Examkin

Soru

Zorluk: ZorSwitching Concepts and VLANs

A network administrator is analyzing frame forwarding behavior on an enterprise Layer 2 switch. Port 5 is an access port assigned to VLAN 25. Port 24 is configured as an 802.1Q trunk port allowing VLANs 10, 25, and 50, with native VLAN set to VLAN 10. The switch receives an Ethernet frame on Port 5 with source MAC address 0011.2233.4455 and destination MAC address 0011.2233.9999. The switch's MAC address table currently contains no entry for destination MAC address 0011.2233.9999. Ports 1 through 10 are active access ports in VLAN 25, while Ports 11 through 20 are active access ports in VLAN 10. Which of the following actions will the switch take to process and forward this frame?

  1. Learns the source MAC address on Port 5 in VLAN 25, then floods the frame out active access ports assigned to VLAN 25 (except Port 5) and out Port 24 with an 802.1Q tag for VLAN 25.Cevap
  2. B
    Floods the frame out all active access ports on the switch regardless of their configured VLAN membership, as unknown unicast traffic bypasses Layer 2 VLAN boundaries.
  3. C
    Forwards the frame out trunk Port 24 untagged, because all flooded traffic sent across 802.1Q trunk links defaults to native VLAN encapsulation.
  4. D
    Drops the frame at the ingress port because Layer 2 switches reject frames whose destination MAC addresses are missing from the Content Addressable Memory (CAM) table.

Cevap

The switch updates its MAC address table with the source MAC address on Port 5 in VLAN 25, then floods the frame out all active access ports in VLAN 25 (excluding receiving Port 5) and transmits it over trunk Port 24 with an 802.1Q VLAN 25 header tag.
When a Layer 2 switch receives an Ethernet frame, it first learns the source MAC address and associates it with the ingress port and VLAN. It then checks its CAM table for the destination MAC address. Because the destination MAC address is not listed, the switch floods the frame out all active ports in the same VLAN (VLAN 25), excluding the port of arrival. When forwarding the frame out an 802.1Q trunk port, the switch inserts an 802.1Q tag specifying VLAN 25 because VLAN 25 is not the configured native VLAN.

Adım Adım Çözüm

1
Examine source MAC learning on ingress
The switch inspects the frame's source MAC address (0011.2233.4455) and maps it to Port 5 in the CAM table under VLAN 25.
Layer 2 switches build their MAC address table dynamically by inspecting source MAC addresses of incoming frames on their ingress ports.
2
Determine destination MAC forwarding lookup
The destination MAC (0011.2233.9999) is absent from the CAM table, triggering unknown unicast flooding within VLAN 25.
If a destination unicast MAC is unknown, the switch forwards the frame to all active ports participating in the same broadcast domain (VLAN), excluding the port on which the frame arrived.
3
Apply 802.1Q trunk tagging rules for egress on Port 24
The frame is forwarded out Port 24 with a VLAN 25 802.1Q header tag.
Port 24 is an 802.1Q trunk allowing VLAN 25. Because VLAN 10 is designated as the native VLAN, frames belonging to VLAN 25 must be explicitly tagged with their VLAN ID upon exiting the trunk interface.

Anahtar Kavram

Switch Frame Forwarding Logic and VLAN Tagging across 802.1Q Trunks
Tahmini Süre:2m 0s
Bu soruyu puanla

Konular

Switching Concepts and VLANs

Bu konudan daha fazla soru