The Cisco perimeter switch must be configured to enforce approved authorizations for controlling the flow of information between interconnected networks in accordance with applicable policy.

Information flow control regulates authorized information to travel within a network and between interconnected networks. Controlling the flow of network traffic is critical so it does not introduce any unacceptable risk to the network infrastructure or data.

An example of a flow control restriction is blocking outside traffic claiming to be from within the organization. For most switches, internal information flow control is a product of system design.

Review the switch configuration to verify that access control lists (ACLs) are configured to allow or deny traffic for specific source and destination addresses as well as ports and protocols. In the example below, ICMP echo and echo-reply packets are allowed for troubleshooting connectivity. WWW traffic is permitted inbound to the NIPRNet host-facing web server (x.12.1.22).

interface GigabitEthernet0/1 description Link to DISN ip address x.12.1.10 255.255.255.0 ip access-group FILTER_PERIMETER in … … … ip access-list extended FILTER_PERIMETER permit tcp any any established permit icmp host x.12.1.9 host x.12.1.10 echo permit icmp host x.12.1.9 host x.12.1.10 echo-reply permit tcp any host x.12.1.22 eq www deny ip any any log-input

If the switch is not configured to enforce approved authorizations for controlling the flow of information between interconnected networks, this is a finding.

Step 1: Configure an ACL to allow or deny traffic as shown in the example below:

R1(config)#ip access-list extended FILTER_PERIMETER R1(config-ext-nacl)#permit tcp any any established R1(config-ext-nacl)#permit tcp host x.12.1.9 host x.12.1.10 eq bgp R1(config-ext-nacl)#permit tcp host x.12.1.9 eq bgp host x.12.1.10 R1(config-ext-nacl)#permit icmp host x.12.1.9 host x.12.1.10 echo R1(config-ext-nacl)#permit icmp host x.12.1.9 host x.12.1.10 echo-reply R1(config-ext-nacl)#permit tcp any host x.12.1.22 eq www R1(config-ext-nacl)#deny ip any any log-input R1(config-ext-nacl)#exit

Step 2: Apply the ACL inbound on all external interfaces.

R2(config)#int g0/0 R1(config-if)#ip access-group FILTER_PERIMETER in R1(config-if)#end