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Syntax Description:

• address— Aggregate IP address.

• mask— Aggregate mask.

• route-map-name— Name of the route map that is used to determine which specific prefixes will be suppressed.

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 the more-specific routes indicated by the route map.

Cisco IOS Software Release: 10.0

Configuration Example: Suppressing a Subset of the More-Specific Routes Used to Form the Aggregate

In Figure 1-8, Router B is generating an aggregate for 172.16.0.0/22 while suppressing one of the more-specific prefixes.

Figure 1-8. Suppressing Specific Prefixes

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 route 172.16.2.0/24:

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 suppress-map suppress

 neighbor 10.1.1.1 remote-as 1

!

access-list 1 permit 172.16.2.0 0.0.0.255

route-map suppress permit 10

 match ip address 1

The form of the route map used permits routes to be suppressed. This example permits prefix 172.16.2.0/24 to be suppressed. The prefixes that are contained in the aggregate that are not specifically matched by the route map will not be suppressed.

Verification

Verify that the aggregate address is in both the Router A and B BGP tables and that the more-specific route 172.16.2.0/24 has been suppressed:

rtrA#show ip bgp

BGP table version is 29, 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

*> 172.16.0.0/22    10.1.1.2                               0 2 i

*> 172.16.1.0/24    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 7, 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

*> 172.16.0.0/22    0.0.0.0                            32768 i

*> 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

*> 172.16.3.0/24    0.0.0.0                  0         32768 i

Examine the specific information for prefix 172.16.2.0/24 on Router B:

rtrB#show ip bgp 172.16.2.0

BGP routing table entry for 172.16.2.0/24, version 7

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 the syntax of your route map and access list.

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