Cisco BGP-4 Command and Configuration Handbook

1-6 aggregate-address address mask summary-only

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. This form of the aggregate-address command advertises the aggregate while suppressing all the more-specific routes.

Cisco IOS Software Release: 10.0

Configuration Example: Advertise an Aggregate While Suppressing the More-Specific Routes

In Figure 1-7, Router B is generating an aggregate for 172.16.0.0/22 while suppressing the specific routes that comprise the aggregate.

Router A router bgp 1 neighbor 10.1.1.2 remote-as 2 ___________________________________________________________________________ Router B 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 ! interface loopback 2 ip address 172.16.2.1 255.255.255.0 ! interface loopback 3 ip address 172.16.3.1 255.255.255.0 ! router bgp 2 network 172.16.0.0 mask 255.255.255.0 network 172.16.1.0 mask 255.255.255.0 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 !

Four loopbacks have been created on Router B to simulate the locally sourced routes that will be aggregated. 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 Router B in order to place more-specific routes of the aggregate into the BGP table. Before aggregating the loopback prefixes, you need to verify that the specific routes are in the BGP tables on Routers A and B:

rtrA# show ip bgp BGP table version is 16, local router ID is 172.17.1.1 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 10.1.1.2 0 0 2 i *> 172.16.1.0/24 10.1.1.2 0 0 2 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 6, 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 0.0.0.0 0 32768 i *> 172.16.1.0/24 0.0.0.0 0 32768 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

Now modify the BGP configuration on Router B to enable the advertisement of the aggregate while suppressing the more-specific routes:

Router B router bgp 2 network 172.16.0.0 mask 255.255.255.0 network 172.16.1.0 mask 255.255.255.0 network 172.16.2.0 mask 255.255.255.0 network 172.16.3.0 mask 255.255.255.0 aggregate-address 172.16.0.0 255.255.252.0 summary-only neighbor 10.1.1.1 remote-as 1

The configuration for Router B contains a network command for every prefix that is part of the aggregate. Because you need only one more-specific route in the BGP table in order to generate the prefix, you could use only one network command. The problem with using only one network command is that if the network goes down, the more-specific route is withdrawn from the BGP table. If the only specific route in the BGP table is withdrawn, the aggregate is withdrawn as well. If you use a network command for every prefix that is contained in the aggregate, the aggregate is advertised as long as one of the more-specific routes is up.

Verification

Verify that the aggregate address is in both the Router A and B BGP tables and that the more-specific routes have been suppressed:

rtrA# show ip bgp BGP table version is 13, local router ID is 172.17.1.1 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 *> 172.16.0.0/22 10.1.1.2 0 2 I ___________________________________________________________________________ rtrB# show ip bgp BGP table version is 11, 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 s> 172.16.0.0/24 0.0.0.0 0 32768 i *> 172.16.0.0/22 0.0.0.0 32768 i s> 172.16.1.0/24 0.0.0.0 0 32768 i s> 172.16.2.0/24 0.0.0.0 0 32768 i s> 172.16.3.0/24 0.0.0.0 0 32768 i

Examine the specific information for one of the specific routes on Router B:

rtrB# show ip bgp 172.16.2.0 BGP routing table entry for 172.16.2.0/24, version 9 Paths: (1 available, best #1) Advertisements of this net are suppressed by an aggregate. Not advertised to any peer Local 0.0.0.0 from 0.0.0.0 (172.16.3.1) Origin IGP, metric 0, localpref 100, weight 32768, valid, sourced, local, best, ref 2

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 there is not at least one more-specific route 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 that the specific routes of the aggregate are being suppressed using the show ip bgp command.