1 AGGREGATE COMMANDSTHERE IS A WIDE RANGE OF AGGREGATE COMMANDS. IT...

There is a wide range of aggregate commands. It is important to

understand how each one works in order to have the desired aggregation

behavior.

The first command is the one used in the previous example:

aggregate-address

address mask

This will advertise the prefix route, and all of the more specific

routes. The command aggregate-address 160.0.0.0 will propagate an addi-

tional network 160.0.0.0 but will not prevent 160.10.0.0 from being also

propagated to RTA. The outcome of this is that both networks 160.0.0.0

and 160.10.0.0 have been propagated to RTA. This is what we mean by

advertising the

prefix and the more specific route.

Please note that you can not aggregate an address if you do not have a

more specific route of that address in the BGP routing table.

For example, RTB can not generate an aggregate for 160.0.0.0 if it does

not have a more specific entry of 160.0.0.0 in its BGP table. The more

specific route could have been injected into the BGP table via incoming

updates from other ASs, from redistributing an IGP or static into BGP or

via the network command (network 160.10.0.0).

In case we would like RTC to propagate network 160.0.0.0 only and NOT the

more specific route then we would have to use the following:

aggregate-address

address mask summary-only

This will a advertise the prefix only; all the more specific routes are

suppressed.

The command aggregate 160.0.0.0 255.0.0.0 summary-only will propagate

network 160.0.0.0 and will suppress the more specific route 160.10.0.0.

Please note that if we are aggregating a network that is injected into

our BGP via the network statement (ex: network 160.10.0.0 on RTB) then

the network entry is always injected into BGP updates even though we are

using the “aggregate summary-only” command. The upcoming CIDR example

discusses this situation.

aggregate-address

address mask as-set

This advertises the prefix and the more specific routes but it includes

as-set information in the path information of the routing updates.

ex: aggregate 129.0.0.0 255.0.0.0 as-set.

This will be discussed in an example by itself in the following sections.

In case we would like to suppress more specific routes when doing the

aggregation we can define a route map and apply it to the aggregates.

This will allow us to be selective about which more specific routes to

suppress.

aggregate-address

address-mask suppress-map map-name

This advertises the prefix and the more specific routes but it

suppresses advertisement according to a route-map. In the previous

diagram, if we would like to aggregate 160.0.0.0 and suppress the more

specific route 160.20.0.0 and allow 160.10.0.0 to be propagated, we can

use the following route map:

route-map CHECK permit 10

match ip address 1

access-list 1 deny 160.20.0.0 0.0.255.255

access-list 1 permit 0.0.0.0 255.255.255.255

Then we apply the route-map to the aggregate statement.

RTC#

router bgp 300

neighbor 3.3.3.3 remote-as 200

neighbor 2.2.2.2 remote-as 100

network 170.10.0.0

aggregate-address 160.0.0.0 255.0.0.0 suppress-map CHECK

Another variation is the: