Logical Packet Flow for a Multilayer Switch

Layer 3 forwarding is based on the destination IP address. Layer 3 forwarding occurs when a packet is routed from a source in one subnet to a destination in another subnet. When a multilayer switch sees its own MAC address in the Layer 2 header, it recognizes that the packet is either destined for itself or has been sent to the default gateway. If the packet is not destined for the multilayer switch, then the destination IP address is compared against the Layer 3 forwarding table for the longest match. In addition, any router ACL checks are performed.

4-16 Building Cisco Multilayer Switched Networks (BCMSN) v3.0 © 2006 Cisco Systems, Inc.

How a Layer 3 Switch Forwards Packets

The table describes how a Layer 3 switch forwards packets.

Step

Action

1.

The Layer 2 engine receives a frame.

2.

The Layer 2 engine performs the input ACL lookup.

3.

The Layer 2 lookup engine recognizes the MAC address of the multilayer switch and therefore determines that the packet is to be switched at Layer 3.

4.

If necessary, an input router ACL check is performed.

5.

The destination IP address is compared against the Layer 3 forwarding table for the longest match.

6.

If necessary, an output router ACL check is performed.

7.

The Layer 2 forwarding engine performs the outbound QoS lookup.

8.

The Layer 2 and Layer 3 header are rewritten.

9.

The Layer 2 forwarding engine forwards the packet.

© 2006 Cisco Systems, Inc. Implementing Inter-VLAN Routing 4-17

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