BGP PATH SELECTION

[gauravprakash]

Hi,

In the decision making of BGP Best Path Selection; one is <<Perfer the path with lowest IGP metric to the BGP next-hop>>. Say one router (A) learns two routes from IBGP neibor ( B & C), and A_B has rip between them and A_C has OSPF between them. How will the BGP speaking router (A) decide which metric is lower to the BGP next hop.

Regrds,

Gaurav

[sundar.palaniappan]

Hi,

In your scenario, Router A would use the OSPF route to get to the BGP next hop as it has a lower admin distance than RIP. The OSPF route is the one that will exist in the routing table of Router A to get to the BGP next hop. Metric comparison will come into play only when both routes belong to the same IGP.

HTH

Sundar

[gauravprakash]

Thx Sundar,

So the neighbor whose next hop is learnt via OSPF will be preferref over that of rip ..thats due to Admin Distance..Right ?

Regards,

G

[sundar.palaniappan]

Your understanding is correct!!

[gauravprakash]

Thx, just an extension to question ...

Say if A learns the next hop for B & C from different routing protocols RIP & OSPF..then will this step be skipped..

Thx again,

G

[shakeelahmadch]

yes ... IGP AD matters

[gauravprakash]

Hi,

Here is a note from RFC 4271 for breaking ties..

<<>

Can u explain in relation to my case of next hops learnt via RIP & OSPF

Regards,

G