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Difficulty: HardL2 Switching Concepts and MAC Table Operation

A Cisco Catalyst switch maintains the following dynamic MAC address table entries across multiple VLANs:

VLANMAC AddressTypePort
100050.56a1.1a01DYNAMICGigabitEthernet0/1
100050.56a2.2b02DYNAMICGigabitEthernet0/2
200050.56a1.1a01DYNAMICGigabitEthernet0/3

A host connected to port GigabitEthernet0/1 sends an ingress Ethernet frame tagged for VLAN 20 with a Source MAC address of 0050.56a1.1a010050.56a1.1a01 and a Destination MAC address of 0050.56a3.3c030050.56a3.3c03.

How does the switch update its MAC address table and handle the forwarding of this ingress frame?

  1. The switch updates the VLAN 20 MAC address table entry for 0050.56a1.1a01 to port GigabitEthernet0/1 while preserving the VLAN 10 entry, and floods the frame out all active VLAN 20 ports except GigabitEthernet0/1.Answer
  2. B
    The switch flushes the VLAN 10 entry for MAC address 0050.56a1.1a01 and drops the frame because the destination MAC address does not exist in VLAN 20.
  3. C
    The switch forwards the frame directly out GigabitEthernet0/3 based on the pre-existing VLAN 20 entry and drops the frame on GigabitEthernet0/1 due to a port mismatch.
  4. D
    The switch updates the VLAN 20 entry to GigabitEthernet0/1 and floods the frame out all active interfaces across all VLANs, including the ingress port GigabitEthernet0/1.

Answer

The switch updates the VLAN 20 MAC table entry for source MAC 0050.56a1.1a01 to GigabitEthernet0/1 while preserving the VLAN 10 entry, and floods the unknown unicast frame out all active interfaces belonging to VLAN 20 except the ingress port GigabitEthernet0/1.
The correct response reflects two core switching mechanics: (1) Per-VLAN MAC learning updates the MAC address table for VLAN 20 on port GigabitEthernet0/1 without impacting entries in other VLANs (such as VLAN 10). (2) Destination MAC address lookup for an unlisted MAC address causes the switch to treat the frame as unknown unicast, flooding it out all active ports in VLAN 20 except the port on which the frame was received.

Step-by-Step Solution

1
Examine the ingress frame's VLAN tag, Source MAC address, and ingress interface.
The ingress frame is tagged for VLAN 20, has Source MAC 0050.56a1.1a01, and arrives on port GigabitEthernet0/1.
Source MAC learning occurs on every incoming frame within the context of the frame's assigned VLAN.
2
Perform MAC address learning/station move update for VLAN 20.
The MAC table entry for VLAN 20 and MAC 0050.56a1.1a01 is updated from GigabitEthernet0/3 to GigabitEthernet0/1. The VLAN 10 table entry remains intact.
Cisco Catalyst switches maintain independent per-VLAN MAC address tables (PVST / per-VLAN learning context). Updating an entry in VLAN 20 does not purge entries in VLAN 10.
3
Perform Destination MAC lookup for 0050.56a3.3c03 in VLAN 20.
Destination MAC 0050.56a3.3c03 is not found in the VLAN 20 MAC address table (Unknown Unicast).
Layer 2 switches check the destination MAC address against the forwarding table for the matching VLAN.
4
Determine the Layer 2 forwarding decision for an unknown unicast frame.
The switch floods the frame out all operational access/trunk interfaces forwarding for VLAN 20, excluding the ingress port GigabitEthernet0/1.
Unknown unicast traffic must be flooded within the broadast domain (VLAN boundary) to reach the intended destination, but ingress interface split-horizon rules prevent returning the frame out its receiving port.

Key Concept

Per-VLAN MAC Address Table Operation and Unknown Unicast Flooding
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Topics

L2 Switching Concepts and MAC Table Operation

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