Cisco 3560G 48TS Software Configuration Guide - Page 667

Configuring Aggregate Addresses, Configuring Routing Domain Confederations

Page 667 highlights

Chapter 30 Configuring IP Unicast Routing 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: Step 1 Step 2 Step 3 Command configure terminal router bgp autonomous-system aggregate-address address mask Step 4 aggregate-address address mask as-set Step 5 Step 6 Step 7 Step 8 Step 9 Step 10 Step 11 aggregate-address address-mask summary-only aggregate-address address mask suppress-map map-name aggregate-address address mask advertise-map map-name aggregate-address address mask attribute-map map-name end show ip bgp neighbors [advertised-routes] copy running-config startup-config 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 Do not use this keyword when aggregating many paths because this route must be continually withdrawn and updated.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. 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 Routing Domain Confederations 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. 78-16156-01 Catalyst 3560 Switch Software Configuration Guide 30-59

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30-59
Catalyst 3560 Switch Software Configuration Guide
78-16156-01
Chapter 30
Configuring IP Unicast Routing
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:
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 Routing Domain Confederations
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.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router bgp
autonomous-system
Enter BGP router configuration mode.
Step 3
aggregate-address
address mask
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.
Step 4
aggregate-address
address mask
as-set
(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.
Step 5
aggregate-address
address-mask
summary-only
(Optional) Advertise summary addresses only.
Step 6
aggregate-address
address mask
suppress-map
map-name
(Optional) Suppress selected, more specific routes.
Step 7
aggregate-address
address mask
advertise-map
map-name
(Optional) Generate an aggregate based on conditions specified by
the route map.
Step 8
aggregate-address
address mask
attribute-map
map-name
(Optional) Generate an aggregate with attributes specified in the
route map.
Step 9
end
Return to privileged EXEC mode.
Step 10
show ip bgp neighbors
[
advertised-routes
]
Verify the configuration.
Step 11
copy running-config startup-config
(Optional) Save your entries in the configuration file.