bgp MED non-transitive

[sarahr202]

Hi everybody.

i just want to confirm my understanding of " non transitive "

When we say bgp 's Med being non-transitive we  are telling the router hey router if you understands this bgp attribute MED , then pass it to your peers( which includes ibgp peers  and ebgp peers in the neighboring AS) . However if you do not understand it please drop it.

Is my understanding correct ?

thanks and have a great day.

[Reza Sharifi · Accepted solution]

Hi Sarah,

Non-transitive means when you learn a med from you neighbor, you will not pass it to another as.

If you are as 2 and you are learning a med from as 1 you will not pass the same med to as 3.

HTH

[Peter Paluch · Accepted solution]

Hello Reza,

Hmmm, are you sure about that? I believe that the transitivity is on a per-peer basis, not on a per-AS. The RFC 4271, Section 5, states:

                                            Unrecognized non-transitive
   optional attributes MUST be quietly ignored and not passed along to
   other BGP peers.

I would therefore say that Sarah's original understading is mostly correct - with the correction that a path with the MED attribute learned by eBGP may be advertised to other iBGP peers including this MED attribute, however, a MED value received from another peering AS must not be propagated to another AS (RFC 4271 Section 5.1.4).

Best regards,

Peter

[Reza Sharifi · Accepted solution]

Hi Peter,

That what I meant to say, but I guess, I didn't mention that when a med is learned from an EBGP peer can be advertised to IBGP peers but not to other EBGP peers.

Thanks,

Reza

[amjadkhan1920]

Dear Raza/sara,

there is something need to point out below thing must be noted

1  case when R1.R2&R3 in AS 1 advertise 192.168.1.0/24  network to AS2 R4 and R5 if you apply the MED on ebgp router which is R3& R4 so the MED will be propagate to R4 as well as whole AS2 

2  case if you apply the MED with route-map on R1  (note network 192.168.1.0 is here on R1)  to the side of R2 & R3 ibgp the MED will be propagate only IBGP and never propagate to AS2 keep in mind

3 case now if we try to propagate the MED  from R2 to side 3 we  will be fail because the network is learn by IBGP 

[amjadkhan1920]

dears,

ok the MED is local and non-transitive but why med influence in IBGP ?

[amjadkhan1920]

and can we say this MED influence from IBGP to EBGP and voice versa with in there AS but not influence to other AS ? means same like split horizon plz correct me