Table of Contents
There are two ways that help us to mitigate the BGPs full-mesh requirement in a network:
- Using BGP route-reflectors
- Using BGP confederation
What is BGP Confederation & How it Works?
In this post, we will understand how BGP confederation works and will also configure as simple topology explaining how to configure the same.
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.
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 neighbor ship in AS 10.
- R4 & R5 will form an IBGP neighbor ship in AS 20.
- R3 and R4 will form EBGP neighbor ship.
Related – BGP Local Preference Cisco
Configuration –
R1:
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:
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:
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:
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:
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:
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 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 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 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 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
I am here to share my knowledge and experience in the field of networking with the goal being – “The more you share, the more you learn.”
I am a biotechnologist by qualification and a Network Enthusiast by interest. I developed interest in networking being in the company of a passionate Network Professional, my husband.
I am a strong believer of the fact that “learning is a constant process of discovering yourself.”
– Rashmi Bhardwaj (Author/Editor)
