Communities:

  • BGP’s way of using route tags.
  • Used to group prefixes together.
    • Advertisement Policy
    • Filtering policy
    • Best Path Selection policy
  • Optional Transitive
    • Not exchanged by default.
    • Needs the additional neighbor command ‘send-community’
  • Standard
    • 4 byte value.
    • Can be decimal or AA:NN
      • AA:NN is enabled with:
        • ‘ip bgp new-format’
        • ‘ip bgp-community new-format

Well-Known:

  • No-Export
    • Does not advertise to EBGP peers.
  • No-Advertise
    • Does not advertise to ANY peer.
  • Local-AS
    • Does not advertise to Confederation EBGP Peers
      • Sub domains.

Configuration:

  • Setup in route-maps.
  • ‘set community {community-number [additive] [well-known-community] | none’
    • Not additive, needs additive keyword.
  • Can delete community by setting as ‘none’
  • Matching communities
    • List Creation
      • Standard list matches community name or number.
        • ‘ip community-list 1 standard permit no-export’
      • Expanded matches regular expression
        • ‘ip community-list expanded AS100 permit 100:[0-9]+
      • Reference from route-map
        • ‘match community AS100’

Extended Communities:

  • Used for specific applications like MPLS L3/L2VPN, EIGRP Cost.

BGP Cost Community:

  • Custom decision process.
  • Only advertised within AS or confederations.
  • Influences BGP path selection
    • Compares before even Weight.
  • EIGRP PE/CE
    • Uses pre-bestpath cost community to encode composite metric into BGP.
  • GSHUT Community
    • Used in conjunction with Graceful Restart
    • Takes restarting peer out of data-path by modifying local preference.
      • Similar to OSPF Max Metric LSA.

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