Cisco BGP-4 Command and Configuration Handbook

1-2 aggregate-address address mask as-set

Syntax Description:

• address ” Aggregate IP address.

• mask ” Aggregate mask.

Purpose: To create an aggregate entry in the BGP table. An aggregate is created only if a more-specific route of the aggregate exists in the BGP table. Without the as-set option, the AS path information for the specific routes forming the aggregate is lost. This form of the aggregate-address command advertises the aggregate while retaining the AS path information for the more-specific routes.

Cisco IOS Software Release: 10.0

Configuration Example: Forming an Aggregate Consisting of Prefixes from Different Autonomous Systems

In Figure 1-4, Router A is learning about networks 172.16.0.0/24 and 172.16.1.0/24 from AS 65530 and networks 172.16.2.0/24 and 172.16.3.0/24 from AS 2.

Router A router bgp 1 neighbor 10.1.1.2 remote-as 2 neighbor 172.17.1.2 remote-as 65530 ___________________________________________________________________________ Router B interface loopback 0 ip address 172.16.2.1 255.255.255.0 ! interface loopback 1 ip address 172.16.3.1 255.255.255.0 ! router bgp 2 network 172.16.2.0 mask 255.255.255.0 network 172.16.3.0 mask 255.255.255.0 neighbor 10.1.1.1 remote-as 1 ___________________________________________________________________________ Router C interface loopback 0 ip address 172.16.0.1 255.255.255.0 ! interface loopback 1 ip address 172.16.1.1 255.255.255.0 ! router bgp 65530 network 172.16.0.0 mask 255.255.255.0 network 172.16.1.0 mask 255.255.255.0 neighbor 172.17.1.1 remote-as 1 !

Four loopbacks have been created ”two on Router B and two on Router C. A BGP router can advertise an aggregate only if at least one specific route of the aggregate is in the BGP table. The BGP network commands are necessary on Routers B and C in order to place more-specific routes of the aggregate into the BGP table. Before aggregating the loopback prefixes, verify that the specific routes are in the BGP tables on Routers A, B, and C:

rtrA# show ip bgp BGP table version is 35, local router ID is 144.223.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 172.16.0.0/24 172.17.1.2 0 0 65530 i *> 172.16.1.0/24 172.17.1.2 0 0 65530 i *> 172.16.2.0/24 10.1.1.2 0 0 2 i *> 172.16.3.0/24 10.1.1.2 0 0 2 i ___________________________________________________________________________ rtrB# show ip bgp BGP table version is 13, local router ID is 172.16.3.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 172.16.0.0/24 10.1.1.1 0 1 65530 i *> 172.16.1.0/24 10.1.1.1 0 1 65530 i *> 172.16.2.0/24 0.0.0.0 0 32768 i *> 172.16.3.0/24 0.0.0.0 0 32768 I ___________________________________________________________________________ rtrC# show ip bgp BGP table version is 35, local router ID is 172.17.1.2 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 172.16.0.0/24 0.0.0.0 0 32768 i *> 172.16.1.0/24 0.0.0.0 0 32768 i *> 172.16.2.0/24 172.17.1.1 0 1 2 i *> 172.16.3.0/24 172.17.1.1 0 1 2 i

Now modify the BGP configuration on Router A to enable the advertisement of the aggregate:

Router A router bgp 1 aggregate-address 172.16.0.0 255.255.252.0 neighbor 10.1.1.2 remote-as 2 neighbor 172.17.1.2 remote-as 65530

The aggregate contains two routes from AS 65530 and two routes from AS 2. Router A should now be advertising the aggregate with itself as the next hop, indicating a loss of AS path information. The AS path information for the aggregate indicates that this prefix originates from AS 1. Routers B and C accept this route because their AS number does not appear in the AS_PATH attribute:

