eBGP Loop

Has anyone seen a situation, where a CPE's advertised route to his eBGP PE, get sent straight back down to the CPE thus causing a loop?
I have it where the PE is setting the Local pref for his own AS to 100 for the inbound prefix, then somehow sending that route back down to the CE with the Local Pref intact. The CPE then thinks this 'new' route with a Local pref of 100 is better, thus causing a loop.
 "Allow-as in 1" is turned on as its a MPLS network, but I don't think seeing the echo route with iBGP parameters attached is normal?

Also I labbed this up, on GNS and cannot replicate the route-feedback. Short of reading the RFC does eBGP use a loop prevention mechanism besides AS-Path in order to not advetise a route on a link in which he has just recieved it? My first guess was no, just AS path, but labbing this up, the PE will not advertise it back to the CPE. Anyfeedback appreciated Thanks


  • Could it be that the PE is receiving this same prefix from elsewhere and is thus selecting that as his best-path...then sending that back to the CPE? The way its supposed to work is if your CPE is sending the PE a route, and this PE is selecting that as his best-path, then it should NOT advertise it back to the CPE. 

    Also, local pref should NOT be sent to an eBGP peer. You are saying that this PE (eBGP peer) is actually attaching local-pref on the prefix its "looping back" to you? 


  • It certainly has only one path to the /24 network, which is via the CPE.

    Here is an extract from the PE:

    3600ME-1#sh ip bgp vpnv4 vrf CUSTOMER_CORE neighbors received-routes | i 
     *                         0 64904 64517 34791 ?
    3600ME-1#sh ip bgp vpnv4 vrf CUSTOMER_CORE neighbors advertised-routes | i
    *>                100      0 64904 64517 34791 ?

    R-01#sh ip bgp

    BGP routing table entry for, version 2234498 <--Looping

    Paths: (3 available, best #1, table default)

    Flag: 0x820

      Advertised to update-groups:


      64900 64904 64517 34791, (received & used) from (

          Origin incomplete, localpref 100,
    valid, external, best  
    routing information!

          Community: 24867:102

      64517 34790 from (

          Origin incomplete, localpref 50,
    valid, external

      64517 34790, (received-only) from (

          Origin incomplete, localpref 100,
    valid, external


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