BGP Route Aggregation: Understanding & Implementation

BGP Route Aggregation: Understanding & Implementation

You use aggregates in order to minimize the size of routing tables.
Aggregation is the process that combines the characteristics of several different routes in such a way that advertisement of a single route is possible.

Aggregate without the summary-only Argument
R1:
conf t
router bgp 1
aggregate-address 172.16.0.0 255.255.0.0
end
wr

Observations:
1. This command advertises the prefix/aggregate route and all the more-specific routes. Hence, the more specific 172.16.2.2/32 and 172.16.3.3/32 routes are still on R4. Issue show ip bgp on R4.
2. You cannot aggregate an address if you do not have a more-specific route of that address in the BGP routing table.

An injection of the more-specific route into the BGP table is possible. The route injection can occur via:
1. Incoming updates from other ASs
2. Redistribution of an IGP or static into BGP
3. The network command, for example, network 172.16.0.0

Aggregate Without the as-set Argument
R1:
conf t
router bgp 1
aggregate-address 172.16.0.0 255.255.0.0 summary-only
end
wr

Observations:
1. This command advertises the prefix/aggregate only. The command suppresses all the more-specific routes. Issue show ip bgp on R4.
2. The more specific 172.16.2.2/32 and 172.16.3.3/32 routes are suppressed on R1. Issue show ip bgp on R1.
3. The aggregate route 172.16.0.0/16 is considered to have originated from AS 1 with origin code IGP.
The route has lost all the specific AS_PATH information of the individual prefixes 172.16.2.2/32, of AS 2, and 172.16.3.3/32, of AS 3. Issue show ip bgp on R4.

Aggregate with the as-set Argument
R1:
conf t
router bgp 1
aggregate-address 172.16.0.0 255.255.0.0 summary-only as-set
end
wr

Observations:
1. With the as-set configuration command, the aggregate inherits the attributes of the more-specific routes. Issue show ip bgp on R4 and check path information.
2. The as-set information becomes important in the avoidance of routing loops because the information records where the route has been.

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