MPLS PART 3 - Important Queestions found at Cisco FAQ

Forwarding Equivalence Class (FEC)

FEC is a group of IP packets which are forwarded in the same manner, over the same path, and with the same forwarding treatment. An FEC might correspond to a destination IP subnet, but it also might correspond to any traffic class that the Edge-LSR considers significant. For example, all traffic with a certain value of IP precedence might constitute a FEC.

Can an LSR transmit/receive a native IP packet (non-MPLS) on an MPLS interface

Yes, if the IP is enabled on the interface. Native packets are received/transmitted as usual. IP is just another protocol. MPLS packets have a different Layer 2 encoding. The receiving LSR is aware of the MPLS packet, based on the Layer 2 encoding.

Can an LSR receive/transmit a labeled packet on a non-MPLS interface?

No. Packets are never transmitted on an interface which is not enabled for that protocol. MPLS has a certain Ether type code associated with it. When a Cisco router receives a packet with an Ether type which is not enabled on the interface, it drops the packet.

What is the range of label values? What label values are reserved? What do the reserved values signify?.

Theoretically, the range is 0 through (2²⁰-1). Label values 0-15 are reserved, and values 4-15 are reserved for future use. Values 0-3 are defined as:

·         A value of 0 represents the "IPv4 Explicit NULL Label". This label indicates that the label stack must be popped, and the packet forwarding must be based on the IPv4 header. This helps to keep Exp bits safe until the egress router. It is used in MPLS based QoS.

·         A value of 1 represents the "Router Alert Label". When a received packet contains this label value at the top of the label stack, it is delivered to a local software module for processing. The actual packet forwarding is determined by the label beneath it in the stack. However, if the packet is forwarded further, the Router Alert Label should be pushed back onto the label stack before forwarding. The use of this label is analogous to the use of the "Router Alert Option" in IP packets (for example, ping with record route option)

·         A value of 2 represents the "IPv6 Explicit NULL Label". It indicates that the label stack must be popped, and the packet forwarding must be based on the IPv6 header.

·         A value of 3 represents the "Implicit NULL Label". This is a label that an LSR can assign and distribute. However, it never actually appears in the encapsulation. It indicates that the LSR pops the top label from the stack and forwards the rest of the packet (labeled or unlabeled) through the outgoing interface (as per the entry in Lfib). Although this value might never appear in the encapsulation, it needs to be specified in the Label Distribution Protocol, so a value is reserved.

Difference between Implicit null and explicit null Label

What protocol and port numbers do LDP and TDP use to distribute labels to LDP/TDP peers?

 LDP uses TCP port 646, and TDP uses TCP port 711. These ports are opened on the router interface only when mpls ip is configured on the interface. The use of TCP as a transport protocol results in reliable delivery of LDP/TDP information with robust flow control and congestion handling mechanisms.

What options are available for load balancing MPLS packets?

A. MPLS packets can be load balanced with the MPLS label information and/or the source and destination address of the essential IP header.

What does LSR do if it received a labelled packet that it does not have a local label for?

If an LSR receives a labelled packet that it does not have local label for , it drops it.

What are benefits of PHP?

PHP- PHP helps an egress PE from and extra LFIB lookup