IBGP Configuration

R1(config)#router bgp 500

R1(config-router)#neighbor 1.1.1.2 remote-as 500

R1(config-router)#neighbor 4.4.4.2 remote-as 500

R1(config-router)#network 10.0.0.0

R1(config-router)#network 1.0.0.0

R1(config-router)#network 4.0.0.0

R1(config-router)#no auto-summary

R1(config-router)#no synchronization

R1(config-router)#exit

R1(config)#

Note: To advertise with default mask value e.g: 10.0.0.0/8 no need to use the mask

Similarly, for R2, R3, and R4 you have to configure.

BGP network Command:

Router(config-router)# network 192.168.1.1 mask 255.255.255.0

Rl#sh ip bgp summary

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

1.1.1.2 4 500 5 5 9 0 0 00:01:40 3

4.4.4.1 4 500 5 5 9 0 0 00:01:17 3

R1#show ip route bgp

B 2.0.0.0/8 [200/0] via 1.1.1.2, 00:03:00

B 3.0.0.0/8 [200/0] via 4.4.4.1, 00:02:46

B 20.0.0.0/8 [200/0] via 1.1.1.2, 00:03:00

B 40.0.0.0/8 [200/0] via 4.4.4.1, 00:02:46

No network “30.0.0.0” in the routing table

R2#sh ip route bgp

B 3.0.0.0/8 [200/0] via 2.2.2.2, 00:03:46

B 4.0.0.0/8 [200/0] via 1.1.1.1, 00:03:51

B 10.0.0.0/8 [200/0] via 1.1.1.1, 00:03:51

11.0.0.0/24 is subnetted, 1 subnets

B 11.0.0.0 [200/0] via 1.1.1.1, 00:03:51

B 30.0.0.0/8 [200/0] via 2.2.2.2, 00:03:46

Here there is no network 40.0.0.0 in the routing table because of BGP SPLIT HORIZON RULE.

BGP Split horizon rules:

An update sent by one IBGP neighbor should not be sent back to another IBGP neighbor.
It is crucial to deploy a BGP split horizon to mitigate the occurrence of routing loops within an autonomous system (AS). In an autonomous system (AS), it is crucial to establish full-mesh IBGP peering among all routers. This ensures the accurate dissemination of BGP route information to all routers within the AS.

Why have these restrictions?

Solution for BGP Split horizon rules:

Route Reflectors and Confederations: To enhance the flexibility of Split Horizon rules, it is recommended to deploy BGP route reflectors or BGP confederations. This feature enables efficient distribution of routes within the Autonomous System (AS), even in cases where the routes have been learned from peers within the AS. However, it is imperative to meticulously configure these mechanisms to prevent any unintended routing loops.
Full mesh neighborship: Every router should be a neighbor of every other router within the AS

IBGP Peering using loopbacks Interfaces