MPLS Fundamentals - Chapter 3 – Forwarding Labeled Packets

• In Cisco IOS, CEF switching is the only IP switching mode that you can use to label packets. Other IP switching modes, such as fast switching, cannot be used, because the fast switching cache does not hold information on labels. Because CEF switching is the only IP switching mode that is supported in conjunction with MPLS, you must turn on CEF when you enable MPLS on the router.
• If a prefix is reachable via a mix of labeled and unlabeled (IP) paths, Cisco IOS does not consider the unlabeled paths for load-balancing labeled packets. That is because in some cases, the traffic going over the unlabeled path does not reach its destination.
• Label 0 is the explicit NULL label, whereas label 3 is the implicit NULL label. Label 1 is the router alert label, whereas label 14 is the OAM alert label. The other reserved labels between 0 and 15 have not been assigned yet.
• The egress LSR signals the penultimate LSR to use implicit NULL by not sending a regular label, but by sending the special label with value 3. The use of implicit NULL at the end of an LSP is called penultimate hop popping (PHP)
• PHP is the default mode in Cisco IOS. In the case of IPv4-over-MPLS, Cisco IOS only advertises the implicit NULL label for directly connected routes and summarized routes.
• The use of the implicit NULL label does not mean that all labels of the label stack must be removed. Only one label is popped off. In any case, the use of the implicit NULL label prevents the egress LSR from having to perform two lookups. Although the label value 3 signals the use of the implicit NULL label, the label 3 will never be seen as a label in the label stack of an MPLS packet. That is why it is called the implicit NULL label.
• In Cisco IOS, however, a safeguard guards against possible routing loops by not copying the MPLS TTL to the IP TTL if the MPLS TTL is greater than the IP TTL of the received labeled packet.
• The default MPLS MTU value of a link equals the MTU value.
• In some Cisco IOS releases, you cannot configure the MPLS MTU to be bigger than the interface MTU.
• Maximum receive unit (MRU) is a parameter that Cisco IOS uses. It informs the LSR how big a received labeled packet of a certain FEC can be that can still be forwarded out of this LSR without fragmenting it. This value is actually a value per FEC (or prefix) and not just per interface.
• Path MTU Discovery is not guaranteed to work in all cases; sometimes the ICMP message does not make it back to the originator. Possible causes for the ICMP message not making it to the originator of the packet are firewalls, access lists, and routing problems.