NetworkTalk & BGP

A.2.c) Multihoming illustration case about peering agreement between ISPs.

A.2.c) Multihoming illustration case about peering agreement between ISPs.

  1. The following situation will help us to highlight why it is important to understand the peering agreements between ISPs.
    1. If ISP-A announces the prefix-A 11.11.232.0/24:
      1. to ISP-B and tags with nothing (meaning LocalPref for prefix-A within ISP-B’s network has been set to 100, the default value)
      2. and to ISP-D by tagging with BGP communities 500:80
    2. See below Figure A.2: prefix-A 11.11.232.0/24 advertisement with community values 300:100 and 500:80                                     

    3. We can foresee that return traffic for 11.11.232.0/24, will come exclusively through ISP-B.
      1. Within ISP-B’s network, prefix-A will be existent through two BGP routes (at least) : 11.11.232.0/24
        1. (a) as-path: 300 1000 with LocalPref 100
        2. (b) as-path: 300 500 1000 with LocalPref 86
          Since LocalPreference has priority on as-path length, packets will be sent to route (a) which is the best, directly to ISP-A.
      2. Inside ISP-D’s network, prefix-A will be present through two BGP routes (at least) :11.11.232.0/24
        1. (a) as-path: 500 1000 with LocalPref 80
        2. (b) as-path: 500 300 1000 with LocalPref 85
          Since LocalPreference has priority on as-path length, packets destined to prefix-A 11.11.232.0/24 will be sent to route (b), directly to ISP-B. Then ISP-B will send this traffic to ISP-A.
    4. Through a correct use of communities values, traffic is coming exclusively through ISP-B’s link.
      The important fact to notice here is that ISP-D sends the traffic destined to ISP-A, not to ISP-A but to ISP-B. And then ISP-B forwards it to ISP-A. -> Of course the customer pays nothing more to ISP-D, since he does not care of the peering exchange agreement between ISP-B and ISP-D.
    5. See below, Figure A.3: Return traffic path for prefix-A where community values are set to 300:100 and 500:80

       

  2. Now, let’s note that if we announce 11.11.232.0/24 through the following way:
    1. ISP-A annouces prefix-A :
      1. to ISP-B and labels with nothing (meaning LocalPref for prefix A within ISP-B’s AS has been set to 100, the default value)
      2. and to ISP-D by tagging with BGP communities 500:90
        Within ISP-D’s network, the prefix-A will be present through two BGP routes (at least) : 11.11.232.0/24
  1. (a) as-path: 500 1000 with LocalPref 90
  2. (b) as-path: 500 300 1000 with LocalPref 85
  • Consequently, ingress traffic for prefix-A will come back through ISP-D and ISP-B at the same time.The reason for that is because within ISP-D’s network, local-preference attribute is higher this time from the customer than the one received from the peer ISP-B (90>85).
    -> In this case, the customer is a victim of a loss of revenue, since he pays bandwidth supposed to be used on ISP-B link, to ISP-D.
  • See below Figure A.4: Return traffic path for the prefix-A 11.11.232.0/24 where community values are set to 300:100 and 500:90

     

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