0 ROUTE MAPSAT THIS POINT I WOULD LIKE TO INTRODUCE ROUTE MAPS BECAU...

6.0 Route Maps

At this point I would like to introduce route maps because they will be

used heavily with BGP. In the BGP context, route map is a method used to

control and modify routing information. This is done by defining condi-

tions for redistributing routes from one routing protocol to another or

controlling routing information when injected in and out of BGP. The for-

mat of the route map follows:

route-map

map-tag [[permit | deny] | [sequence-number]]

The map-tag is just a name you give to the route-map. Multiple instances

of the same route map (same name-tag) can be defined. The sequence number

is just an indication of the position a new route map is to have in the

list of route maps already configured with the same name.

For example, if I define two instances of the route map, let us call it

MYMAP, the first instance will have a sequence-number of 10, and the

second will have a sequence number of 20.

route-map MYMAP permit 10

(first set of conditions goes here.)

route-map MYMAP permit 20

(second set of conditions goes here.)

When applying route map MYMAP to incoming or outgoing routes, the first

set of conditions will be applied via instance 10. If the first set of

conditions is not met then we proceed to a higher instance of the route

map.

The conditions that we talked about are defined by the match and set

configuration commands. Each route map will consist of a list of match

and set configuration. The match will specify a match criteria and set

specifies a set action if the criteria enforced by the match command are

met.

For example, I could define a route map that checks outgoing updates and

if there is a match for IP address 1.1.1.1 then the metric for that

update will be set to 5. The above can be illustrated by the following

commands:

match ip address 1.1.1.1

set metric 5

Now, if the match criteria are met and we have a permit then the routes

will be redistributed or controlled as specified by the set action and we

break out of the list.

If the match criteria are met and we have a deny then the route will not

be redistributed or controlled and we break out of the list.

If the match criteria are not met and we have a permit or deny then the

next instance of the route map (instance 20 for example) will be checked,

and so on until we either break out or finish all the instances of the

route map. If we finish the list without a match then the route we are

looking at will not be accepted nor forwarded.

One restriction on route maps is that when used for filtering BGP updates

(as we will see later) rather than when redistributing between protocols,

you can NOT filter on the inbound when using a “match” on the ip address.

Filtering on the outbound is OK.

The related commands for match are:

match as-path

match community

match clns

match interface

match ip address

match ip next-hop

match ip route-source

match metric

match route-type

match tag

The related commands for set are:

set as-path

set automatic-tag

set community

set clns

set interface

set default interface

set ip next-hop

set ip default next-hop

set ip precedence

set tos

set level

set local-preference