CCIE SP MPLS FAQ: Frame-mode MPLS Operation
Figure: SuperNet Service Provider Network
Q1. MPLS can function using two different modes. What are they?
Answer: Frame-mode—Labeled packets are exchanged as frames at Layer 2
Q2. When using frame-mode MPLS, where is the label header carried within the frame?
Answer: The label header is inserted between the Layer-2 and Layer-3 headers.
Q3. Describe the format of an MPLS label header.
Answer: 20 bits label 3 bits EXP field 1 bit S 8 bits TTL field
Q4. In a LAN environment, how does the receiving router identify an MPLS packet?
Answer: Labeled packets can be identified by their ethertype value of 0x8847 or 0x8848.
Q5. List three different actions an MPLS LSR can perform on a labeled packet when running in frame-mode.
Answer: Pop—Removes the top label and forwards as either an IP or MPLS labeled packet. Swap—Replaces the top label with another value. Push—Places another label or labels on the top of the label stack. Aggregate—The egress LSR has to perform additional L3 lookup in its IP forwarding table to successfully forward the packet. Untag—Similar to Pop, but the result of the Pop operation is an unlabeled IP datagram. This operation is used on egress circuits toward non-MPLS nodes.
Q6. Name two label-distribution protocols used in conjunction with unicast destination-based routing.
Answer: Tag Distribution Protocol (TDP). Label Distribution Protocol (LDP).
Q7. Which label distribution method does a frame-mode implementation use?
Answer: Unsolicited downstream label distribution.
Q8. Which label allocation method does a frame-mode implementation use?
Answer: Independent control label allocation.
Q9. Which label retention method does a frame-mode implementation use?
Answer: Liberal label retention.
Q10. How is penultimate hop popping requested through the use of TDP or LDP?
Answer: By using an implicit-null label value to indicate that the upstream LSR should remove the top label from the MPLS label stack.