Default Route

Purpose

Per definition a default route means "route all packets this way unless there is a specific route for the destination IP address". A default route is expressed in IPv4 as 0.0.0.0/0 and as ::/0 in IPv6.

It is completely legitimate if a transit provider announces default routes to their customers. In case the customers router has limited memory for the routing table, it can simply only accept the default route, blocking all other more specific routes.

On the other hand, if you want the full routing table, you should not accept any default route from your transit providers. You must never accept a default route from any non-transit-provider neighbor.

Configuration

Cisco IOSBird2

ip prefix-list default-route-v4 permit 0.0.0.0/0
ipv6 prefix-list default-route-v6 permit ::/0

Route-Map statement for allowing only default routes in:

route-map prefixes-in permit 10
  match ip address prefix-list default-route-v4
  match ipv6 address prefix-list default-route-v6

Route-Map statement for not allowing default routes in: ``` route-map prefixes-in deny 10 match ip address prefix-list default-route-v4 match ipv6 address prefix-list default-route-v6 ````

function reject_default_route4()
{
  if net = 0.0.0.0/0 then {
    # optional logging:
    # print "Reject: Defaultroute: ", net, " ", bgp_path;
    reject;
  }
}
function reject_default_route6()
{
  if net = ::/0 then {
    # optional logging:
    # print "Reject: Defaultroute: ", net, " ", bgp_path;
    reject;
  }
}
filter import_ipv4 {
  reject_default_route4();
  ...
  accept;
}
filter import_ipv6 {
  reject_default_route6();
  ...
  accept;
}