Split horizon
Split horizon rule is to prevent routing loop. In iBGP a router which learns prefixes from a BGP speaker will not advertise to BGP speaker, this makes a full mesh iBGP peering necessary in order for other BGP speakers to know the route.
The number of TCP sessions can be calculated with this formula n(n-1)/2 where n is the number of BGP speaker. Let say if there are 10 BGP speakers the number of TCP sessions needed in a full mesh peering is 45.
Without route reflector
Without route reflector these scenarios will occur in ibgp peering:
1. R6 learns the route of R7 and R8. If iBGP peering is not fully meshed, R8 only knows the route advertised by R6 and will not receive route advertised by R7. R7 will not receive route advertised by R8 but only receives route advertised by R6.
2. In order for R7 and R8 to know each other route, in a situation without route reflector, R7 must also peer with R8. R7 and R8 will require to have an additional iBGP tcp session.
Route reflector
In order to reduce the number of TCP sessions as the network grow, route reflector can be used. Route reflector bypass the split horizon rule, route learned from one BGP speaker will be advertised to another BGP speaker, the BGP speakers however will peer to route reflector only.
With route reflector, R7 and R8 only require to peer with R6, where R6 is the route reflector. R7 and R8 only require one iBGP TCP session to R6. R7 and R8 will learn each other routes through R6.
R8-CLIENT#sh ip route bgp
172.31.0.0/24 is subnetted, 4 subnets
B 172.31.7.0 [200/0] via 7.7.7.7, 00:30:52
B 172.31.6.0 [200/0] via 6.6.6.6, 00:30:28
B 172.31.9.0 [20/0] via 9.9.9.9, 00:27:54
10.0.0.0/30 is subnetted, 2 subnets
B 10.1.1.0 [200/0] via 7.7.7.7, 00:27:49
R8-CLIENT#
Line 3 and 7 are routes advertised by R7-CLIENT.
R7-CLIENT#show ip route bgp
172.31.0.0/24 is subnetted, 4 subnets
B 172.31.6.0 [200/0] via 6.6.6.6, 00:32:36
B 172.31.9.0 [20/0] via 9.9.9.9, 00:29:01
B 172.31.8.0 [200/0] via 8.8.8.8, 00:39:23
10.0.0.0/30 is subnetted, 2 subnets
B 10.1.1.4 [200/0] via 8.8.8.8, 00:30:09
R7-CLIENT#
Line 3 and 7 are routes advertised by R8-CLIENT.
Without route reflector R7-CLIENT and R8-CLIENT must peer to each other in order to receive each other’s route.
In BGP the most important thing to remember is the BGP speakers must have reachability to one another.
Entire lab configurations
RR-R6#sh running-config
Building configuration...
Current configuration : 1624 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname RR-R6
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
memory-size iomem 5
ip cef
!
!
!
!
ip auth-proxy max-nodata-conns 3
ip admission max-nodata-conns 3
!
multilink bundle-name authenticated
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
archive
log config
hidekeys
!
!
!
!
!
!
!
!
interface Loopback0
ip address 6.6.6.6 255.255.255.255
ip router isis cyruslab
!
interface Loopback6
ip address 172.31.6.1 255.255.255.0
!
interface FastEthernet0/0
no ip address
shutdown
duplex auto
speed auto
!
interface Serial0/0
bandwidth 2000
ip address 192.168.1.1 255.255.255.252
ip router isis cyruslab
clock rate 2000000
!
interface FastEthernet0/1
no ip address
shutdown
duplex auto
speed auto
!
interface Serial0/1
bandwidth 2000
ip address 192.168.1.9 255.255.255.252
ip router isis cyruslab
clock rate 2000000
!
interface Serial0/2
no ip address
shutdown
clock rate 2000000
!
router isis cyruslab
net 49.0001.0000.0000.0006.00
!
router bgp 65000
no synchronization
bgp log-neighbor-changes
network 172.31.6.0 mask 255.255.255.0
neighbor 7.7.7.7 remote-as 65000
neighbor 7.7.7.7 update-source Loopback0
neighbor 7.7.7.7 route-reflector-client
neighbor 8.8.8.8 remote-as 65000
neighbor 8.8.8.8 update-source Loopback0
neighbor 8.8.8.8 route-reflector-client
no auto-summary
!
ip forward-protocol nd
!
!
ip http server
no ip http secure-server
!
!
!
!
!
!
!
control-plane
!
!
!
!
!
!
!
!
!
!
line con 0
logging synchronous
line aux 0
line vty 0 4
!
!
end
RR-R6#
R7-CLIENT#sh run
Building configuration...
Current configuration : 1756 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname R7-CLIENT
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
memory-size iomem 5
ip cef
!
!
!
!
ip auth-proxy max-nodata-conns 3
ip admission max-nodata-conns 3
!
multilink bundle-name authenticated
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
archive
log config
hidekeys
!
!
!
!
!
!
!
