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Difficulty: Very hardL2 Switching Concepts and MAC Table Operation

A Cisco Catalyst switch operating with default MAC address table aging timer settings (300 seconds) has an existing dynamic entry for Host A (MAC: 0010.a412.1111) on interface GigabitEthernet0/1 with a current age of 295 seconds. At time t=0t = 0 seconds, Host B (MAC: 0010.a412.2222) connected to GigabitEthernet0/2 sends a unicast frame destined to Host A. Host A sends no traffic during this period. At time t=10t = 10 seconds, Host B sends a second unicast frame destined to Host A. How does the switch process the frame sent at t=10t = 10 seconds?

  1. The switch floods the frame out all operational ports within the VLAN except GigabitEthernet0/2 because Host A's entry aged out 5 seconds prior.Answer
  2. B
    The switch forwards the frame directly out GigabitEthernet0/1 because the frame sent at t = 0 seconds refreshed Host A's MAC address table aging timer.
  3. C
    The switch drops the frame at t = 10 seconds because the destination MAC address is absent from the MAC table and ARP resolution is incomplete.
  4. D
    The switch forwards the frame out GigabitEthernet0/1 and transmits a LLDP/CDP topology query to verify Host A's connectivity.

Answer

The switch floods the frame out all operational ports within the VLAN except GigabitEthernet0/2 because Host A's entry aged out 5 seconds prior.
Switches update and refresh MAC address table entries strictly based on the Source MAC address of ingress frames. When Host B sends a frame destined for Host A at t = 0 seconds, Host A's entry timer is NOT reset because Host A did not transmit any frame. Consequently, Host A's dynamic entry reaches the 300-second threshold at t = 5 seconds and is removed from the MAC table. When Host B sends another frame at t = 10 seconds, the switch finds no matching destination entry and floods the unknown unicast frame out all active ports in the VLAN except the receiving port (GigabitEthernet0/2).

Step-by-Step Solution

1
Analyze MAC address table aging mechanics for the frame transmitted at t = 0 seconds.
Host B (0010.a412.2222) is learned on GigabitEthernet0/2. The frame is forwarded out GigabitEthernet0/1 using Host A's existing MAC table entry.
MAC address aging timers are refreshed ONLY when a frame is received containing that MAC address as the Source MAC. Destined (destination MAC) lookups do not reset the aging timer.
2
Calculate the expiration time of Host A's MAC address entry.
Host A's entry had an age of 295 seconds at t = 0. With a 300-second default timer and no incoming frames originating from Host A, the entry expires at t = 5 seconds (295 + 5 = 300 seconds).
Host A remained silent, so no frames with Source MAC 0010.a412.1111 entered the switch to reset the timer.
3
Evaluate the switch forwarding logic for the frame sent at t = 10 seconds.
At t = 10 seconds, the destination MAC 0010.a412.1111 is no longer in the MAC address table. The switch performs unknown unicast flooding.
When a destination unicast MAC address is not present in the L2 MAC table, Layer 2 switches flood the frame out all active interfaces in the same VLAN except the ingress port.

Key Concept

L2 MAC Table Aging and Unknown Unicast Flooding Logic
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Topics

L2 Switching Concepts and MAC Table Operation

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