Quick Study Notes: BGP network statement, AS numbers, path attributes

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More on BGP next hop: If route recursion toward the next hop of a BGP prefix is not successful, the route cannot be considered for best path selection, which also implies that it cannot be installed in the routing table or advertised to any other BGP peer. Example: You will see 5.5.5.5 (inaccessible) and Not advertised to any peer in the show ip bgp <prefix> command output.


BGP network statement:
  • enables BGP on an interface and advertises the connected network
  • controls the BGP networks to originate from this router
  • must be an exact match for a route to be installed in the BGP table


BGP timers (default):
  • keepalive: frequency with which KEEPALIVE messages are sent to the peer (60 seconds)
  • holdtime: interval after not receiving a KEEPALIVE message that the peer is declared dead (180 seconds)
  • advertisement interval: a minimum delay between BGP routing updates (30 seconds for eBGP, 0 seconds fro iBGP)
  • scan timer: periodic measure for next hop validation (60 seconds)


BGP AS numbers (2-byte):
  • 0: reserved
  • 1-64495: public AS numbers (special 23456/RFC6793)
  • 64496-64511: reserved for use in documentation/sample code
  • 64512-65534: private AS numbers
  • 65535: reserved


BGP AS numbes (4-byte):
  • 65536-65551: reserved for use in documentation and sample code
  • 65552-131071: reserved
  • 131072-4199999999: public AS numbers
  • 4200000000-4294967294: private AS numbers
  • 4294967295: reserved


BGP AS numbers (4-byte) expressed in asdot notation:
  • 1.0-1.15: reserved for use in documentation and sample code
  • 1.16-1.65535: reserved
  • 2.0-64086.55903: public AS numbers
  • 64086.55904-65535.65534: private AS numbers
  • 65535.65535: reserved


BGP path attributes:
  • well-known mandatory: must be recognized by all BGP implementations and must be present in an UPDATE
  • well-known discretionatry: must be recognized by all BGP implementations but need not be present in an UPDATE
  • optional transitive: might or might not be recognized by BGP implementations but is passed  on to other BGP routers
  • optional non-transitive: might or might not be recognized by BGP implementations and is not passed on to other BGP routers


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