BGP: Confederations
· Confederation is simply dividing the AS into Sub-ASs.
· Inside a Sub-AS, full-mesh required.
· If we can’t have full-mesh, we can use Route Reflectors inside a Sub-AS.
· Confederation EBGP connection acts like regular EBGP connection regarding TTL, because Confederation EBGP connection also has TTL of 1.
· Next-hop will not change while advertising to Confederation EBGP peer. (NEXT-HO gets changed in regular EBGP update)
· MED value is advertised to Confederation EBGP peer. (MED is non-transitive attribute, so it will not be advertised in regular EBGP update)
· Local Preference also advertised to Confederation EBGP peer. (Local Preference value will not leave AS).
· Let me take a small topology to configure this
· AS 400: R4
· AS 100: R1, R2, R3
· Confederation AS 65531: R1 & R2
· Confederation AS 65532: R3
· EBGP: R2 – R4
· Confederation EBGP: R2 – R3
· Confederation IBGP: R1 – R2
R4#show run | begin router bgp
router bgp 400
no synchronization
bgp router-id 4.4.4.4
bgp log-neighbor-changes
network 4.4.4.4 mask 255.255.255.255
neighbor 100.1.24.2 remote-as 100
no auto-summary
!
R2#show run | begin router bgp
router bgp 65531
no synchronization
bgp router-id 2.2.2.2
bgp log-neighbor-changes
bgp confederation identifier 100
bgp confederation peers 65532
network 2.2.2.2 mask 255.255.255.255
neighbor 100.1.12.1 remote-as 65531
neighbor 100.1.23.3 remote-as 65532
neighbor 100.1.24.4 remote-as 400
no auto-summary
!
R1#show run | begin router bgp
router bgp 65531
no synchronization
bgp router-id 1.1.1.1
bgp log-neighbor-changes
bgp confederation identifier 100
network 1.1.1.1 mask 255.255.255.255
neighbor 100.1.12.2 remote-as 65531
no auto-summary
!
R3#show run | begin router bgp
router bgp 65532
no synchronization
bgp router-id 3.3.3.3
bgp log-neighbor-changes
bgp confederation identifier 100
bgp confederation peers 65531
network 3.3.3.3 mask 255.255.255.255
neighbor 100.1.23.2 remote-as 65531
no auto-summary
!
R4#show ip bgp sum
BGP router identifier 4.4.4.4, local AS number 400
BGP table version is 5, main routing table version 5
4 network entries using 468 bytes of memory
4 path entries using 208 bytes of memory
4/3 BGP path/bestpath attribute entries using 496 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 1196 total bytes of memory
BGP activity 4/0 prefixes, 4/0 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
100.1.24.2 4 100 27 26 5 0 0 00:03:52 3
R4#
R2#show ip bgp sum
BGP router identifier 2.2.2.2, local AS number 65531
BGP table version is 5, main routing table version 5
4 network entries using 468 bytes of memory
4 path entries using 208 bytes of memory
5/4 BGP path/bestpath attribute entries using 620 bytes of memory
2 BGP AS-PATH entries using 48 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 1344 total bytes of memory
BGP activity 4/0 prefixes, 4/0 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
100.1.12.1 4 65531 8 10 5 0 0 00:04:02 1
100.1.23.3 4 65532 7 9 5 0 0 00:03:50 1
100.1.24.4 4 400 8 10 5 0 0 00:03:55 1
R2#
R1#show ip bgp sum
BGP router identifier 1.1.1.1, local AS number 65531
BGP table version is 3, main routing table version 3
4 network entries using 468 bytes of memory
4 path entries using 208 bytes of memory
5/2 BGP path/bestpath attribute entries using 620 bytes of memory
2 BGP AS-PATH entries using 48 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 1344 total bytes of memory
