CISC-RT-000240 - The Cisco perimeter router must be configured to deny network traffic by default and allow network traffic by exception - deny rule
A deny-all, permit-by-exception network communications traffic policy ensures that only connections that are essential and approved are allowed. This requirement applies to both inbound and outbound network communications traffic. All inbound and outbound traffic must be denied by default. Firewalls and perimeter routers should only allow traffic through that is explicitly permitted. The initial defense for the internal network is to block any traffic at the perimeter that is attempting to make a connection to a host residing on the internal network. In addition, allowing unknown or undesirable outbound traffic by the firewall or router will establish a state that will permit the return of this undesirable traffic inbound.
This requirement is not applicable for the DODIN Backbone. Step 1: Configure an inbound ACL to deny all other traffic by default as shown in the example below. R1(config)#ip access-list extended EXTERNAL_ACL R1(config-ext-nacl)#permit tcp any any established R1(config-ext-nacl)#permit tcp host x.11.1.1 eq bgp host x.11.1.2 R1(config-ext-nacl)#permit tcp host x.11.1.1 host x.11.1.2 eq bgp R1(config-ext-nacl)#permit icmp host x.11.1.1 host x.11.1.2 echo R1(config-ext-nacl)#permit icmp host x.11.1.1 host x.11.1.2 echo-reply ... ... ... R1(config-ext-nacl)#deny ip any any log-input Step 2: Apply the ingress filter to all external interfaces R1(config)#int g0/2 R1(config-if)#ip access-group EXTERNAL_ACL in