Oeq - Bgp attribute


Q. Which BGP attributes can be used to modified the Path for the incoming traffic; for the Outgoing traffic and for both incoming and outgoing as well

Ans:

Incoming : AS-path, MED, type of origin


Outgoing : Weight, Local Preference

both : BGP Next Hop, BGP community

Any thoughts ????

Comments

  • Incoming - AS-Path

    Outgoing - Weight / LPF

    Both - MED


    Can any one confirm this?

  • ORIGIN wouldn't be deterministic to rely on. Eg.: what if the origin is IGP already?

    I would say:

    Incoming: MED, AS-PATH

    Outgoing: Weight, Local-Preference, MED

    Both: MED

    MED is not commonly used to interfere with outgoing traffic selection, but it is possible.

  • I am not sure how do we use MED to influence outbound traffic...

    could you please explain :)

    For inbound traffic, we can also use route aggregation.

     

  • Dears,

    lets differntiate between the eBGP and iBGP path selection ,since some attributes could be propagated to the eBGP and others not

    Meanwhile,

    1) for the eBGP peering:

    - inbound Direction: AS-Path, MED, Origin and Next hop attribute (using outbound route-map)

    - outbound direction: LP,Weight,MED,AS-path,Origin and next hop attributes (using inbound route map with the eBGP peer)

     

    2) for the iBGP peering:

    - inbound Direction: LP, MED, Origin and Next hop attribute (using outbound route-map with the iBGP peer)

    - outtound direction: LP,Weight,MED,Origin and next hop attributes (using inbound route map with the iBGP peer)

    However, you have to consider too that changing some attributes with the eBGP peer might affect the iBGP peer if it is propagated to it

    example: if you changed the AS-ptah/MED/Origin/LP or next hop for the route recived from the eBGP peer ,hence this change will affect the downstream iBGP routers decision too.

    Thanks and hope this being informative for you,

    Regards,

    Kayoub

    CCIE#27007

  • Hi John

    to answer this question , i think we have to think first of which BGP attributes (from those used for path seection) are be propagated to both internal and external updates

    AS-Path
    origin
    MED

    AS-Path is the prefered attribute to affect incoming traffic esp in internet-wide and is propagated to internal neighbors

    MED is used between adjacent ASs so it can be used to affect incoming traffic between adjacent ASAs and is propagated to internal neighbor as metric  

    origin can be used for both inbound and outbound route selection but it is has limited options (external - internal - incomplete)

    so i think the correct answer will be AS-Path , MED and origin

    I excluded NH because mostly it is changed in AS border and it isn't NH itself that makes a path prefered over other , its the IGP cost to the NH that makes the decision so the correct statement here should "IGP cost for BGP NH " and that isn't BGP attribute

    Thanks
    Hekas

     

     

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