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Syntax Description:

• AS-number— AS number used with EBGP neighbors.

• 1_or_more_AS-numbers— AS number(s) of directly connected peers that are in a different sub-AS.

Purpose: IBGP neighbors do not propagate routing information learned from one IBGP neighbor to another IBGP neighbor. If you are running IBGP, every IBGP speaker must have a connection to every other IBGP speaker in the AS. This becomes a scaling problem as the number of IBGP speakers increases. The number of IBGP connections for n speakers is [n(n–1)]/2. Table 3-1 lists the number of connections needed for two to ten IBGP speakers.

Table 3-1. IBGP Connections Needed for a Full Mesh

Number of IBGP Speakers
Number of Connections

2
1

3
3

4
6

5
10

6
15

7
21

8
28

9
36

10
45

A confederation is one technique used to overcome the scaling issue with IBGP. The AS is divided into multiple subautonomous systems. Within a confederation sub-AS, a full IBGP mesh is required. BGP connections between confederations behave like EBGP peers, but they exchange routing information as if they were using IBGP. This means that the BGP attributes next hop, metric, and local preference are preserved. To an EBGP neighbor, the confederation appears as a single AS.

Cisco IOS Software Release: 10.3

Configuration Example: BGP Confederation

Autonomous system 1 in Figure 3-7 contains five BGP routers. For an IBGP full mesh, we would need ten IBGP connections. In order to reduce the number of BGP connections within the AS, a BGP confederation is used. AS 1 is divided into three subautonomous systems using AS numbers from the private AS range 64512 to 65535.

Figure 3-7. BGP Confederation

Router A

interface Serial0

 ip address 193.16.0.2 255.255.255.252

!

router bgp 65530

 bgp confederation identifier 1

 bgp confederation peers 65531

 neighbor 193.16.0.1 remote-as 65531

___________________________________________________________________________

Router B

interface Ethernet1/0

 ip address 172.16.0.1 255.255.255.252

!

interface Ethernet1/2

 ip address 172.16.0.17 255.255.255.252

!

interface Serial2/0

 ip address 193.16.0.1 255.255.255.252

 clockrate 64000

!

router bgp 65531

 bgp confederation identifier 1

 bgp confederation peers 65530 65532

 neighbor 172.16.0.2 remote-as 65531

 neighbor 172.16.0.18 remote-as 65532

 neighbor 193.16.0.2 remote-as 65530

___________________________________________________________________________

Router C

interface Ethernet1/0

 ip address 172.16.0.2 255.255.255.252

!

router bgp 65531

 bgp confederation identifier 1

 neighbor 172.16.0.1 remote-as 65531

___________________________________________________________________________

Router D

interface Ethernet1/0

 ip address 172.16.0.14 255.255.255.252

!

interface Ethernet1/2

 ip address 172.16.0.18 255.255.255.252

!

interface Serial2/0

 ip address 193.16.0.9 255.255.255.252

 clockrate 64000

!

router bgp 65532

 bgp confederation identifier 1

 bgp confederation peers 65531

 neighbor 172.16.0.13 remote-as 65532

 neighbor 172.16.0.17 remote-as 65531

 neighbor 193.16.0.10 remote-as 2

___________________________________________________________________________

Router E

interface Ethernet2/1

 ip address 172.16.0.13 255.255.255.252

!

router bgp 65532

 bgp confederation identifier 1

 neighbor 172.16.0.14 remote-as 65532

___________________________________________________________________________

Router F

interface Serial0

 ip address 193.16.0.10 255.255.255.252

!

router bgp 2

 neighbor 193.16.0.9 remote-as 1

The configuration of a BGP confederation is relatively straightforward. The BGP process number used for each router in the confederation is the AS number used to identify the sub-AS:

router bgp 65530, 65531, or 65532

Every router in the confederation is configured with the AS number that you want to use with EBGP peers in this case, AS 1:

bgp confederation identifier 1

Finally, if a router has BGP connections to routers in a different sub-AS, you must use the bgp confederation peers command:

Router A

router bgp 65530

 bgp confederation peers 65531

___________________________________________________________________________

Router B

router bgp 65531

 bgp confederation peers 65530 65532

___________________________________________________________________________

Router D

router bgp 65532

 bgp confederation peers 65531

Routes advertised by BGP within the confederation carry the AS number of each sub-AS that the route advertisement has passed through. For example, create a loopback interface on Router A, and advertise this prefix in BGP:

