AS PATH Length

Purpose

The AS PATH in the DFZ can become very long. At some point this can become an issue (with some Vendors/OSes). It should be considered to limit the maximum length of the AS PATH.

Description

Configuration

Cisco IOSCisco IOS XRJuniper JunosBird2

router bgp 64500
 bgp maxas-limit 50

route-policy AS-PATH-WAY-TOO-LONG
 if as-path length ge 50 then
  drop
 endif
end-policy
router bgp 64500
 neighor 198.51.100.1
  address-family ipv4 unicast
   route-policy AS-PATH-WAY-TOO-LONG in
 neighor 2001:db8::1
  address-family ipv6 unicast
   route-policy AS-PATH-WAY-TOO-LONG in
  exit
 exit
exit

[edit policy-options]
    as-path AS-PATH-MAX-LENGTH ".{50,}";

[edit policy-options policy-statement 4-BASE-IN]
    term AS-PATH-WAY-TOO-LONG {
      from {
        as-path AS-PATH-MAX-LENGTH;
      } 
      then reject;
    }

function reject_long_aspaths()
{
  if ( bgp_path.len > 50 ) then {
    # optional logging:
    # print "Reject: Too long AS path: ", net, " ", bgp_path;
    reject;
  }
}
filter import_all {
  reject_long_aspaths();
  ...
  accept;
}