MPLS Configuration Basics 1319■ Conservative: In this mode, an LSR keeps only label to FEC bindings that arefrom its next hops for the FECs.In liberal mode, an LSR can adapt to route changes quickly; while in conservativemode, there are less label to FEC bindings for an LSR to advertise and keep.The conservative label retention mode is usually used together with the DoD modeon LSRs with limited label space.Basic concepts for label switching■ Next hop label forwarding entry (NHLFE): Operation to be performed on thelabel, which can be Push or Swap.■ FEC to NHLFE map (FTN): Mapping of a FEC to an NHLFE at the ingress node.■ Incoming label map (ILM): Mapping of each incoming label to a set of NHLFEs.The operations performed for each incoming label includes Null and Pop.Label switching processEach packet is classified into a certain FEC at the ingress LER. Packets of the sameFEC travel along the same path in the MPLS domain, that is, the same LSP. Foreach incoming packet, an LSR examines the label, uses the ILM to map the label toan NHLFE, replaces the old label with a new label, and then forwards the labeledpacket to the next hop.PHP As described in “Architecture of MPLS” on page 1314, each transit LSR on anMPLS network forwards an incoming packet based on the label of the packet,while the egress LER removes the label from the packet and forwards the packetbased on the network layer destination address.In fact, on a relatively simple MPLS application network, the label of a packet isuseless for the egress, which only needs to forward the packet based on thenetwork layer destination address. In this case, the penultimate hop popping (PHP)feature can pop the label at the penultimate node, relieving the egress of the labeloperation burden and improving the packet processing capability of the MPLSnetwork.TTL Processing in MPLS MPLS TTL processing involves two aspects: TTL propagation and ICMP responsepath.IP TTL propagationAn MPLS label contains an 8-bit long TTL field, which has the same meaning asthat of an IP packet.According to RFC 3031 “Multiprotocol Label Switching Architecture”, when anLSR labels a packet, it copies the TTL value of the original IP packet or the upperlevel label to the TTL field of the newly added label. When an LSR forwards alabeled packet, it decrements the TTL value of the label at the stack top by 1.When an LSR pops a label, it copies the TTL value of the label at the stack top backto the TTL field of the IP packet or lower level label.TTL can be used not only to prevent routing loops, but to implement the tracertfunction:
