BGP Confederation Explained

There are two ways that help us to mitigate the BGPs full-mesh requirement in a network: Using BGP route-reflectors and Using BGP confederation. In this post, we will understand how BGP confederation works and will also configure as simple topology explaining how to configure the same.

What is BGP Confederation & How it Works?

BGP confederations allow us to divide a single AS into one or more ASs and assign the whole group to a single AS. Each smaller AS will need to have a full mesh inside its own AS.

Related – BGP Interview Questions

Even though this ASs will have EBGP peers to ASs within the confederation, they exchange routing as if they were using IBGP; next hop, metric and local preference information is preserved.

To the outside world, the confederation (the group of ASs) will look like a single AS.

BGP Confederation: Topology

We will use the below topology to configure the BGP confederation:

• In the above topology routers R2, R3, R4 & R5 are part of the same AS 100.
• We have divided AS 100 into two smaller AS 10 and 20 with two routers each.
• R2 & R3 will form an IBGP neighborship in AS 10.
• R4 & R5 will form an IBGP neighborship in AS 20.
• R3 and R4 will form EBGP neighborship.

Related – BGP Local Preference Cisco

Configuration

R1

interface Loopback0
ip address 1.1.1.1 255.255.255.255

!

interface FastEthernet0/0

ip address 9.9.12.1 255.255.255.0

!

router bgp 200

bgp log-neighbor-changes

network 1.1.1.1 mask 255.255.255.255

neighbor 9.9.12.2 remote-as 100

R2

interface Loopback0
ip address 9.9.0.2 255.255.255.255

ip ospf 1 area 0

!

interface FastEthernet0/0

ip address 9.9.12.2 255.255.255.0

!

interface FastEthernet1/0

ip address 9.9.23.2 255.255.255.0

ip ospf 1 area 0

!

router bgp 10

bgp log-neighbor-changes

bgp confederation identifier 100

bgp confederation peers 20

neighbor 9.9.0.3 remote-as 10

neighbor 9.9.0.3 update-source Loopback0

neighbor 9.9.12.1 remote-as 200

R3

interface Loopback0
ip address 9.9.0.3 255.255.255.255

ip ospf 1 area 0

!

interface FastEthernet0/0

ip address 9.9.23.3 255.255.255.0

ip ospf 1 area 0

!

interface FastEthernet1/0

ip address 9.9.34.3 255.255.255.0

ip ospf 1 area 0

!

router bgp 10

bgp log-neighbor-changes

bgp confederation identifier 100

bgp confederation peers 20

neighbor 9.9.0.2 remote-as 10

neighbor 9.9.0.2 update-source Loopback0

neighbor 9.9.0.4 remote-as 20

neighbor 9.9.0.4 ebgp-multihop 2 # Uses EBGP multihop as R4 is treated as a EBGP neighbor inside a confederation and Loopback of R4 is one hop away from R3.

neighbor 9.9.0.4 update-source Loopback0

R4

interface Loopback0
ip address 9.9.0.4 255.255.255.255

ip ospf 1 area 0

!

interface FastEthernet0/0

ip address 9.9.34.4 255.255.255.0

ip ospf 1 area 0

!

interface FastEthernet1/0

ip address 9.9.45.4 255.255.255.0

ip ospf 1 area 0

!

router bgp 20

bgp log-neighbor-changes

bgp confederation identifier 100

bgp confederation peers 10

neighbor 9.9.0.3 remote-as 10

neighbor 9.9.0.3 ebgp-multihop 2

neighbor 9.9.0.3 update-source Loopback0

neighbor 9.9.0.5 remote-as 20

neighbor 9.9.0.5 update-source Loopback0

R5

interface Loopback0
ip address 9.9.0.5 255.255.255.255

ip ospf 1 area 0

!

interface FastEthernet0/0

ip address 9.9.45.5 255.255.255.0

ip ospf 1 area 0

!

interface FastEthernet1/0

ip address 9.9.56.5 255.255.255.0

!

