MPLS-Enabled Applications: Emerging Developments and New Technologies

11.8. FAILURE NOTIFICATION MECHANISMS

An important requirement for Layer 2 transport schemes is to provide a mechanism for a PE to indicate to a local CE that there is a problem with the connection to one of the remote CEs. For example, there may not be connectivity to the remote PE due to problems in the service provider's network, or the link between the remote PE and remote CE could be down.

First of all, how does a PE become aware of such connectivity problems? As described in a previous section, the BGP scheme provides a circuit status vector so that a PE can advertise to remote PEs the state of its local PE-CE circuits and the state of its LSPs to remote PEs. Earlier versions of the LDP scheme stipulated that a PE should withdraw the VPN label that it advertises to a remote PE if there are problems of this nature. More recently, a Status TLV has been added to the LDP scheme that allows one PE to signal to the remote PE the status of its connectivity. The action taken by a PE if there is a connectivity problem depends on the Layer 2 media type.

In the ATM case, operations, administration and management (OAM) cells can be generated by the PE and sent to the local CE. This action tells the ATM CE equipment that there is a problem with the VC or VP in question, so the CE will stop sending traffic on that connection. In the case where pseudowires are provided on a per-VC basis, AIS F5 OAM cells can be sent, and in the case where pseudowires are provided on a per-VP basis, AIS ...