rtrA# show ip bgp BGP table version is 6, local router ID is 144.223.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 172.16.0.0/24 172.17.1.2 0 0 65530 i *> 172.16.0.0/22 0.0.0.0 32768 i *> 172.16.1.0/24 172.17.1.2 0 0 65530 i *> 172.16.2.0/24 10.1.1.2 0 0 2 i *> 172.16.3.0/24 10.1.1.2 0 0 2 I rtrA# show ip bgp 172.16.0.0 255.255.252.0 BGP routing table entry for 172.16.0.0/22, version 6 Paths: (1 available, best #1, table Default-IP-Routing-Table) Advertised to non peer-group peers: 10.1.1.2 172.17.1.2 Local, (aggregated by 1 144.223.1.1) 0.0.0.0 from 0.0.0.0 (144.223.1.1) Origin IGP, localpref 100, weight 32768, valid, aggregated, local, atomic- aggregate, best ___________________________________________________________________________ rtrB# show ip bgp BGP table version is 8, local router ID is 172.16.3.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 172.16.0.0/24 10.1.1.1 0 1 65530 i *> 172.16.0.0/22 10.1.1.1 0 1 i *> 172.16.1.0/24 10.1.1.1 0 1 65530 i *> 172.16.2.0/24 0.0.0.0 0 32768 i *> 172.16.3.0/24 0.0.0.0 0 32768 I ___________________________________________________________________________ rtrC# show ip bgp BGP table version is 8, local router ID is 172.16.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 172.16.0.0/24 0.0.0.0 0 32768 i *> 172.16.0.0/22 172.17.1.1 0 1 i *> 172.16.1.0/24 0.0.0.0 0 32768 i *> 172.16.2.0/24 172.17.1.1 0 1 2 i *> 172.16.3.0/24 172.17.1.1 0 1 2 i

Now use the as-set option on Router A in order to preserve AS path information:

Router A router bgp 1 aggregate-address 172.16.0.0 255.255.252.0 as-set neighbor 10.1.1.2 remote-as 2 neighbor 172.17.1.2 remote-as 65530

Verification

Verify that the aggregate address is in the BGP table on Router A and that the AS path information has been retained:

rtrA# show ip bgp BGP table version is 10, local router ID is 144.223.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 172.16.0.0/24 172.17.1.2 0 0 65530 i *> 172.16.0.0/22 0.0.0.0 32768 {65530,2} i *> 172.16.1.0/24 172.17.1.2 0 0 65530 i *> 172.16.2.0/24 10.1.1.2 0 0 2 i *> 172.16.3.0/24 10.1.1.2 0 0 2 i

The as-set option causes Router A to preserve the AS path information for the aggregate:

rtrA# show ip bgp 172.16.0.0/22 BGP routing table entry for 172.16.0.0/22, version 10 Paths: (1 available, best #1, table Default-IP-Routing-Table) Advertised to non peer-group peers: 10.1.1.2 172.17.1.2 {65530,2}, (aggregated by 1 144.223.1.1) 0.0.0.0 from 0.0.0.0 (144.223.1.1) Origin IGP, localpref 100, weight 32768, valid, aggregated, local, atomic- aggregate, best

Routers B and C should reject the aggregate because their AS number is now contained in the AS path attribute:

rtrB# show ip bgp BGP table version is 9, local router ID is 172.16.3.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 172.16.0.0/24 10.1.1.1 0 1 65530 i *> 172.16.1.0/24 10.1.1.1 0 1 65530 i *> 172.16.2.0/24 0.0.0.0 0 32768 i *> 172.16.3.0/24 0.0.0.0 0 32768 I ___________________________________________________________________________ rtrC# show ip bgp BGP table version is 9, local router ID is 172.16.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 172.16.0.0/24 0.0.0.0 0 32768 i *> 172.16.1.0/24 0.0.0.0 0 32768 i *> 172.16.2.0/24 172.17.1.1 0 1 2 i *> 172.16.3.0/24 172.17.1.1 0 1 2 i

Troubleshooting

Step 1. Verify that the BGP neighbors are in the Established state using the show ip bgp neighbors command.

If the neighbor relationship is not in the Established state, see section 8-23.

Step 2. Ensure that at least one specific route in the BGP table is contained in the range of addresses that you want to aggregate using the show ip bgp command.

Step 3. If at least one more-specific route is in the BGP table, go to Step 5.

Step 4. If at least one more-specific route is not in the BGP table, do the following:

- ” Check the syntax (address and mask) of your BGP network command. Go to Step 2.

- ” If you're redistributing routes (connected, static, or from an IGP), make sure you are using the no auto-summary command. Check the syntax of your redistribution command(s). Go to Step 2.

Step 5. Verify that no filters are blocking the aggregate from being advertised.

Step 6. Verify the syntax of your route map and access list.