering. Policy routing may modify the next hop, while route-filtering my simply set a metric for the route thats being redistributed.

Hope you get the difference between them.

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