Remote gateway case

Configuring ACL "101" (example below) and applying it outbound on VLAN 1 in the figure below includes the router gateway (10.0.8.1) needed by devices on other networks. This can prevent the switch from sending ARP and other routing messages to the gateway router to support traffic from authorized remote networks.

In Inadvertently blocking a gateway, this ACE (see data in bold below) denies access to the 10 Net's 10.0.8.1 router gateway needed by the 20 Net (Subnet mask is 255.255.255.0).See: example

Inadvertently blocking a gateway

To avoid inadvertently blocking the remote gateway for authorized traffic from another network (such as the 20 Net in this Example:):

Procedure
  1. Configure an ACE that specifically permits authorized traffic from the remote network.
  2. Configure narrowly defined ACEs to block unwanted IP traffic that would otherwise use the gateway; such ACEs might deny traffic for a particular application, particular hosts, or an entire subnet.
  3. Configure a "permit any" ACE to specifically allow any IP traffic to move through the gateway.

ACE blocking an entire subnet

switch(config)# access-list config

ip access-list extended "101"
  deny ip 0.0.0.0 255.255.255.255 10.0.8.30 0.0.0.255
  permit ip 0.0.0.0 255.255.255.255 0.0.0.00 255.255.255.255
  exit