A service provider is using GRE for his transport tunnel on the Alcatel-Lucent 7750 SR. How can traffic be marked as it traverses the service provider's network? (Choose two)

Question

Options

• AUsing dot1p bits of the 802.1q Ethernet frame header
• BUsing DSCP bits within the ToS field of the IP packet header.
• CUsing the IP precedence bits within the ToS field of the IP packet header.
• DUsing the CLP bit of the ATM cell header.
• EUsing the EXP bits of the MPLS transport label header.

• A
  72% (26)
• C
  8% (3)
• D
  17% (6)
• E
  3% (1)

Explanation

When GRE is used as the transport tunnel, traffic traverses the SP network encapsulated inside an outer IP packet carried over Ethernet - so marking must happen at the layers that are actually visible to the network: Layer 2 (dot1p in the 802.1q VLAN tag, A) and Layer 3 (DSCP bits in the outer IP header's ToS field, B). The 7750 SR supports both mechanisms to influence QoS treatment across Ethernet and IP infrastructure.

Why the distractors are wrong:

• C (IP precedence): IP precedence is the legacy 3-bit QoS field that DSCP superseded. The 7750 SR uses DSCP for IP marking, making B the correct Layer 3 answer.
• D (ATM CLP bit): The Cell Loss Priority bit belongs to ATM cell headers - an entirely different technology not in play in a GRE/IP/Ethernet scenario.
• E (MPLS EXP bits): EXP (now called TC) bits are used when the transport is MPLS, not GRE. The question explicitly states GRE is the transport tunnel, so MPLS label fields are irrelevant.

Memory tip: Match the marking field to the encapsulation technology - GRE rides on IP and Ethernet, so use DSCP (IP) and dot1p (Ethernet). If it were MPLS, you'd pick EXP; if it were ATM, CLP. One transport, one set of tools.

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