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LDP-IGP Synchronization

LDP is often used to establish MPLS LSPs throughout a complete network domain using an IGP such as OSPFv2 or IS-IS. In such a network, all links in the domain have IGP adjacencies as well as LDP adjacencies. LDP establishes the LSPs on the shortest path to a destination as determined by IP forwarding.

MPLS data packets can be discarded in these networks when the network IGP is operational on a link for which LDP is not fully operational, because there is no coupling between the LDP operational state and the IGP. When LDP is not fully operational, LDP is considered to not be synchronized with the IGP.

This issue is especially significant for applications such as a core network that does not employ BGP. Another example is an MPLS VPN where each given PE router depends on the availability of a complete MPLS forwarding path to the other PE routers for each VPN that it serves. This means that along the shortest path between the PE routers, each link must have an operational hello adjacency and an operational LDP session, and MPLS label bindings must have been exchanged over each session.

When LDP has not completed exchanging label bindings with an IGP next hop, traffic is discarded if the head end of the LSP forwards traffic because the LSP is assumed to be in place. The following are some examples of when this can happen.

When an LDP hello adjacency or an LDP session with a peer is lost due to some error while the IGP still points to that peer. IP forwarding of traffic continues on the IGP link associated with the LDP peer rather than being shifted to another IGP link with which LDP is synchronized.
When a new IGP link comes up, causing the next hop to a certain destination to change in the IGP’s SPF calculations. Although the IGP might be up on the new link, LDP might not have completed label exchange for all the routes. This condition might be transient or due to a misconfiguration.

The LDP protocol is unable to indicate to a dependent service the availability of an uninterrupted LSP to the desired destination. LDP-IGP synchronization minimizes this disruption due to LDP not being operational on a link for which the IGP is operational. When synchronization is in effect, the IGP advertises the maximum possible cost or metric for that link. If an alternative next hop exists for traffic, the IGP can choose that next hop for forwarding. If LDP is operational for that next hop, then no traffic is discarded.

The IGP does not advertise the original cost or metric for the link until either LDP label exchange has been completed with the peer on the link or a configured amount of time has passed (the holddown period).

With synchronization configured, LDP notifies the IGP to advertise the maximum cost for the link when one of the following triggering events takes place:

The LDP hello adjacency goes down.
The LDP session goes down.
LDP is configured on an interface.

If the holddown timer has been configured, the timer starts when the triggering event takes place. When the timer expires, LDP notifies the IGP to resume advertising the original cost.

If the holddown timer has not been configured, the IGP waits (endlessly) until bindings have been received from downstream routers for all the FECs that have a next hop on that interface. Only after that takes place does LDP notify the IGP to bring down the cost on the interface.

LDP-IGP synchronization is supported only for directly connected peers and links with the platform label space.

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