In BGP there are two loop prevention mechanism:

• for EBGP there is AS-Path attribute which states that router will drop BGP advertisement when it sees it own AS number in AS path attribute
• for IBGP there is split horizon rule which states that update sent by one IBGP neighbor should be not send to another IBGP neighbor

BGP Split horizon rule

The split horizon rule is there for loop prevention. Within an AS the AS-PATH number doesnt change. So if you got looped topology you may create a loop without this rule. The real problem arises if the originator router delete the bgp advertisement, but because of absance of control mechanism in BGP the router can be fooled by a copy of advertisement received in same time from another node.

We can prevent this behavior by two ways:

• create full mesh bgp neighborship between nodes
• because full bgp mesh is quite annoying to do there is another way called route reflectors.