BGP MED Attribute and IBGP

omago · Mon Mar 11, 2013 2:00 pm

I am a little bit confused with Mikrotik's BGP MED attribute implementation.
Two observations from the LAB (tested both v5.24 and v6.0rc11):

1) MED works ok for EBGP sessions. But when propagating external routes to internal BGP (IBGP) neighbors, MED attributes are deleted.

2) When a Mikrotik router receives the same route from an EBGP neighbor and from an IBGP neighbor ( Cisco, by example, propagates MED attr. over IBGP sessions), MED values are not evaluated as part of the BGP best route selection process.

Are these observations correct ?
Is this the way MED is implemented in RouterOS's BGP ?

mrz · Mon Mar 11, 2013 2:08 pm

From the manual:
http://wiki.mikrotik.com/wiki/Manual:BG ... _Algorithm

The router compare MED attribute only for paths that have the same neighboring (leftmost) AS. Paths without explicit MED value are treated as with MED of 0

omago · Mon Mar 11, 2013 5:36 pm

Thank you for your quick response but let me explain my question with more detail.
Imagine this scenario with 2 routers (R1 & R2) in AS65001 and another 2 (R3 & R4) in AS65002:

R1[AS65001] ------- R3[AS65002]------R4[AS65002]---------R2[AS65001]

R1 announces a route to 10.10.12.0/24 to R3 with MED=101
R2 announces same route to R4 with MED=102

1) If R4 is a Mikrotik router it receives the route correctly form R2 with MED=102
but when propagating it to it's IBGP peer R3 it sends it with MED=0

[admin@R4] > /routing bgp advertisements print R3 detail 

 peer="R3" prefix=10.10.12.0/24 nexthop=10.10.34.4 as-path="65001" 
   origin=incomplete local-pref=100

2) And if R3 is a router that propagates MED to IBGP peers (as Cisco or Juniper do) at R4 we have 2 routes to 10.10.12.0/24
both with the same:

- Weight
- Local Pref
- AS Path
- Origin

 
[admin@R4] > /ip route print detail 
Flags: X - disabled, A - active, D - dynamic, 
C - connect, S - static, r - rip, b - bgp, o - ospf, m - mme, 
B - blackhole, U - unreachable, P - prohibit 
0 ADb  dst-address=10.10.12.0/24 gateway=10.10.24.2 
        gateway-status=10.10.24.2 reachable via  ether2 distance=20 scope=40 
        target-scope=10 bgp-as-path="65001" bgp-med=102 bgp-origin=incomplete 
        received-from=R2 

 1  Db  dst-address=10.10.12.0/24 gateway=10.10.34.3 
        gateway-status=10.10.34.3 reachable via  ether3 distance=200 scope=40 
        target-scope=30 bgp-as-path="65001" bgp-local-pref=100 bgp-med=101 
        bgp-origin=incomplete received-from=R3

Then, according to http://wiki.mikrotik.com/wiki/Manual:BG ... _Algorithm, MED should be compared and route 1 with MED=101 selected as best. But our LAB shows thar route 0 with MED=102 is the active route.

From 1) and 2) it looks like MED in Mikrotik is used only in EBGP: It is not propagated to IBGP peers and it's value is ignored when a route from an IBGP peer is one of the candidates.

I'd like to know if this is a design decision in RouterOS or a bug.

mrz · Mon Mar 11, 2013 5:43 pm

MED will not be used for routes received from iBGP peer.

omago · Mon Mar 11, 2013 5:53 pm

OK. Not a bug then.

RFC4451 writes only a "MAY" on that but it will be great if MED could be propagated and used also in iBGP.

Thank you.
Best Regards.

BGP MED Attribute and IBGP

BGP MED Attribute and IBGP

Re: BGP MED Attribute and IBGP

Re: BGP MED Attribute and IBGP

Re: BGP MED Attribute and IBGP

Re: BGP MED Attribute and IBGP