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Difficulty: MediumL2 Switching Concepts and MAC Table Operation

A Cisco Catalyst switch receives an Ethernet frame on interface FastEthernet0/10. The frame has a source MAC address of 0011.2233.4455 and a destination MAC address of 00aa.bbcc.dd88. The switch MAC address table already contains an active dynamic entry for 0011.2233.4455 mapped to interface FastEthernet0/10, but contains no entry for 00aa.bbcc.dd88. Interfaces FastEthernet0/10 through FastEthernet0/15 are active access ports assigned to VLAN 10, while interface FastEthernet0/16 is an access port assigned to VLAN 20. Which action does the switch perform upon receiving this frame?

  1. Refreshes the aging timer for MAC 0011.2233.4455 on FastEthernet0/10 and floods the frame out interfaces FastEthernet0/11 through FastEthernet0/15.Answer
  2. B
    Drops the frame immediately because the destination MAC address is missing from the MAC address table.
  3. C
    Floods the frame out all active switch interfaces across all VLANs, including FastEthernet0/10 and FastEthernet0/16.
  4. D
    Forwards the frame directly to the default gateway router to resolve the destination MAC address.

Answer

The switch refreshes the aging timer for the existing source MAC entry on interface FastEthernet0/10 and floods the frame out all other active interfaces in VLAN 10 (FastEthernet0/11 through FastEthernet0/15).
When a switch receives a frame, it inspects the source MAC address to update or refresh its MAC address table entry. Next, it looks up the destination MAC address. Because the destination MAC is not in the table, the switch treats the frame as an unknown unicast frame and floods it out all operational interfaces in the same VLAN (VLAN 10), except for the receiving interface (FastEthernet0/10).

Step-by-Step Solution

1
Examine the source MAC address and update the MAC address table.
The source MAC address 0011.2233.4455 is already mapped to interface FastEthernet0/10. The switch resets/refreshes the aging timer for this entry.
MAC learning occurs on every incoming frame. If the entry exists on the correct port, its aging timer is reset.
2
Examine the destination MAC address to determine the forwarding decision.
The destination MAC address 00aa.bbcc.dd88 is not present in the MAC address table (unknown unicast).
Switches use MAC table lookup for unicast forwarding. Absence of an entry triggers unknown unicast flooding.
3
Identify the target egress interfaces for flooding.
The frame is flooded out FastEthernet0/11 through FastEthernet0/15. It is excluded from FastEthernet0/10 (ingress port) and FastEthernet0/16 (different VLAN).
Flooding is constrained within the ingress VLAN boundary and strictly avoids reflecting traffic back out the receiving interface.

Key Concept

Layer 2 Unknown Unicast Flooding and MAC Table Aging
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Topics

L2 Switching Concepts and MAC Table Operation

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