NetworkTalk & BGP

A.5.a) Ebgp loop prevention mechanism based on as-path attribute

A.5.a) Ebgp loop prevention mechanism based on as-path attribute

  1. When an ISP origins or advertises a network to the Internet, its as-number is inserted in the as-path. In some situations, it happens that one of its peers or a customer advertises to him this same network originated from its AS. Therefore, he can receive a prefix in which his as-number is present in the as-path. In this case, the prefix and the BGP update are silently ignored.
    This situation is common when customers and its upstream providers are linked in a way that the global connectivity is fully meshed, closing the loop.
  2. See below Figure A.8: BGP loop prevention mechanism between external AS

     

  3. This mechanism is easy to apprehend, but it can get complicated when it is processing not through your AS, but within your peer’s network. It can create many surprises, as soon as you design your traffic engineering and your matrix case redundancy.
    1. ISP-B proposes the following communities which allow customers to set local preference about their advertised network, within ISP-B’s AS:
      1. 300:70 set local preference 70
      2. 300:80 set local preference 80
      3. 300:90 set local preference 90
      4. local pref for routes learned from peers : 86
    2. ISP-C proposes the following communities which allow customers to set local preference about their advertised network, within ISP-C’s AS:
      1. 400:70 set local preference 70
      2.  local pref for routes learned from peers : 90
    3. ISP-D proposes the following communities which allow customers to set local preference about their advertised network, within ISP-D’s AS:
      1. 500:70 set local preference 70
      2. 500:80 set local preference 80
      3. 500:90 set local preference 90
      4. local pref for routes learned from peers set to 85
  4. We announce prefix-C with the value 500:100, 300:80 and 400:70. Within the bgp table of ISP-D Ebgp speaker, the route for prefix 11.11.229.0/24 is learned by 3 ways:
    • 11.11.229.0/24, localpref 100, as-path: 1000.
      • Best (higher localpref)
    • 11.11.229.0/24, localpref 85, as-path: 300 – 500 – 1000.
      • rejected(as-loop issue)
    • 11.11.229.0/24, localpref 85, as-path: 400 – 500 – 1000.
      • rejected(as-loop issue)

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