Router A

interface loopback 0

 ip address 150.150.150.1 255.255.255.0

!

router bgp 65530

 network 150.150.150.0 mask 255.255.255.0

Now trace the route through the confederation to Router F:

rtrA#show ip bgp

BGP table version is 33, local router ID is 193.16.0.2

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

*> 150.150.150.0/24 0.0.0.0                  0         32768 I

___________________________________________________________________________________

rtrB#show ip bgp

BGP table version is 6, local router ID is 172.16.88.4

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

*> 150.150.150.0/24 193.16.0.2              20    100      0 (65530) I

___________________________________________________________________________________

rtrD#show ip bgp

BGP table version is 10, local router ID is 172.16.88.1

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

*> 150.150.150.0/24 193.16.0.2              20    100      0 (65531 65530) I

___________________________________________________________________________________

rtrF#show ip bgp

BGP table version is 23, local router ID is 193.16.0.10

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

*> 150.150.150.0/24 193.16.0.9                             0 1 i

Within the confederation, each sub-AS that the route has traversed is contained in the AS-path attribute. Outside the confederation, the AS-path attribute contains only the AS number of the confederation identifier.

Verification

Examine the neighbor relationship between BGP routers in a different sub-AS. For this case, we will examine the relationship between Routers A and B:

rtrA#show ip bgp neighbors

BGP neighbor is 193.16.0.1,  remote AS 65531, external link

 Index 1, Offset 0, Mask 0x2

  BGP version 4, remote router ID 172.16.88.4

  Neighbor under common administration

  BGP state = Established, table version = 30, up for 00:58:23

  Last read 00:00:24, hold time is 180, keepalive interval is 60 seconds

___________________________________________________________________________

rtrB#show ip bgp neighbors 193.16.0.2

BGP neighbor is 193.16.0.2,  remote AS 65530, external link

  BGP version 4, remote router ID 193.16.0.2

  Neighbor under common administration

  BGP state = Established, up for 01:05:34

  Last read 00:00:34, hold time is 180, keepalive interval is 60 seconds

The BGP neighbor relationship between Routers A and B is external because they are in a different sub-AS. The neighbors are under a common administration because they are confederation peers. The BGP relationship between neighbors in the same sub-AS is a normal IBGP relationship, as shown by the output for Routers B and C:

rtrB#show ip bgp neighbors 172.16.0.2

BGP neighbor is 172.16.0.2,  remote AS 65531, internal link

  BGP version 4, remote router ID 172.16.88.3

  BGP state = Established, up for 01:09:25

  Last read 00:00:25, hold time is 180, keepalive interval is 60 seconds

___________________________________________________________________________

rtrC#show ip bgp neighbors

BGP neighbor is 172.16.0.1,  remote AS 65531, internal link

  BGP version 4, remote router ID 172.16.88.4

  BGP state = Established, up for 01:10:50

  Last read 00:00:50, hold time is 180, keepalive interval is 60 seconds

Finally, examine the relationship between Routers D and F:

rtrD#show ip bgp neighbors 193.16.0.10

BGP neighbor is 193.16.0.10,  remote AS 2, external link

  BGP version 4, remote router ID 193.16.0.10

  BGP state = Established, up for 1d02h

  Last read 00:00:15, hold time is 180, keepalive interval is 60 seconds

___________________________________________________________________________

rtrF#show ip bgp neighbors

BGP neighbor is 193.16.0.9,  remote AS 1, external link

 Index 1, Offset 0, Mask 0x2

  BGP version 4, remote router ID 172.16.88.1

  BGP state = Established, table version = 21, up for 1d02h

  Last read 00:00:09, hold time is 180, keepalive interval is 60 seconds

Router F sees router D as belonging to AS 1, the confederation identifier. The sub-AS numbers are hidden from true external peers.

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. For IBGP and loopbacks, see section 8-33.

Step 2. Verify the syntax of the confederation commands. Each router in the confederation should use the command bgp confederation identifier as-number. BGP connections between subautonomous systems should use the command bgp confederation peers 1_or_more_AS-numbers.

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