MPLS LDP Loop Detection

There is a possibility of looping between LSPs established in MPLS. It is possible to detect and prevent looping LSPs in LDP using the loop detection mechanism. It is necessary to configure the LDP loop detection mechanism on each LSR in order to make it work. LDP loop detection configuration may differ between LDP peers, however, in order to establish an LDP session.

Fig 1.1- MPLS topology

There are two modes for detecting LDP loops:

Maximum hop count: Messages that request labels or map labels can include information about their hop count, which increments by one with each hop. A loop is considered when this value exceeds the specified limit, and an LSP cannot be established.

Path vector: In label request or label mapping messages, path vector tables can be used to convey path information. If the MPLS LSR ID is already present in the path vector table of such a message, the LSR checks it. An attempt to establish an LSP will fail if either of the following occurs

• The path vector table already contains the LSR's MPLS LSR ID.
• The path's hop count exceeds the predetermined threshold.

The LSR updates the path vector table with the MPLS LSR ID if it is not already there.

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