The SR OS supports the synchronization of an IGP and LDP based on a solution described in RFC 5443, which consists of setting the cost of a restored link to infinity to give both the IGP and LDP time to converge. When a link is restored after a failure, the IGP sets the link cost to infinity and advertises it. The actual value advertised in OSPF is 0xFFFF (65535). The actual value advertised in an IS-IS regular metric is 0x3F (63) and in IS-IS wide-metric is 0xFFFFFE (16777214). This synchronization feature is not supported on RIP interfaces.
When the LDP synchronization timer subsequently expires, the actual cost is put back and the IGP readvertises it and uses it at the next SPF computation. The LDP synchronization timer is configured using the following command:
config>router>if> [no] ldp-sync-timer seconds
The SR OS also supports an LDP End of LIB message, as defined in RFC 5919, that allows a downstream node to indicate to its upstream peer that it has advertised its entire label information base. The effect of this on the IGP-LDP synchronization timer is described below.
If an interface belongs to both IS-IS and OSPF, a physical failure causes both IGPs to advertise an infinite metric and to follow the IGP-LDP synchronization procedures. If only one IGP bounces on this interface or on the system, then only the affected IGP advertises the infinite metric and follows the IGP-LDP synchronization procedures.
Next, an LDP Hello adjacency is brought up with the neighbor. The LDP synchronization timer is started by the IGP when the LDP session to the neighbor is up over the interface. This is to allow time for the label-FEC bindings to be exchanged.
When the LDP synchronization timer expires, the link cost is restored and is readvertised. The IGP announces a new best next hop and LDP uses it if the label binding for the neighbor’s FEC is available.
If the user changes the cost of an interface, the new value is advertised at the next flooding of link attributes by the IGP. However, if the LDP synchronization timer is still running, the new cost value is only advertised after the timer expires. The new cost value is also advertised after the user executes any of the following commands:
If the user changes the value of the LDP synchronization timer parameter, the new value takes effect at the next synchronization event. If the timer is still running, it continues to use the previous value.
If parallel links exist to the same neighbor, then the bindings and services should remain up as long as there is one interface that is up. However, the user-configured LDP synchronization timer still applies on the interface that failed and was restored. In this case, the router only considers this interface for forwarding after the IGP readvertises its actual cost value.
The LDP End of LIB message is used by a node to signal completion of label advertisements, using a FEC TLV with the Typed Wildcard FEC element for all negotiated FEC types. This is done even if the system has no label bindings to advertise. The SR OS also supports the Unrecognized Notification TLV (RFC 5919) that indicates to a peer node that it ignores unrecognized status TLVs. This indicates to the peer node that it is safe to send End of LIB notifications even if the node is not configured to process them.
The behavior of a system that receives an End of LIB status notification is configured through the CLI on a per-interface basis:
config>router>if>[no] ldp-sync-timer seconds end-of-lib
If the end-of lib option is not configured, then the LDP synchronization timer is started when the LDP Hello adjacency comes up over the interface, as described above. Any received End of LIB LDP messages are ignored.
If the end-of-lib option is configured, then the system behaves as follows on the receive side:
The ldp-sync-timer is started.
If LDP End of LIB Typed Wildcard FEC messages are received for every FEC type negotiated for a specified session to an LDP peer for that IGP interface, the ldp-sync-timer is terminated and processing proceeds as if the timer had expired, that is, by restoring the IGP link cost.
If the ldp-sync-timer expires before the LDP End of LIB messages are received for every negotiated FEC type, then the system restores the IGP link cost.
The receive side drops any unexpected End of LIB messages.
If the end-of-lib option is configured, then the system also sends out an End of LIB message for prefix and P2MP FECs after all FECs are sent for all peers that have advertised the Unrecognized Notification Capability TLV.
See the 7450 ESS, 7750 SR, 7950 XRS, and VSR Router Configuration Guide for the CLI command descriptions for LDP-IGP Synchronization.