BGP basic doubt

[Arjun Dabol]

Hi experts

i am confused with 3 topics :

1. BGP Split horizon

2. BGP Syncronization

3. BGP loop prevention

they all are inter related ? can someone explain plz ?

[Georg Pauwen]

Hello,

the iBGP split-horizon rule - an iBGP router will not advertise iBGP routes to other iBGP neighbors - actually is designed for loop prevention.

The synchronization rule is meant to prevent blackholing. If an AS passes traffic from another AS to a third AS, BGP should not advertise a route before all routers in your AS learn about the route via IGP. BGP waits until IGP propagates the route within the AS and then advertises it to external peers.

So all three are inter-related in the sense that all are meant to ensure proper route propagation and path selection...

[Arjun Dabol]

Thanks a lot...I read that synchronization rule is no more required or new IOS do not need this..why is that ?

[Georg Pauwen]

Hello,

synchronization (or not) depends on the setup of your network. To start out with, it is only relevant if your AS is a transit AS. If you still have non-iBGP routers in your AS, you should still enable synchronization.

[cofee]

Wanted to add this in addition to Georg's comments.

Paths from an external BGP (eBGP) neighbor if the local autonomous system (AS) appears in the AS_PATH ------ Such paths are denied upon ingress into the router and are not even installed in the BGP Routing Information Base (RIB).

* Above method is used by ebgp peers for loop prevention/split horizon. 

[Jon Marshall]

Just to add that BGP loop prevention is implemented using the AS path in the BGP route advertisements.

When an EBGP speaking router advertises routes to an EBGP peer it adds it's own AS to the path. If it receives a route with it's own AS in the path then it will drop it because this could be a potential routing loop.

You can override this behaviour in certain circumstances if you need to.

This is also why you have the IBGP rule Georg outlined, because when IBGP routers advertise routes to IBGP peers they do not modify the AS path so they have no way of telling if there is a loop in the network.

Jon