BGP activity 4/0 prefixes, 4/0 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
100.1.12.2 4 65531 10 8 3 0 0 00:04:03 3
R1#
R3#show ip bgp sum
BGP router identifier 3.3.3.3, local AS number 65532
BGP table version is 3, main routing table version 3
4 network entries using 468 bytes of memory
4 path entries using 208 bytes of memory
4/2 BGP path/bestpath attribute entries using 496 bytes of memory
2 BGP AS-PATH entries using 48 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 1220 total bytes of memory
BGP activity 4/0 prefixes, 4/0 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
100.1.23.2 4 65531 9 7 3 0 0 00:03:51 3
R3#
R4#show ip bgp
BGP table version is 5, local router ID is 4.4.4.4
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 1.1.1.1/32 100.1.24.2 0 100 i
*> 2.2.2.2/32 100.1.24.2 0 0 100 i
*> 3.3.3.3/32 100.1.24.2 0 100 i
*> 4.4.4.4/32 0.0.0.0 0 32768 i
R4#
R2#show ip bgp
BGP table version is 5, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*>i1.1.1.1/32 100.1.12.1 0 100 0 i
*> 2.2.2.2/32 0.0.0.0 0 32768 i
*> 3.3.3.3/32 100.1.23.3 0 100 0 (65532) i
*> 4.4.4.4/32 100.1.24.4 0 0 400 i
R2#
R1#show ip bgp
BGP table version is 3, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 1.1.1.1/32 0.0.0.0 0 32768 i
*>i2.2.2.2/32 100.1.12.2 0 100 0 i
* i3.3.3.3/32 100.1.23.3 0 100 0 (65532) i
* i4.4.4.4/32 100.1.24.4 0 100 0 400 i
R1#
R3#show ip bgp
BGP table version is 3, local router ID is 3.3.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* 1.1.1.1/32 100.1.12.1 0 100 0 (65531) i
*> 2.2.2.2/32 100.1.23.2 0 100 0 (65531) i
*> 3.3.3.3/32 0.0.0.0 0 32768 i
* 4.4.4.4/32 100.1.24.4 0 100 0 (65531) 400 i
R3#
Observations:
· All BGP Neighbors are Up.
· Next-hop is not changed for Confederation EBGP update. (next-hop will change for regular EBGP updates).
· For the prefix 4.4.4.4/32 which is advertised by R4, R2 didn’t change the next-hop value while forwarding the update to R3, which is Confederation EBGP peer.
· Let me configure MED and Local Preference values to see what happens
R4(config)#route-map MED
R4(config-route-map)#set metric 444
R4(config-route-map)#router bgp 400
R4(config-router)#neighbor 100.1.24.2 route-map MED out
R4(config-router)#
R2(config)#route-map LOCAL
R2(config-route-map)#set local-preference 222
R2(config-route-map)#router bgp 65531
R2(config-router)#neighbor 100.1.24.4 route-map LOCAL in
R2(config-router)#
R2#show ip bgp
BGP table version is 5, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*>i1.1.1.1/32 100.1.12.1 0 100 0 i
*> 2.2.2.2/32 0.0.0.0 0 32768 i
*> 3.3.3.3/32 100.1.23.3 0 100 0 (65532) i
*> 4.4.4.4/32 100.1.24.4 444 222 0 400 i
R2#
R1#show ip bgp
BGP table version is 5, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 1.1.1.1/32 0.0.0.0 0 32768 i
*>i2.2.2.2/32 100.1.12.2 0 100 0 i
* i3.3.3.3/32 100.1.23.3 0 100 0 (65532) i
* i4.4.4.4/32 100.1.24.4 444 222 0 400 i
R1#
R3#show ip bgp
BGP table version is 5, local router ID is 3.3.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* 1.1.1.1/32 100.1.12.1 0 100 0 (65531) i
*> 2.2.2.2/32 100.1.23.2 0 100 0 (65531) i
*> 3.3.3.3/32 0.0.0.0 0 32768 i
* 4.4.4.4/32 100.1.24.4 444 222 0 (65531) 400 i
R3#
Observations:
· MED is advertised to Confederation EBGP Peer
· Local Preference also advertised to Confederation EBGP Peer
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