Dual-home PEs importing their own RT back to CE?

link in SP track but maybe should be here, anyway


is there a benefit and disadvantages to importing its own RT back to CE?

like in dual-home settup with R1 and R2 as CE in AS X;

Ra and Rb as PE in SP area; R1 links with Ra and R2 with Rb.

PE routers Ra and Rb have same RD and exporting RT a:a and importing back RT a:a. Do we really need  import a:a?

What if change RD of Ra and b to be different?


  • MartinlMartinl ✭✭✭

    it seems that number of routes is duplicated on PE. Ra puts Rb routes under its RD.  new routes are exact copy of what was already there but next-hop is via Rb (internal). stll Ra preffers external routes to R1, CE. 

  • JoeMJoeM ✭✭✭

    Hi Martin,

    I am trying to follow what you are seeing.   Output?

    A BGP expert like Peety can give a better answer, and I would love to hear the advantages/disadvantages.   I don't see a problem with the export/imports being the same for CE's within a single customer's network -- especially if there are not a lot of prefixes being traded between sites.

    I have seen customers where the CE's have the same RT export/import.  I have always thought of this as simplicity or just not allocating numerous AS numbers for smaller sites.   Of course  Allow-AS or AS-Override is needed to allow the imports.

  • MartinlMartinl ✭✭✭

    it looks like double redunndancy to me and not needed.  R1 is already backup to R2 and vice versa.  if R1 is importing its own routes back into AS , R2 sees them as duplicates.  and vice versa.

    i will try to lab this

  • JoeMJoeM ✭✭✭

    If you are only dealing with a single CE, I cannot think of a scenario where there is an advantage to importing the same RT (from and to same CE device).  

    But by default, it still should not be an issue, because the CE should not import routes when it sees its own AS number. The commands allow-as or as-override would be explicit commands that would only be used if there were remote sites with the same AS, right?



  • MartinlMartinl ✭✭✭

    yes, as-override is configured nad neighbor SOO X:Y  on provider side must be to avoid loops.

  • Is the CE running Multi VRF CE and learning routes from the other CE via its MPLS connection? If I understand it to be this way, disable the RT values, on Multi VRF CE they aren't needed. Site of Origin should preven the routes from getting propagated. 

    I don't want to start offering add'l feedback since I don't know the design. If it is a single CE connected to 2 PEs, SIte of Origin should be enough. 

    Can you provide background info on this, I'm having a hard time putting the pieces together.

  • MartinlMartinl ✭✭✭

    Top PEs, Ra and PE Rb, have same RDs and exporting Green RT from Customeer X into MPLS VPN core.  I have seen some topologies where those PEs (Ra and Rb) are also importing same Green RTs back to X.  Why would one import its own routes back in?



  • JoeMJoeM ✭✭✭

    Hi Martin,

    By default, they woud not import their own routes back in.   Because the CE's would see their own AS number (loop prevention).

    If allowas-in is used on the CPE's, then the engineer would just need to understand what they are doing. I do not see a benefit or a goal for purposefully accepting already known routes.   Not optimal, but not a huge issue.

    NOTE: my experience with Juniper, is that by default the PE will not advertise the AS to the same AS.   Needs an extra config line to make that happen.

    I do not see AS numbers on this BGP map.  The way it is drawn, I am guessing that it is the same provider, and the CPE's are using same AS# .  Is this correct?



  • MartinlMartinl ✭✭✭

    scenario 1: 

    Different AS for X and Y. but still top PE import and export both Green routes, aka route-target both 1:2

    I know, i do not see reason for it but still some examples are with importing /exporting own routes.

    scenario 2:

    Same AS for X and Y; Here PE routers need to do As-override and add SOO to prevnet loops, correct?  

    Same issue, is there a need for exporting and importing same routes ?  

    maybe redundancy?


  • MartinlMartinl ✭✭✭

    this my old post but I think found answer in CCIE BGP Multipath for MPLS Layer 3 VPN -high from Brian's RS Adv. tech course. Will try to summarize it later and post it here

  • JoeMJoeM ✭✭✭

    We will be waiting. This is a thread from 8 months ago. ;-)

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