The aggregate-address Command
AS SET BGP Route Aggregation allows network administrators to advertise a summarized route instead of advertising multiple individual routes, which can help conserve bandwidth and reduce processing overhead on routers. When you issue the aggregate-address command without any arguments, no individual route attributes (such as AS PATH or community) are inherited, resulting in a loss of granularity. This loss of granularity can impact the ability to make detailed routing decisions.
The aggregate-address address mask [as-set] [summary-only] [suppress-map map-name] [advertise-map map-name] [attribute-map map-name] command in Border Gateway Protocol (BGP) allows the aggregation of specific routes into one aggregate route. This command is useful for reducing the size of the routing table and improving overall network efficiency.
The optional as-set keyword can be used with the aggregate-address command to maintain the BGP path information history. By using the as-set keyword with the aggregate-address command, network administrators can preserve the BGP path information history, which can be valuable for troubleshooting and analyzing network performance. This allows for more detailed analysis and understanding of the routes being advertised while benefiting from the reduced size of the routing table and improved network efficiency.
AS SET attributes from the component aggregate routes are copied to the aggregate route as the router generates it. The original aggregate prefix AS Path settings are saved in the AS SET portion of the AS Path. Even if multiple ASs are listed, the AS SET BGP, which is displayed within brackets, only counts as one hop.
Aggregate Route
The AS SET BGP aggregation route simplifies the representation of multiple AS hops in the routing table, making it more efficient and concise. Additionally, this allows for easier tracking and identification of the origin ASs that contributed to the aggregate route.
By retaining the AS Path information in the AS SET BGP, network administrators can trace the aggregate route and identify any potential issues or bottlenecks in the network. This information is crucial for troubleshooting and optimizing network performance.
Route Aggregation (RA), also known as BGP Route Summarization or summary route, is a method of reducing the size of the routing table by announcing to other ASes the entire address block received from the Regional Internet Registry (RIR). Non-aggregation routing, in which individual sub-prefixes of the address block are announced to BGP peers, is the inverse of aggregated route RA, which shrinks the global BGP routing table, reducing router workload and saving network bandwidth.
AS SET BGP Route Aggregation Example
Consider the following topology below.
Router1 Configuration
Router(config)#ip cef Router(config)#interface gigabitEthernet0/0 Router(config-if)#ip address 192.168.10.1 255.255.255.0 Router(config)#interface gigabitEthernet0/1 Router(config-if)#ip address 192.168.20.1 255.255.255.0 Router(config)#interface gigabitEthernet0/2 Router(config-if)#ip address 192.168.30.1 255.255.255.0 Router(config)#router bgp 1 Router(config-router)#neighbor 192.168.10.2 remote-as 2 Router(config-router)#neighbor 192.168.20.2 remote-as 3 Router(config-router)#neighbor 192.168.30.2 remote-as 4
Router2 Configuration
Router(config)#ip cef Router(config)#interface loopback0 Router(config-if)#ip address 172.168.10.10 255.255.255.255 Router(config)#interface gigabitEthernet0/0 Router(config-if)#ip address 192.168.10.2 255.255.255.0 Router(config)#router bgp 2 Router(config-router)#network 172.168.10.10 mask 255.255.255.255 Router(config-router)#neighbor 192.168.10.1 remote-as 1
Router3 Configuration
Router(config)#ip cef Router(config)#interface loopback0 Router(config-if)#ip address 172.168.20.20 255.255.255.255 Router(config)#interface gigabitEthernet0/1 Router(config-if)#ip address 192.168.20.2 255.255.255.0 Router(config)#router bgp 3 Router(config-router)#network 172.168.20.20 mask 255.255.255.255 Router(config-router)#neighbor 192.168.20.1 remote-as 1
Router4 Configuration
Router(config)#ip cef Router(config)#interface gigabitEthernet0/2 Router(config-if)#ip address 192.168.30.2 255.255.255.0 Router(config)#router bgp 4 Router(config-router)#neighbor 192.168.30.1 remote-as 1
With AS-SET BGP
Add the AS-SET parameter on Router1
Router1(config)#router bgp 1 Router1(config-router)#aggregate-address 192.168.0.0 255.255.0.0 summary-only as-set
Router4#show ip bgp
BGP table version is 11, local router ID is 192.168.30.2
Status codes: suppressed, damped, history, valid, > best, i - internal,
RIB-failure, Stale, multipath, backup-path, RT-Filter,
best-external, additional-path, RIB-compressed,
secondary path,
Network Next Hop Metric LocPrf Weight Path
*> 192.168.0.0 192.168.30.1 0 0 1 {2,3} i
This route efficiently summarizes multiple, hidden sub-routes within the 192.168.30.0/24 network. Its AS Path reveals it traversed ASes 1, 2, and 3, while the AS_SET {AS1 AS2 AS3} clarifies all contributing ASes, aiding troubleshooting and optimizing network performance.
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