router bgp 20

bgp log-neighbor-changes

bgp confederation identifier 100

bgp confederation peers 10

neighbor 9.9.0.4 remote-as 20

neighbor 9.9.0.4 update-source Loopback0

neighbor 9.9.56.6 remote-as 200

R6

interface Loopback0
ip address 6.6.6.6 255.255.255.255

!

interface FastEthernet0/0

ip address 9.9.56.6 255.255.255.0

!

router bgp 200

bgp log-neighbor-changes

network 6.6.6.6 mask 255.255.255.255

neighbor 9.9.56.5 remote-as 100

Logs

R3 forms IBGP with R2 and EBGP with R4:

R3#sh ip bgp summary

BGP router identifier 9.9.0.3, local AS number 10
BGP table version is 1, main routing table version 1

1 network entries using 144 bytes of memory

1 path entries using 80 bytes of memory

1/0 BGP path/bestpath attribute entries using 136 bytes of memory

1 BGP AS-PATH entries using 24 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

BGP using 384 total bytes of memory

BGP activity 1/0 prefixes, 1/0 paths, scan interval 60 secs

Neighbor        V     AS       MsgRcvd MsgSent   TblVer  InQ OutQ  Up/Down   State/PfxRcd

9.9.0.2         4     10       5          4        1      0     0          00:01:15     1

9.9.0.4         4     20       4          4        1      0     0           00:01:07    0

And Similarly R4 forms IBGP with R5 and EBGP with R3:

R4#sh ip bgp summary

BGP router identifier 9.9.0.4, local AS number 20
BGP table version is 1, main routing table version 1

1 network entries using 144 bytes of memory

1 path entries using 80 bytes of memory

1/0 BGP path/bestpath attribute entries using 136 bytes of memory

1 BGP AS-PATH entries using 24 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

BGP using 384 total bytes of memory

BGP activity 1/0 prefixes, 1/0 paths, scan interval 60 secs

Neighbor        V     AS    MsgRcvd  MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd

9.9.0.3         4       10      5       4         1      0    0         00:01:42        0

9.9.0.5         4        20     6       4         1      0    0          00:01:50        1

But for the external routers R1 and R6, the remote AS will be seen as a single AS 100 and they form neighbor ship with R2 and R5 respectively.

R1#sh ip bgp summary

BGP router identifier 1.1.1.1, local AS number 200
BGP table version is 2, main routing table version 2

1 network entries using 144 bytes of memory

1 path entries using 80 bytes of memory

1/1 BGP path/bestpath attribute entries using 136 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

BGP using 360 total bytes of memory

BGP activity 1/0 prefixes, 1/0 paths, scan interval 60 secs

Neighbor        V       AS      MsgRcvd  MsgSent   TblVer  InQ OutQ Up/Down        State/PfxRcd

9.9.12.2        4       100     5          7         2      0     0         00:03:02        0

R6#sh ip bgp summary

BGP router identifier 6.6.6.6, local AS number 200
BGP table version is 2, main routing table version 2

1 network entries using 144 bytes of memory

1 path entries using 80 bytes of memory

1/1 BGP path/bestpath attribute entries using 136 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

BGP using 360 total bytes of memory

BGP activity 1/0 prefixes, 1/0 paths, scan interval 60 secs

Neighbor        V      AS         MsgRcvd    MsgSent   TblVer  InQ OutQ   Up/Down     State/PfxRcd

9.9.56.5        4     100          6           7        2      0      0          00:03:09        0

Hope you would have understood all about BGP Confederation.

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ABOUT THE AUTHOR

Rashmi Bhardwaj

Founder of AAR TECHNOSOLUTIONS, Rashmi is an evangelist for IT and technology. With more than 12 years in the IT ecosystem, she has been supporting multi domain functions across IT & consultancy services, in addition to Technical content making.

You can learn more about her on her linkedin profile – Rashmi Bhardwaj