Content

| 5.2 |  | Cisco Express Forwarding |
| 5.2.6 |  | Packet flow for Layer 2 and Layer 3 forwarding decisions |

This section details the data flow through the Layer 2 and Layer 3 forwarding process on a Catalyst 6000 Supervisor Engine 2. The process is fundamentally unchanged between a Layer 2 and Layer 3 decision. It is a function of whether the Layer 3 data exists. In other words, if the MSFC2 is present, the Layer 3 path is valid. Otherwise, forwarding decisions are made based purely on Layer 2 forwarding information. This discussion details the data path for Layer 2 and Layer 3 lookups.

The CEF forwarding process is detailed in Figure . The steps are as follows.

Step 1
The Layer 3 forwarding engine and Layer 2 forwarding engine ASICs receive the packet headers from the data bus.

Step 2
The following operations occur in parallel:

- The Layer 2 forwarding engine performs a lookup on the destination MAC address.
- The Layer 2 forwarding engine ASIC performs any necessary input security ACL lookups.
- The Layer 2 forwarding engine ASIC performs any necessary input QoS/ACL lookups.
- The Layer 3 forwarding engine ASIC performs a FIB table lookup.
- The Layer 3 forwarding engine ASIC performs a NetFlow table lookup.

Step 3
The following operations occur in parallel:

- The input ACL and QoS results from the Layer 2 forwarding engine ASIC are forwarded to the Layer 3 forwarding engine ASIC.
- The Layer 3 forwarding engine ASIC sends the destination VLAN information for the packet to the Layer 2 forwarding engine ASIC.

Step 4
The following operations occur in parallel:

- The Layer 3 forwarding engine ASIC performs the adjacency lookup.
- The Layer 2 forwarding engine ASIC performs the outbound security ACL lookup.
- The Layer 2 forwarding engine ASIC performs the outbound QoS ACL lookup.

Step 5
The Layer 2 forwarding engine sends the results of the security and QoS ACL lookups to the Layer 3 forwarding engine ASIC.

Step 6
The following operations occur in parallel:

- The Layer 3 forwarding engine ASIC generates the rewrite result and sends it to the Layer 2 forwarding engine ASIC.
- The Layer 3 forwarding engine ASIC updates the adjacency table statistics as necessary.
- The Layer 3 forwarding engine ASIC updates the NetFlow table statistics as necessary.

The Layer 2 forwarding engine looks up the destination MAC address received from the Layer 3 forwarding engine ASIC. It then chooses between a Layer 2 and a Layer 3 result and sends the result onto the results bus, an out-of-band control-plane mechanism used for this purpose.

Figure summarizes the CEF process with a traffic flow from a source workstation (on the right side of the diagram) to a destination workstation (on the left side of the diagram). The critical elements of the diagram are the FIB and adjacency entries of the intermediate routers along the path. All the IP addresses and MAC addresses referenced by the FIB and adjacency tables are indicated as the packet traverses the network.