MPLS Fundamentals - Chapter 2 – Review Questions

1. Name the four fields that are part of a label?

- Label (first 20bits)

- EXP (next 3 bits)

- BoS (next 1 bit)

- TTL (next 8bits)

In all 32 bits.

 

2. How many labels can reside in a label stack?

- No limit, but you seldom see labels more than 4 in a label stack.

 

3. In which layer does MPLS fit in the OSI reference model?

- The MPLS label sits between the frame (layer 2) & the transported protocol packet (layer3), hence it can be conceived to be at layer 2.5

 

4. Which table does an LSR use to forward labeled packets?

- Label Information Base (LIB) is used to store all the remote labels received from multiple adjacent LSR's. On the other hand the Label Forwarding Instance Base (LFIB) consists of only those labels from the LIB which are being currently used for taking forwarding decisions.

 

5. What type of interfaces in Cisco IOS uses the Downstream-on-Demand label distribution mode and the per-interface label space?

- In Cisco IOS, all interfaces except LC-ATM interfaces use the UD label distribution mode. All LC-ATM interfaces use the DoD label distribution mode.

 

6. Why does the MPLS label have a Time To Live (TTL) field?

- Bits 24 to 31 are the eight bits used for Time To Live (TTL). This TTL has the same function as the TTL found in the IP header. It is simply decreased by 1 at each hop, and its main function is to avoid a packet being stuck in a routing loop. If a routing loop occurs and no TTL is present, the packet loops forever. If the TTL of the label reaches 0, the packet is discarded.