Configuring Aggregate Addresses - Cisco Catalyst 3550 series Software Configuration Manual

Configuring BGP

Configuring Aggregate Addresses

Classless interdomain routing (CIDR) enables you to create aggregate routes (or supernets) to minimize
the size of routing tables. You can configure aggregate routes in BGP either by redistributing an
aggregate route into BGP or by creating an aggregate entry in the BGP routing table. An aggregate
address is added to the BGP table when there is at least one more specific entry in the BGP table.
Beginning in privileged EXEC mode, use these commands to create an aggregate address in the routing
table:
Command
Step 1
configure terminal
Step 2 router bgp autonomous-system
Step 3
aggregate-address address mask
Step 4
aggregate-address address mask as-set
Step 5 aggregate-address address-mask
summary-only
Step 6 aggregate-address address mask
suppress-map map-name
Step 7 aggregate-address address mask
advertise-map map-name
Step 8
aggregate-address address mask
attribute-map map-name
Step 9
end
Step 10
show ip bgp neighbors [advertised-routes]
Step 11 copy running-config startup-config
To delete an aggregate entry, use the no aggregate-address address mask router configuration
command. To return options to the default values, use the command with keywords.
Configuring a Routing Domain Confederation
One way to reduce the IBGP mesh is to divide an autonomous system into multiple subautonomous
systems and to group them into a single confederation that appears as a single autonomous system. Each
autonomous system is fully meshed within itself and has a few connections to other autonomous systems
in the same confederation. Even though the peers in different autonomous systems have EBGP sessions,
they exchange routing information as if they were IBGP peers. Specifically, the next hop, MED, and
local preference information is preserved. You can then use a single IGP for all of the autonomous
systems.
Catalyst 3550 Multilayer Switch Software Configuration Guide
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Purpose
Enter global configuration mode.
Enter BGP router configuration mode.
Create an aggregate entry in the BGP routing table. The aggregate
route is advertised as coming from the AS, and the atomic aggregate
attribute is set to indicate that information might be missing.
(Optional) Generate AS set path information. This command
creates an aggregate entry following the same rules as the previous
command, but the advertised path will be an AS_SET consisting of
all elements contained in all paths. Do not use this keyword when
aggregating many paths because this route must be continually
withdrawn and updated.
(Optional) Advertise summary addresses only.
(Optional) Suppress selected, more specific routes.
(Optional) Generate an aggregate based on conditions specified by
the route map.
(Optional) Generate an aggregate with attributes specified in the
route map.
Return to privileged EXEC mode.
Verify the configuration.
(Optional) Save your entries in the configuration file.
Chapter 31 Configuring IP Unicast Routing
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