Communities-2

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.