Cisco Systems OL-4580-01 manual Configuring IP Unconditional Bridging, 6-10

Chapter 6 Configuring the Internet Protocol

Configuring IP Unconditional Bridging

Caution Enabling the CSS to forward the subnet broadcast can make the subnet susceptible to “smurf” attacks; an attacker sends an ICMP echo request frame using a subnet broadcast address as a destination and a forged address as the source.

If a “smurf” attack is successful, all the destination subnet hosts reply to the echo and flood the path back to the source. By disabling subnet broadcast forwarding, the original echo never reaches the hosts.

Configuring IP Unconditional Bridging

By default, the routing table lookup of a destination path by the CSS on received packets overrides bridging decisions to be made for those packets. If the routing table specifies that the CSS use a different physical Ethernet port than what is specified for port bridging, the CSS ignores the bridging decision. If you have a network that you want to bridge through the CSS to an upstream router, you may want to force the CSS to make a bridging decision on the received packets instead of making a routing table decision.

Use the ip uncond-bridgingglobal configuration command to always make a bridging decision on the received packets. With this command, the bridging decision always takes precedence over a routing table decision.

For example:

(config)# ip uncond-bridging

To restore the default behavior of the CSS, enter:

(config)# no ip uncond-bridging

Configuring IP Opportunistic Layer 3 Forwarding

The CSS opportunistic Layer 3 forwarding feature allows the CSS to reduce the number of network device hops for certain packets or flows. The CSS forwards packets at Layer 3 if the destination MAC address in the Ethernet header is the CSS MAC address. Use the ip opportunistic command to enable opportunistic Layer 3 forwarding and allow the CSS to make Layer 3 forwarding decisions even if the Layer 2 packet destination MAC address does not belong to the CSS.

	Cisco Content Services Switch Routing and Bridging Configuration Guide
6-10	OL-4580-01