Configuring Multicast LDP Link Protection
A Layer Distribution Protocol (LDP) label-switched path (LSP) that is point to multipoint can be used to send traffic from a single root or ingress node to a number of leaf or egress nodes traversing one or more transit nodes. Multicast Label Distribution Protocol (MLDP) link protection enables fast reroute of traffic carried over point-to-multipoint LDP LSPs in case of a link failure. When one of the links of the point-to-multipoint tree fails, the subtrees might get detached until the IGP reconverges and MLDP initiates label mapping using the best path from the downstream router to the new upstream router.
To protect the traffic flowing through the LDP point-to-multipoint LSP, you can configure an explicit tunnel for traffic to be re-routed in the event of link failure. The explicit path has to terminate on the next downstream router, and the reverse path forwarding for the traffic should be successful.
You can configure MLDP link protection using dynamic RSVP LSPs or a regular LDP of unidirectional paths using hop-by-hop routing. Dynamic RSVP LSPs are used as a bypass tunnel. The RSVP LSP's explicit route object (ERO) is calculated using Constrained Shortest Path First (CSPF) with the constraint as the link to avoid. The LSP is signaled and torn down dynamically whenever link protection is necessary. A targeted adjacency to the downstream label-switching router (LSR) is created (if none is preconfigured already) for two reasons:
- Keeping the session up after link failure.
- Using the point-to-multipoint label received during the session to send traffic to the downstream LSR using RSVP LSP as a bypass tunnel.
It is possible to configure a remote neighbor to the LSR by configuring LDP tunneling on RSVP LSPs, or LDP-based virtual private LAN service (VPLS), or on Layer 2 circuits, or LDP session protection.
To enable MLDP link protection, Junos OS supports the make-before-break (MBB) feature to ensure minimum packet loss when attempting to signal a new LSP path before tearing down the old LSP path.
An LSR selects its upstream LSR as its next hop to the root of a point-to-multipoint LSP. When the best path to reach the root changes, the LSR chooses a new upstream LSR. During this transition, the LSP might go down temporarily resulting in packet loss until the LSP reconverges to the new upstream LSR. By configuring MBB, you can minimize packet loss during reconvergence. In addition, there might be scenarios where the best path from LSR to the root changes and yet the LSP continues to forward packets to the earlier next hop to the root. In such cases, a new LSP must be established before the old LSP is brought down to minimize the duration of packet loss. If a link fails, the downstream LSR continues to receive and forward packets to other downstream LSRs as it continues to receive packets from the RSVP LSP.
 | Note: You must configure link protection for the LDP interface using the link-protection statement at the [edit protocols ldp] hierarchy level before configuring MBB. |
To configure make before break, include the make-before-break statement at the [edit protocols ldp] hierarchy level:
When you include the make-before-statement in the configuration, the LDP LSR advertises that it is capable of handling MBB point-to-multipoint LSPs configured using the p2mp configuration statement at the [edit protocols ldp] hierarchy level.
You can include the following options for the make-before-break statement:
- switchover-delay—Specify a value from 1 through 300 seconds to change switchover delay for a point-to-multipoint LSP from the old LSR to the new upstream LSR. The default value is 30 seconds. If an MBB acknowledgement is received on a point of local repair (PLR) router, the PLR waits for the specified seconds to switch its upstream LSR from the old LSR to the new LSR.
- timeout—Specify a value from 1 through 300 seconds to change make-before-break timeout for point-to-multipoint LSPs. The default value is 30 seconds. Even if an MBB acknowledgment is not received for a point-to-multipoint LSP before the specified timeout period expires, the LSR performs an MBB switchover from the old LSR to the new upstream LSR.
