3Com MSR 50 Series, Руководство по настройке

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Configuring MPLS Basic Capability 1325
Maximum hop count
A label request message or label mapping message can include information about
its hop count, which increments by 1 for each hop. When this value exceeds the
specified limit, LDP considers that a loop is present and the attempt to establish an
LSP fails.
Path vector
A label request message or label mapping message can include path information
in the format of path vector table. Whenever such a message reaches an LSR, the
LSR checks the path vector table of the message to see whether its MPLS LSR ID is
already there. If either of the following cases occurs, the attempt to establish an
LSP fails:
■
The MPLS LSR ID of the LSR is already in the path vector table.
■
Hop counts of the path exceeds the specified limit.
If the MPLS LSR ID of the LSR is not in the path vector table, the LSR adds it into
the table.
LDP GR
n
For details about Graceful Restart (GR), refer to “GR Overview” on page 1957 .
During MPLS LDP session establishment, the LDP devices need to perform Fault
Tolerance (FT) and GR capability negotiation. Only when both devices support GR,
can the established session be FT/GR capable. To support GR, a GR device must
backup the FECs and label information.
When an LDP session is GR capable:
1
Whenever the GR restarter restarts, the GR helper will detect that the related LDP
session is down and will keep its neighborship with the GR restarter and retain
information about the session until the reconnect timer times out.
2
If the GR helper receives a session request from the GR restarter before the
reconnect timer times out, it retains the LSP and label information of the session
and restores the session with the GR restarter. Otherwise, it deletes all the LSP and
label information associated with the session.
3
After the session recovers, the GR restarter and GR helper activate their neighbor
liveness timer and recovery timer, restore all the LSP information relative to this
session, and send to each other label mapping and label request messages.
4
Upon receipt of the mapping messages from each other, the GR restarter and GR
helper delete the LSP stale flag and will delete all the LSP information of the
session after the neighbor liveness timer and recovery timer times out.
To summarize, during a graceful restart, the LSP information is preserved for the
forwarding plane and therefore MPLS packets can be forwarded without
interruption.
Configuring MPLS
Basic Capability
You must configure MPLS basic capability on all routers intended to engage in
MPLS forwarding within an MPLS domain, and you must configure MPLS basic
capability before configuring any other MPLS features.