CCIE SP MPLS FAQ: Advanced MPLS Topics

CCIE SP MPLS FAQ: Advanced MPLS Topics

Q1. Is it possible to restrict which prefixes have labels assigned to them so that label switching occurs to only certain destination prefixes?

Answer: No. Labels are assigned to all internal prefixes, although it is possible to filter the distribution of these prefixes, which provides the desired result.

Q2. What is the maximum frame size permissible across an Ethernet segment, and how can introducing MPLS affect forwarding across this type of media?

Answer: 1518 bytes. By introducing MPLS, you can exceed the maximum permissible frame size through the addition of one or more MPLS labels.

Q3. Which mechanism can you deploy to help determine the maximum allowable Path MTU for a packet?

Answer: Path MTU discovery as described in RFC 1191.

Q4. What is the maximum size datagram, which does not support Path MTU discovery, that hosts can send?

Answer: 576 octets.
Figure: Path MTU Discovery Mechanism

Q5. What is the purpose of the DF bit?

Answer: To determine whether a packet can be fragmented by a receiving LSR. If the bit is set, the packet cannot be fragmented and must be dropped if it exceeds the maximum MTU of the outgoing link.

Q6.In a frame-mode MPLS implementation, how are loopsdetected in the forwarding plane?

Answer: By examination of the TTL field within the MPLS header. If the TTL field reaches 0, the packet is dropped.

Q7. Is it possible to detect loops in the forwarding plane by using the TTL field in a cell-mode implementation of MPLS?

Answer: No. There is no concept of TTL within an ATM header.

Q8. Using the hop-count TLV, an ATM-LSR can detect a loop in the control plane. What is the maximum number of permissible hops within a Cisco implementation, and can you change this maximum?

Answer: 254 hops. You can change the maximum by using the mpls ldp maxhops command.

Q9. When using traceroute across an MPLS network, how can ICMP messages be forwarded back to the source of the traceroute?

Answer: By reusing the original label stack, the original destination can forward the packet back across the MPLS network to the traceroute’s source.

Q10. What action does an aggregation LSR take if no label exists on an incoming packet?

Answer: It routes the packet based on its destination IP address.

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