This feature allows a downstream LSR to send a label binding to two upstream LSR nodes, but only accept traffic as follows:
for normal operation, traffic is accepted from the ILM on the interface to the primary next hop of the root LSR for the point-to-multipoint FEC
for failure operation, traffic is accepted from the ILM on the interface to the backup next hop
A candidate upstream LSR node must be either an ECMP next hop or an LFA next hop. Either option allows the downstream LSR to perform a fast switchover and to source the traffic from another upstream LSR while IGP is converging because of a failure of the LDP session of the upstream peer, which is the primary next hop of the root LSR for the point-to-multipoint FEC. That is, the candidate upstream LSR node provides upstream FRR node protection for the mLDP FEC packets.
Multicast LDP fast upstream switchover is illustrated in Figure: Multicast LDP Fast Upstream Switchover. LSR U is the primary next hop for the root LSR R of the point-to-multipoint FEC. LSR U' is an ECMP or LFA backup next hop for the root LSR R of the same point-to-multipoint FEC.
In Figure: Multicast LDP Fast Upstream Switchover, downstream LSR Z sends a label mapping message to both upstream LSR nodes, and programs the primary ILM on the interface to LSR U and the backup ILM on the interface to LSR U'. The labels for the primary and backup ILMs must be different. Thus LSR Z attracts traffic from both ILMs. However, LSR Z blocks the ILM on the interface to LSR U' and only accepts traffic from the ILM on the interface to LSR U.
If the link to LSR U fails, or LSR U fails, causing the LDP session to LSR U to go down, LSR Z detects the failure and reverse the ILM blocking state. In addition, LSR Z immediately starts receiving traffic from LSR U' until IGP converges and provides a new primary next hop and a new ECMP or LFA backup next hop, which may or may not be on the interface to LSR U'. When IGP convergence is complete, LSR Z updates the primary and backup ILMs in the datapath.
LDP uses the interface of either an ECMP next hop or an LFA next hop to the root LSR prefix, whichever is available, to program the backup ILM. However, ECMP next hop and LFA next hop are mutually exclusive for a specific prefix. IGP installs the ECMP next hop in preference to the LFA next hop as a prefix in the routing table manager (RTM).
If one or more ECMP next hops for the root LSR prefix exist, LDP picks the interface for the primary ILM based on the rules of mLDP FEC resolution specified in RFC 6388, Label Distribution Protocol Extensions for Point-to-Multipoint and Multipoint-to-Multipoint Label Switched Paths:
The candidate upstream LSRs are numbered from lowest to highest IP address.
The following hash is performed:
H = (CRC32(Opaque Value)) modulo N
where N is the number of upstream LSRs
The Opaque Value is the field in the point-to-multipoint FEC element immediately after the Opaque Length field. The Opaque Length indicates the opaque value used in this calculation.
The selected upstream LSR U is the LSR that has the number H.
LDP then picks the interface for the backup ILM using the following new rules:
if (H + 1 < NUM_ECMP) {
// If the hashed entry is not last in the next hops then pick up the next as backup.
backup = H + 1;
} else {
// Wrap around and pick up the first.
backup = 1;
}
In some topologies, it is possible that no ECMP or LFA next hop is found. In this case, LDP programs the primary ILM only.