BGP multihoming without as prepend or local preference

[vovochka83]

What will happen to subnet 192.168.1.1/24 if there is not as prepend or local preference for all three routers R1, R2, R3? On all there routers i have advertised the same subnet to ISP A, how the incoming/outgoing path will be choose?

[MUKUL JOSHI]

Hi Chin,

Incoming traffic here depends on the attributes that you have applied in the OUTBOUND direction to these three routers for that given subnet OR the attributes that you have applied in INBOUND direction at other end.

Outgoing traffic depens on the attributes applied in the inbound direction on these three routers OR in the outbound direction at other end.

Now in this scenario Your subnet is advertised to ISP A from all three routers  and policies at ISP end will decide the traffic flow.

HTH

MJ

[Richard Burts · Accepted solution]

If R1 and R2 and R3 each advertise the same subnet to ISP A and do not apply any parameter to make the advertisement more or less attractive then ISP A will receive 3 equivalent advertisements of the network. The BGP process of ISP A will choose one of the advertisements. I am sending a link to an article that describes the BGP path selection process. If you look in this article you will find that the process at ISP A will get down to step 10 and will choose the advertisement that was received first. And if perhaps two of the routers sent the advertisement at the same time then step 11 says that it will choose the advertisement from the peer with the lowest RID.

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml

That should clear up how packets are sent from ISP A to the network. The other question you ask is how is traffic sent from the network to ISP A And the answer to that is that it depends on what the hosts on the network have configured as their default gateway.

HTH

Rick