Cisco Systems OL-4580-01 manual Example of Opportunistic Layer 3 Forwarding

Chapter 6 Configuring the Internet Protocol

Configuring IP Opportunistic Layer 3 Forwarding

For example, Figure 6-1shows a CSS connected to VLAN1 and VLAN2. Each VLAN has an end station and an uplink to Router1. End stations A and B both point to Router1 as their default router. When End Station A transmits a packet to End Station B, it uses its default route to Router1. The packet contains Router1’s destination MAC address. A traditional Layer 2 device forwards the packet to Router1, and Router1 forwards the packet to End Station B on VLAN2.

Using opportunistic Layer 3 forwarding, the CSS inspects the IP packet header to determine the destination IP address. Instead of forwarding the packet to Router1, the CSS forwards the packet directly to End Station B. Because the CSS handles the packet only once, the router and uplink are not used and network resources are conserved.

Figure 6-1 Example of Opportunistic Layer 3 Forwarding

Internet

Router1

(default)

Subnet

CSS

VLAN1

End Station A

VLAN2

End Station B

49383

The options for this global configuration mode command are as follows:

•local (default) - Applies opportunistic Layer 3 forwarding if the destination IP address belongs to a node that resides on one of the subnets directly attached to the CSS and the CSS is aware of an ARP resolution for that node. Because the local option is the default, use the no ip opportunistic command to reconfigure IP opportunistic Layer 3 forwarding to the local setting.

•all - Applies opportunistic Layer 3 forwarding if the destination IP address matches any entry in the CSS routing table. We do not recommend this option if the topology includes multiple routers and the CSS does not know all of the routes the routers are aware of.

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