PLANET XGS3-24042 - Manual (Page 659)

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 Ordered Mode: For a FEC label mapping of a LSR, the LSR only advertise the mapping to its
upstream when it already has the label mapping of the FEC next-hop, or when it is the egress router
of the FEC. The label advertisement of a flow starts from the egress router of this FEC flow, binding
routers from downstream to upstream, thus to guarantee the mapping between labels and the flow is
complete and coherent in the whole network. The ordered mode can prevent loop more effectively.
 Independent Mode: LSR doesn’t have to wait for the label of the FEC next-hop to advertise labels to
its peer. It can notify label mapping to the LSR connected to it at any time. This mode may cause the
LSR advertise a label to its upstream before receiving one from its downstream. This mode can
accelerate the creation and aggregation of LSP.
Requirements for LSR to be an Egress router:
 The FEC quotes the LSR address;
 The FEC next-hop router locates outside the label switching network;
 The FEC unit passes the route area, such as another OSPF SUMMAERY domain, or another
autonomy system of OSPF, BGP.
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 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