This next section walks through the switching process and focuses on order
of operations. The order things happen is extremely important for two
reasons. First, order of events is good test material. Second, understanding
the processing order allows you to evaluate how the various filtering and
forwarding mechanisms interact (examples include error checking, accesslists,
VLAN access-lists, routing, and QoS).
Understanding the Switching Process
Steps involved in layer 2 forwarding are as follows:
�¡ Input
1. Receive frame.
2. Verify frame integrity.
3. Apply inbound VLAN ACL (Virtual Local Area Network Access
List).
4. Look up destination MAC (Media Address Code).
■ Output
1. Apply outbound VLAN ACL.
2. Apply outbound QoS ACL.
3. Select output port.
4. Queue on port.
5. Rewrite.
6. Forward.
Steps involved in layer 3 forwarding are as follows:
■ Input
1. Receive frame.
2. Verify frame integrity.
3. Apply inbound VLAN ACL.
4. Look up destination MAC.
■ Routing
1. Input ACL.
2. Switch if entry cached.
3. Identify exit interface and next-hop address using routing table.
4. Output ACL.
■ Output
1. Apply outbound VLAN ACL.
2. Apply outbound QoS ACL.
3. Select output port.
4. Queue on port.
5. Rewrite source and destination MAC, IP checksum and frame
check sequence, and decrement TTL (Time to Live field in the IP
header).
6. Forward.