!
interface Loopback0
ip address 7.7.7.7 255.255.255.255
ip router isis cyruslab
!
interface Loopback7
ip address 172.31.7.1 255.255.255.0
!
interface FastEthernet0/0
no ip address
shutdown
duplex auto
speed auto
!
interface Serial0/0
bandwidth 2000
ip address 192.168.1.2 255.255.255.252
ip router isis cyruslab
clock rate 2000000
!
interface FastEthernet0/1
no ip address
shutdown
duplex auto
speed auto
!
interface Serial0/1
bandwidth 2000
ip address 192.168.1.5 255.255.255.252
ip router isis cyruslab
clock rate 2000000
!
interface Serial0/2
bandwidth 2000
ip address 10.1.1.1 255.255.255.252
clock rate 2000000
!
router isis cyruslab
net 49.0001.0000.0000.0007.00
is-type level-1
!
router bgp 65000
no synchronization
bgp log-neighbor-changes
network 10.1.1.0 mask 255.255.255.252
network 172.31.7.0 mask 255.255.255.0
neighbor 6.6.6.6 remote-as 65000
neighbor 6.6.6.6 update-source Loopback0
neighbor 6.6.6.6 next-hop-self
neighbor 9.9.9.9 remote-as 65100
neighbor 9.9.9.9 ebgp-multihop 2
neighbor 9.9.9.9 update-source Loopback0
no auto-summary
!
ip forward-protocol nd
ip route 9.9.9.9 255.255.255.255 10.1.1.2
!
!
ip http server
no ip http secure-server
!
!
!
!
!
!
!
control-plane
!
!
!
!
!
!
!
!
!
!
line con 0
exec-timeout 0 0
logging synchronous
line aux 0
line vty 0 4
!
!
end
R7-CLIENT#
R8-CLIENT#sh run
Building configuration...
Current configuration : 1757 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname R8-CLIENT
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
memory-size iomem 5
ip cef
!
!
!
!
ip auth-proxy max-nodata-conns 3
ip admission max-nodata-conns 3
!
multilink bundle-name authenticated
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
archive
log config
hidekeys
!
!
!
!
!
!
!
!
interface Loopback0
ip address 8.8.8.8 255.255.255.255
ip router isis cyruslab
!
interface Loopback8
ip address 172.31.8.1 255.255.255.0
!
interface FastEthernet0/0
no ip address
shutdown
duplex auto
speed auto
!
interface Serial0/0
bandwidth 2000
ip address 192.168.1.10 255.255.255.252
ip router isis cyruslab
clock rate 2000000
!
interface FastEthernet0/1
no ip address
shutdown
duplex auto
speed auto
!
interface Serial0/1
bandwidth 2000
ip address 192.168.1.6 255.255.255.252
ip router isis cyruslab
clock rate 2000000
!
interface Serial0/2
bandwidth 2000
ip address 10.1.1.5 255.255.255.252
clock rate 2000000
!
router isis cyruslab
net 49.0001.0000.0000.0008.00
is-type level-1
!
router bgp 65000
no synchronization
bgp log-neighbor-changes
network 10.1.1.4 mask 255.255.255.252
network 172.31.8.0 mask 255.255.255.0
neighbor 6.6.6.6 remote-as 65000
neighbor 6.6.6.6 update-source Loopback0
neighbor 6.6.6.6 next-hop-self
neighbor 9.9.9.9 remote-as 65100
neighbor 9.9.9.9 ebgp-multihop 2
neighbor 9.9.9.9 update-source Loopback0
no auto-summary
!
ip forward-protocol nd
ip route 9.9.9.9 255.255.255.255 10.1.1.6
!
!
ip http server
no ip http secure-server
!
!
!
!
!
!
!
control-plane
!
!
!
!
!
!
!
!
!
!
line con 0
exec-timeout 0 0
logging synchronous
line aux 0
line vty 0 4
!
!
end
R8-CLIENT#
EXTERNAL-R9#sh run
Building configuration...
Current configuration : 1567 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname EXTERNAL-R9
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
memory-size iomem 5
ip cef
!
!
!
!
ip auth-proxy max-nodata-conns 3
ip admission max-nodata-conns 3
!
multilink bundle-name authenticated
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
!
archive
log config
hidekeys
!
!
!
!
!
!
!
!
interface Loopback0
ip address 9.9.9.9 255.255.255.255
!
interface Loopback9
ip address 172.31.9.1 255.255.255.0
!
interface FastEthernet0/0
no ip address
shutdown
duplex auto
speed auto
!
interface Serial0/0
bandwidth 2000
ip address 10.1.1.2 255.255.255.252
clock rate 2000000
!
interface FastEthernet0/1
no ip address
shutdown
duplex auto
speed auto
!
interface Serial0/1
bandwidth 2000
ip address 10.1.1.6 255.255.255.252
clock rate 2000000
!
interface Serial0/2
no ip address
shutdown
clock rate 2000000
!
router bgp 65100
no synchronization
bgp log-neighbor-changes
network 172.31.9.0 mask 255.255.255.0
neighbor 7.7.7.7 remote-as 65000
neighbor 7.7.7.7 ebgp-multihop 2
neighbor 7.7.7.7 update-source Loopback0
neighbor 8.8.8.8 remote-as 65000
neighbor 8.8.8.8 ebgp-multihop 2
neighbor 8.8.8.8 update-source Loopback0
no auto-summary
!
ip forward-protocol nd
ip route 0.0.0.0 0.0.0.0 10.1.1.5
ip route 0.0.0.0 0.0.0.0 10.1.1.1
!
!
ip http server
no ip http secure-server
!
!
!
!
!
!
!
control-plane
!
!
!
!
!
!
!
!
!
!
line con 0
exec-timeout 0 0
logging synchronous
line aux 0
line vty 0 4
!
!
end
EXTERNAL-R9#
cyruslab 