Examkin

Question

Difficulty: HardInterswitch Connectivity and Trunking (802.1Q)

A Cisco Catalyst switch receives an untagged Ethernet frame on an interface configured as an 802.1Q trunk port. Place the operational steps performed by the switch in the correct order from ingress frame arrival to egress transmission.

  1. 1The switch receives an untagged Ethernet frame on a physical interface operating in 802.1Q trunk mode.
  2. 2The switch internally tags/associates the incoming frame with the native VLAN configured on the ingress trunk port.
  3. 3The switch evaluates the ingress port's allowed VLAN list to verify that the native VLAN is permitted to cross the interface.
  4. 4The switch performs a Layer 2 MAC address table lookup within the context of the assigned native VLAN to determine the egress destination port.
  5. 5The switch transmits the frame out an egress trunk port without an 802.1Q header because the frame belongs to that egress port's native VLAN.

Answer

The correct sequence is: (1) Frame reception on 802.1Q trunk, (2) Internal classification into ingress native VLAN, (3) Ingress trunk allowed VLAN list verification, (4) MAC address table lookup within native VLAN context, and (5) Egress transmission untagged on matching native VLAN trunk.
When an untagged frame is received on an 802.1Q trunk port, the switch first accepts the frame and assigns it internally to the interface's native VLAN. It then validates whether that native VLAN is permitted according to the trunk's allowed VLAN configuration. Next, it performs a Layer 2 MAC table lookup restricted to that native VLAN's broadcast domain to select the egress port. Finally, when transmitting out another trunk port configured with the same native VLAN, the switch leaves the frame untagged according to 802.1Q specification.

Step-by-Step Solution

1
Identify the ingress frame state upon arrival.
The frame arrives on the 802.1Q trunk interface lacking a 4-byte 802.1Q VLAN header.
802.1Q trunks expect untagged frames to belong to the designated native VLAN.
2
Map untagged ingress traffic to the port's native VLAN.
The switch internal ASIC assigns the frame to the configured native VLAN ID of the receiving trunk port.
All frames moving through a Catalyst switch fabric must be associated with a specific VLAN domain.
3
Check interface trunk filtering rules (allowed VLAN list).
The switch confirms that the native VLAN ID is present in the `switchport trunk allowed vlan` filter for that interface.
If the native VLAN was explicitly removed from the allowed list, the switch would drop the frame at ingress.
4
Determine the Layer 2 egress destination.
The switch queries its MAC address table within the scope of the assigned native VLAN.
VLAN isolation requires that MAC lookups occur strictly within the MAC table instance dedicated to that specific VLAN.
5
Apply egress 802.1Q tagging rules prior to transmission.
The frame is forwarded out the destination trunk port untagged because its VLAN matches the destination port's configured native VLAN.
Standard IEEE 802.1Q behavior specifies that frames belonging to a trunk's native VLAN are transmitted across the link without an 802.1Q tag header.

Key Concept

IEEE 802.1Q Native VLAN Ingress Classification, Filtering, and Egress Tagging Lifecycle
Estimated Time:2m 0s
Rate this question

Topics

Interswitch Connectivity and Trunking (802.1Q)

More questions from this topic