Tier 1 ASNs

As a Tier 2/3 Network, you may want to filter out routes that have traversed Tier 1 ASNs at IXPs or from customers, as you probably don't want to provide transit for Tier 1 Networks or accept transit for Tier 1 Networks from your customers.

Caution

You should only apply this filter on your customers and IXP peers, not on your upstream transit providers.

Be aware that you need to manually check the prefix list as you could peer with for instance Microsoft of other parties on the list. So you need to do a quick sanity check on the AS numbers to fit your need.

Bird2

define TRANSIT_ASNS = [
    174,                  # Cogent
    701,                  # UUNET
    1299,                 # Telia
    2914,                 # NTT Ltd.
    3257,                 # GTT Backbone
    3320,                 # Deutsche Telekom AG (DTAG)
    3356,                 # Level3
    3491,                 # PCCW
    4134,                 # Chinanet
    5511,                 # Orange opentransit
    6453,                 # Tata Communications
    6461,                 # Zayo Bandwidth
    6762,                 # Seabone / Telecom Italia
    6830,                 # Liberty Global
    7018                  # AT&T
];

function reject_transit_paths()
int set transit_asns;
{
  transit_asns = TRANSIT_ASNS;
  if (bgp_path ~ transit_asns) then {
    print "Reject: Transit ASNs found on IXP Peering: ", net, " ", bgp_path;
    reject;
  }
}

protocol bgp example_ix_rs {
  ipv4 {
    import filter {
      reject_transit_paths();
      # other filters
      accept;
    };
  }
  ipv6 {
    import filter {
      reject_transit_paths();
      # other filters
      accept;
    };
  }
}