BGP Path selection

[4everlearning]

Hi Guys,

I have a 1G link between my CE and MPLS PE running eBGP. I will add another 10G link as a preparation to offload the 1G link in the future. 

Now the new link will share the same subnet range with the old link since the old link will be migrated to the new one later. Also both the 1G and 10G links will be binded to the same VRF. 

If I bring  up the new eBGP connection for the new 10G between the CE and PE without advertising anything from the 10G link (advertisement will remain only from the existing 1G for now) what will be the behaviour from PE side ? will the PE keep sending everything to the !G link? or would it send to both links the 1G and 10G? or would it prefer the 10G? 

Thanks,

[rais · Accepted solution]

Once the eBGP connection has been brought up, the default behavior is to send all BGP routes out. An export policy will be needed to not allow any updates sent out the new link.

HTH.

[4everlearning]

Thanks Rais, 

So if no export policy was applied, the BGP advertisement from PE will be sent to both links. In this case what would be the behaviour? would the traffic be load blancing on both or would the CE prefer the 10G link and just send traffic to PEs through the 10G link? 

Thanks,

[rais · Accepted solution]

That's correct. Once the advertisement is on both links it will follow the normal BGP path selection. Most likely in your case the lowest interface address of the two links would win and become the active path for the data.  BGP is oblivious of bandwidth in this case.

Thanks.