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Difficulty: MediumL2 Switching Concepts and MAC Table Operation

An engineer connects a new host to interface FastEthernet0/12 on a Cisco Catalyst switch. The host immediately sends a unicast Ethernet frame intended for a server whose MAC address is already dynamically learned on interface FastEthernet0/18. Arrange the operational steps in the exact chronological order that the switch executes upon receiving this frame.

  1. 1The switch receives the incoming Ethernet frame on interface FastEthernet0/12.
  2. 2The switch inspects the frame header to record the source MAC address and map it to interface FastEthernet0/12 in the MAC address table.
  3. 3The switch performs a lookup in the MAC address table using the frame's destination MAC address.
  4. 4The switch matches the destination MAC address to the existing entry for interface FastEthernet0/18.
  5. 5The switch forwards the frame directly out interface FastEthernet0/18 without flooding other active ports.

Answer

The correct operational sequence begins with frame reception on FastEthernet0/12, followed by source MAC address learning, destination MAC lookup in the table, identifying the egress port match on FastEthernet0/18, and finally unicast forwarding out FastEthernet0/18.
Layer 2 switch operation dictates a strict sequence of events upon frame arrival: frame ingress occurs first, followed immediately by source MAC address learning (adding or updating the source MAC and interface association in the MAC address table). Next, the switch performs a table lookup for the destination MAC address, identifies the matching egress interface (FastEthernet0/18), and lastly forwards the unicast frame directly out that specific interface.

Step-by-Step Solution

1
Identify the ingress port event.
The switch receives the raw frame on FastEthernet0/12.
Frame processing cannot occur until the physical interface receives and buffers the frame.
2
Perform ingress source MAC learning.
The switch associates the frame's source MAC address with interface FastEthernet0/12 in its Layer 2 MAC address table.
Cisco switches always process source MAC learning prior to destination forwarding logic.
3
Perform destination MAC table lookup.
The switch checks its MAC address table for the frame's destination address.
The destination MAC determines whether the frame will be unicast forwarded, flooded, or filtered.
4
Evaluate the table lookup result.
An existing mapping points to interface FastEthernet0/18.
Since the server's MAC was previously learned, the destination lookup succeeds.
5
Execute frame forwarding.
The switch transmits the frame solely out interface FastEthernet0/18.
Known unicast frames are forwarded strictly to the matching egress port, preventing unnecessary broadcast domain traffic.

Key Concept

L2 Switch Ingress Processing, MAC Address Learning, and Known Unicast Forwarding
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Topics

L2 Switching Concepts and MAC Table Operation

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