Q/A: MPLS Architecture
Name the four fields that are part of a label.
Answer: The four fields that are part of a label are a 20-bit label value, 3 experimental bits, 1 Bottom of Stack bit, and an 8-bit TTL field.
How many labels can reside in a label stack?
Answer: Any number of labels can reside in a label stack.
In which layer does MPLS fit in the OSI reference model?
Answer: MPLS fits in no category of the OSI reference model. The best description for MPLS would be Layer 2.5.
Which table does an LSR use to forward labeled packets?
Answer: The LSR uses the LFIB table to forward labeled packets.
What type of interfaces in Cisco IOS use the Downstream-on-Demand label distribution mode and the per-interface label space?
Answer: LC-ATM interfaces use the Downstream-on-Demand label distribution mode and the per-interface label space.
Why does the MPLS label have a Time To Live (TTL) field?
Answer: As the packets are labeled, the IP TTL can no longer be used. A mechanism is still needed to avoid a packet circulating in a loop forever.
Taken from MPLS Fundamental
