3Com S7906E - Switch, Configuration Manual

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Maximum hop count
A label request message or label mapping message may contain information about its hop count, which
increments by 1 for each hop. When this value reaches 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 may contain path information in the form of path
vector list. When such a message reaches an LSR, the LSR checks the path vector list of the message
to see whether its MPLS LSR ID is in the list. If either of the following cases occurs, the attempt to
establish an LSP fails:
z
The MPLS LSR ID of the LSR is already in the path vector list.
z
The hop count of the path reaches the specified limit.
If the LSR does not find its MPLS LSR ID in the path vector list, it adds the ID into the list.
LDP Label Filtering
By default, the LDP protocol accepts all label bindings received from the downstream neighbors, and
then advertises the label bindings to each upstream neighbor based on the split horizon mechanism.
The LDP label filtering feature allows the LDP protocol to accept and advertise label bindings selectively.
It provides two filtering mechanisms, label acceptance control (or inbound filtering) for the inbound
direction and label advertisement control (or outbound filtering) for the outbound direction, as described
below.
Label acceptance control
On an LSR, after a label acceptance control (or inbound filtering) policy is configured for a downstream
neighbor, label binding acceptance on the LSR is controlled, that is, for the label bindings that are
received from the specified downstream neighbor, the LSR accepts only those matching the specified
prefix list.
Figure 1-8
Network diagram for label acceptance control
Label
advertisement
Label control policy on device C:
Accept labels that match prefix list
x.x.x.x/m from downstream
neighbor device B .
Downstream
device A
Upstream
device C
Downstream
device B
Label
advertisement
Label advertisement control
With a label advertisement control (or outbound filtering) policy configured, an LSR advertises the label
bindings for the specified address prefixes to the specified upstream neighbors.