Analyzing the Forwarding Path

Analyzing the forwarding path of a network device is an important part of network troubleshooting. The forwarding path is the sequence of operations a device performs on a packet when it receives it on an input interface and before it sends it out on an output interface. This sequence of operations may include the following:

Input processing: The device performs checks to verify that the packet is valid and conforms to the protocol standards. The packet is checked for errors, and its header fields are parsed to determine the packet’s destination address.

Forwarding decision: Based on the destination address, the device makes a decision on where to send the packet. This could involve a lookup in a routing table or a lookup in a forwarding table (for switches).

Switching: The device performs switching operations based on the forwarding decision. This could involve updating the destination MAC address in the Ethernet header, adding or removing VLAN tags, or performing other operations as needed.

Output processing: The packet is prepared for transmission on the output interface. This could involve adding a checksum or other control information to the packet, as well as performing any necessary buffering or queuing operations.

Transmission: The packet is transmitted on the output interface.

When troubleshooting network problems, it is important to understand the forwarding path and where packets may be dropping or getting delayed. Tools such as packet capture and analysis can help identify where packets are getting lost or delayed in the forwarding path. Additionally, analyzing the device’s forwarding tables or routing tables can help identify misconfigurations or routing problems that could be impacting packet forwarding.

The switch forwarding logic can be summarized as

Determine the VLAN in which the frame should be forwarded. On access interfaces, this is based on the access VLAN associated with the incoming interface.
Look for the frame’s destination MAC address in the MAC address table, but only for entries in the VLAN identified in step 1. If the destination MAC is
- Found (unicast), forward the frame out the only interface listed in the matched address table entry.
- Not found (unicast), flood the frame out all other access ports (except the incoming port) in that same VLAN.
- Broadcast or multicast, flood the frame out all other access ports (except the incoming port) in that same VLAN.