Operation Manual – MPLS Basics
H3C S9500 Series Routing Switches
Chapter 1 MPLS Basics Configuration
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Liberal: In this mode, an LSR keeps any received label to FEC binding regardless
of whether the binding is from its next hop for the FEC or not.
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Conservative: In this mode, an LSR keeps only label to FEC bindings that are from
its next hops for the FECs.
In liberal mode, an LSR can adapt to route changes quickly; while in conservative mode,
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 mode on
LSRs with limited
Label space and LDP identifier
.
IV. Basic concepts for label switching
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Next hop label forwarding entry (NHLFE): Operation to be performed on the label,
which can be Push or Swap.
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FEC to NHLFE map (FTN): Mapping of a FEC to an NHLFE at the ingress node.
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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.
V. Label switching process
Each packet is classified into a certain FEC at the ingress LER. Packets of the same
FEC travel along the same path in the MPLS domain, that is, the same LSP. For each
incoming packet, an LSR examines the label, uses the ILM to map the label to an
NHLFE, replaces the old label with a new label, and then forwards the labeled packet to
the next hop.
1.2.2 PHP
As described in
Architecture of MPLS
, each transit LSR on an MPLS 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 packet based on the network layer destination
address.
In fact, on a relatively simple MPLS application network, the label of a packet is useless
for the egress, which only needs to forward the packet based on the network 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 label operation burden and
improving the packet processing capability of the MPLS network.
1.2.3 TTL Processing in MPLS
MPLS TTL processing involves two aspects: TTL propagation and ICMP response
path.
I. IP TTL propagation
An MPLS label contains an 8-bit long TTL field, which has the same meaning as that of
an IP packet.
