When you enable MPLS LDP MIB notification functionality by issuing the
snmp-server enable traps mpls ldp command, notification messages are generated and sent to a designated NMS in the network to signal the occurrence of specific events within Cisco software.
The MPLS LDP MIB objects that announce LDP status transitions and event notifications include the following:
mplsLdpSessionUp--This message is generated when an LDP entity (a local LSR) establishes an LDP session with another LDP entity (an adjacent LDP peer in the network).
mplsLdpSessionDown--This message is generated when an LDP session between a local LSR and its adjacent LDP peer is terminated.
The up and down notifications indicate the last active interface in the LDP session.
The value of the path vector limit can range from 0 to 255; a value of 0 indicates that loop detection is off; any value other than 0 up to 255 indicates that loop detection is on and, in addition, specifies the maximum number of hops through which an LDP message can pass before a loop condition in the network is sensed.
We recommend that all LDP-enabled routers in the network be configured with the same path vector limit. Accordingly, the mplsLdpPathVectorLimitMismatch object exists in the MPLS LDP MIB to provide a warning message to the NMS when two routers engaged in LDP operations have a dissimilar path vector limits.
mplsLdpFailedInitSessionThresholdExceeded--This message is generated when a local LSR and an adjacent LDP peer attempt to set up an LDP session between them, but fail to do so after a specified number of attempts. The default number of attempts is 8. This default value is implemented in Cisco software and cannot be changed by either the CLI or an SNMP agent.
Eight failed attempts to establish an LDP session between a local LSR and an LDP peer, due to any type of incompatibility between the devices, causes this notification message to be generated.
In general, Cisco routers support the same features across multiple platforms. Therefore, the most likely incompatibility to occur between Cisco LSRs is a mismatch of their respective ATM VPI/VCI label ranges.
For example, if you specify a range of valid labels for an LSR that does not overlap the range of its adjacent LDP peer, the routers try eight times to create an LDP session between themselves before the mplsLdpFailedInitSessionThresholdExceeded notification is generated and sent to the NMS as an informational message.
Operationally, the LSRs whose label ranges do not overlap continue their attempt to create an LDP session between themselves after the eight retry limit is exceeded. In such cases, the LDP threshold exceeded notification alerts the network administrator to the existence of a condition in the network that may warrant attention.
RFC 3036, LDP Specification , details the incompatibilities that can exist between Cisco routers and/or other vendor LSRs in an MPLS network. Among such incompatibilities, for example, are the following:
- Nonoverlapping ATM VPI/VCI ranges (as noted above) or nonoverlapping Frame-Relay DLCI ranges between LSRs attempting to set up an LDP session
- Unsupported label distribution method
- Dissimilar protocol data unit (PDU) sizes
- Dissimilar LDP feature support