Information
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 switches 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 switch will establish a state that will permit the return of this undesirable traffic inbound.
Solution
Step 1: Configure an inbound ACL to deny all other traffic by default as shown in the example below:
SW1(config)#ip access-list extended EXTERNAL_ACL
SW1(config-ext-nacl)#permit tcp any any established
SW1(config-ext-nacl)#permit icmp host x.11.1.1 host x.11.1.2 echo
SW1(config-ext-nacl)#permit icmp host x.11.1.1 host x.11.1.2 echo-reply
...
...
...
SW1(config-ext-nacl)#deny ip any any log-input
Step 2: Apply the ingress filter to all external interfaces.
SW1(config)#int g0/2
SW1(config-if)#ip access-group EXTERNAL_ACL in