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Difficulty: HardL2 Switching Concepts and MAC Table Operation

A Cisco Catalyst switch contains the following entry in its MAC address table for VLAN 10:

VLANMAC AddressTypePort
10000a.1111.2222DynamicGigabitEthernet0/1
10000b.3333.4444DynamicGigabitEthernet0/2

A network technician disconnects the host with MAC address 000a.1111.2222 from port GigabitEthernet0/1 and reconnects it to port GigabitEthernet0/3. The host immediately transmits an Ethernet frame destined for MAC address 000b.3333.4444 into port GigabitEthernet0/3 on VLAN 10. How will the switch handle MAC address learning and frame forwarding for this ingress traffic?

  1. It updates the MAC address table entry for 000a.1111.2222 to associate with GigabitEthernet0/3 and forwards the frame exclusively out GigabitEthernet0/2.Answer
  2. B
    It drops the frame because the source MAC address is already dynamically mapped to a different physical interface.
  3. C
    It forwards the frame out GigabitEthernet0/2 while simultaneously flooding an ARP request out all active VLAN 10 interfaces to clear the old port binding.
  4. D
    It floods the frame out all active interfaces assigned to VLAN 10 except GigabitEthernet0/3 to locate the new path to 000b.3333.4444.

Answer

The switch updates the MAC address table entry for 000a.1111.2222 to point to GigabitEthernet0/3 and forwards the frame directly out GigabitEthernet0/2.
When an Ethernet frame arrives on port GigabitEthernet0/3, the switch reads the source MAC address 000a.1111.2222. Recognizing that this address was previously assigned to GigabitEthernet0/1, it performs a station move update by rewriting the MAC table entry to GigabitEthernet0/3. Next, the switch looks up the destination MAC address 000b.3333.4444. Since an entry already exists pointing to GigabitEthernet0/2, the switch forwards the frame as a known unicast transmission exclusively out GigabitEthernet0/2.

Step-by-Step Solution

1
Inspect the ingress frame's source MAC address and ingress port.
Source MAC 000a.1111.2222 is received on GigabitEthernet0/3.
Layer 2 switches perform MAC learning by examining the source MAC address of every incoming frame.
2
Update the MAC address table for station movement.
The dynamic entry for 000a.1111.2222 is updated from GigabitEthernet0/1 to GigabitEthernet0/3.
When a source MAC address is seen on a new interface, the switch updates the existing table mapping immediately.
3
Lookup the destination MAC address 000b.3333.4444 in the MAC address table.
Destination MAC 000b.3333.4444 is matched to egress port GigabitEthernet0/2.
Known unicast destination addresses result in point-to-point forwarding to the registered port.
4
Forward the Ethernet frame.
The frame is forwarded solely out GigabitEthernet0/2.
Because the destination port is known and distinct from the ingress port, no flooding or dropping occurs.

Key Concept

L2 Switching Station Movement and Known Unicast Forwarding
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Topics

L2 Switching Concepts and MAC Table Operation

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