How do Ethernet virtual circuits provide a way for service providers to maximize the use of VLAN tags?

Question

Options

• AThey add an additional tag to VLANs that allows up to two switch ports to use the same
• BThey redefine the VLAN tag to include classification, forwarding, and QoS using MPLS labels
• CThey separate the classification and forwarding concepts for VLAN tagging which allows
• DThey assign VLAN IDs to VTP domains so that the same VLAN ID are used more than once

• A
  12% (3)
• B
  8% (2)
• C
  76% (19)
• D
  4% (1)

Explanation

Option C is correct because Ethernet Virtual Circuits (EVCs) use a technique often called QinQ (802.1ad tag stacking) - they add an outer "service tag" (S-tag) for provider forwarding while preserving the customer's inner VLAN tag (C-tag). By decoupling classification (identifying which customer/service a frame belongs to) from forwarding (how the provider transports it), the same VLAN IDs (1–4094) can be reused across many different customers simultaneously, dramatically expanding effective VLAN capacity.

Why the distractors are wrong:

• A is incorrect because the benefit of tag stacking is not limited to two ports - it scales across the entire provider network for many customers.
• B is incorrect because EVCs use Ethernet-based tag stacking (802.1ad), not MPLS labels; MPLS is a separate Layer 2.5 technology with its own label mechanism.
• D is incorrect because VTP (VLAN Trunking Protocol) is a Cisco LAN management protocol and has no role in EVC service provider architectures.

Memory tip: Think of it as a "double envelope" - the customer's VLAN ID is the inner envelope, and the provider's service tag is the outer envelope. Because the outer envelope keeps customers separate, thousands of customers can all use "inner envelope #100" without conflict.

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