User Manual

80-13
Label Retention Mode
Label Retention Mode determines how the LSR handles the currently useless mapping from label to FEC it
received. In DU mode, the upstream LSR may receive a large number of <FEC, label> map sets from the
downstream LSR, in which case, only when the FEC in the map set is the local FEC next-hop of the upstream
LSR, this map set is meaningful for the label forwarding. MPLS defines two label retention modes to
determine the processing of currently useless map set.
 Conservative Mode: the LSR will reserve the label mapping received from the neighbor LSR no
matter the neighbor is its next-hop or not. The advantage of this mode is that it only creates and
maintain the labels that meaningful for data forwarding, a very significant feature when the label
space is limited (ATM switching).
 Liberal Mode: the LSR only save label maps from the neighbor LSR which is its next-hop. The
advantage of this mode is that the expense of processing route changes is very low; and the
disadvantage is many useless labels will be advertised and maintained.
In the Liberal label retention mode, LSR can adapt rapidly to route changes; in the Conservative mode, LSR
can distribute and save relatively less labels. The Conservative retention mode, together with the DoD mode,
usually applies to LSR with limited label space.
Some Basic Concepts of Label Switching
 NHLFE: Next Hop Label Forwarding Entry. It is used to describe the operation to the label, including
Push and Swap.
 FTN (FEC to NHLFE map): the process of mapping FEC to NHLFE on the Ingress router.
 ILM (Incoming Label Map): the process of mapping received labels to NHLFE by LSR.
The Label Switching Process
The Ingress LER divides the packets entering the network into FECs. The packets belonging to the same FEC
will follow the same path - LSP, in the MPLS domain. LSR will distribute a label for the incoming FEC packet
and forward it through the corresponding interface.
The detailed process of label switch is as follows:
 All LSRs along the LSP will create an ILM first, the entries in which are the rule of mapping the
incoming labels.
 LSR will map the labels of received packets to NHLFE;
 LSR will find the corresponding NHLFE in the LIB based on the label, replace it with the new label
and then forward the label packet.
80.1.4 LDP Session
There are four steps to establish a LDP session:
 Discover
 Establish and maintain the session
 Create LSP
 Cancel the session