Graceful Restart On Switches With Redundant Cmms - Alcatel-Lucent OmniSwitch 6850-48 Network Configuration Manual

MPLS Overview
LDP signaling works with the MPLS label manager to manage the relationships between labels and the
corresponding FEC. For service-based FECs, LDP works in tandem with the Service Manager to identify
the Virtual Private LAN Services (VPLSs) to signal.
An MPLS label identifies a set of actions that are performed on an incoming packet. The FEC is identified
through the signaling protocol (in this case, LDP) and allocated a label. The mapping between the label
and the FEC is communicated to the forwarding plane.
When an unlabeled packet ingresses the OmniSwitch router, the packet is associated with a FEC. The
appropriate label is imposed on the packet, and the packet is forwarded. Other actions that can take place
before a packet is forwarded are imposing additional labels, other encapsulations, learning actions, etc.
When all actions associated with the packet are completed, the packet is forwarded.
When a labeled packet ingresses the router, the label or stack of labels indicates the set of actions associ-
ated with the FEC for that label or label stack. The actions are preformed on the packet and then the packet
is forwarded.

Graceful Restart on Switches with Redundant CMMs

A chassis-based switch with two Chassis management Modules (CMMs) can support redundancy where if
the primary CMM fails or goes offline for any reason, the secondary CMM is instantly notified. The
secondary CMM automatically assumes the primary role. This switch between the primary and secondary
CMMs is known as takeover.
When a takeover occurs, an MPLS router must re-establish full adjacencies with all its previously fully
adjacent neighbors. This time period between the restart and the re-establishment of adjacencies is termed
graceful restart.
This implementation of MPLS provides a graceful restart mechanism for the LDP component of MPLS.
This mechanism is supported only for planned takeovers (e.g., the users performs the takeover), not
unplanned takeovers (e.g., the primary CMM unexpectedly fails) or when a link goes down between two
routers.
In the network illustration below, a helper router, Router Y, monitors the network for topology changes.
As long as there are none, it continues to advertise its LDP adjacencies as if the restarting router, Router
X, had remained in continuous LDP operation (i.e., Router Y continues to list an adjacency to Router X
over network segment S, regardless of the adjacency's current synchronization state).
page 10-14 OmniSwitch AOS Release 6 Network Configuration Guide
Configuring MPLS
September 2009