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Difficulty: MediumL2 Switching Concepts and MAC Table Operation

A Cisco switch receives an Ethernet frame on interface GigabitEthernet0/1 assigned to VLAN 10. The frame features a source MAC address of 0050.56a1.1111 and a destination MAC address of 0050.56b2.2222. The switch MAC address table currently contains an entry for 0050.56b2.2222 associated with interface GigabitEthernet0/3 on VLAN 10, but has no existing entry for 0050.56a1.1111. Which two actions does the switch take upon processing this ingress frame? (Select two.)

  1. It records an entry in the MAC address table mapping source MAC address 0050.56a1.1111 to interface GigabitEthernet0/1 on VLAN 10.Answer
  2. It forwards the frame directly out interface GigabitEthernet0/3 without transmitting it on any other port.Answer
  3. C
    It floods the frame out all active interfaces in VLAN 10 except for the ingress interface GigabitEthernet0/1.
  4. D
    It drops the frame immediately because the incoming source MAC address is not pre-registered in the switch configuration.
  5. E
    It generates an ICMP Destination Unreachable control packet and transmits it back out interface GigabitEthernet0/1.

Answer

The switch learns the source MAC address by associating 0050.56a1.1111 with ingress port GigabitEthernet0/1 in VLAN 10, and forwards the frame directly out egress port GigabitEthernet0/3 based on the known destination MAC table entry.
Layer 2 switches process Ethernet frames in two distinct phases. First, the switch reads the frame's source MAC address (0050.56a1.1111) and updates its MAC address table by binding that address to ingress port GigabitEthernet0/1 under VLAN 10. Second, the switch checks its MAC table for the destination MAC address (0050.56b2.2222). Since an entry already exists pointing to GigabitEthernet0/3 in VLAN 10, the switch forwards the frame directly out interface GigabitEthernet0/3.

Step-by-Step Solution

1
Examine incoming frame headers for source MAC address processing.
The switch reads source MAC 0050.56a1.1111 on ingress interface GigabitEthernet0/1 (VLAN 10).
Layer 2 switches inspect the source MAC address of every ingress frame to dynamically build and maintain the MAC address table.
2
Update the MAC address table.
A new dynamic MAC address table entry is created: MAC 0050.56a1.1111 → GigabitEthernet0/1 (VLAN 10).
The source MAC was not previously in the table, so a new mapping is recorded to enable future return traffic forwarding.
3
Examine incoming frame headers for destination MAC lookup.
The switch searches the MAC address table for destination MAC 0050.56b2.2222 in VLAN 10.
Destination lookup determines whether the frame requires point-to-point forwarding or unknown unicast flooding.
4
Forward the frame to the destination interface.
The frame is forwarded directly out GigabitEthernet0/3.
Because destination MAC 0050.56b2.2222 is known in the MAC table on port GigabitEthernet0/3, unicast forwarding is performed without flooding.

Key Concept

Layer 2 Ethernet switching operates on a two-step process per frame: (1) Learn source MAC address to ingress port mapping, and (2) Forward based on destination MAC address lookup (known unicast forwarding vs. unknown unicast flooding).
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Topics

L2 Switching Concepts and MAC Table Operation

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