This feature is an enhanced graceful handling capability that is supported only among SR OS based implementations. If LDP tries to resolve a FEC over a link or a targeted session and it runs out of data path or CPM resources, it puts the LDP/T-LDP session into overload state. As a result, it releases to its LDP peer the labels of the FECs which it could not resolve and also sends an LDP notification message to all LDP peers with the new status load of overload for the FEC type which caused the overload. The notification of overload is per FEC type, that is, unicast IPv4, P2MP mLDP and so on, and not per individual FEC. The peer which caused the overload and all other peers stop sending any new FECs of that type until this node updates the notification stating that it is no longer in overload state for that FEC type. FECs of this type previously resolved and other FEC types to this peer and all other peers continues to forward traffic normally.
After the user has taken action to free resources up, the overload state of the LDP/T-LDP sessions toward its peers must be manually cleared.
The enhanced mechanism is enabled instead of the base mechanism only if both LSR nodes advertise this new LDP capability at the time the LDP session is initialized. Otherwise, they continue to use the base mechanism.
This feature operates among SR OS LSR nodes using a couple of private vendor LDP capabilities:
The first one is the LSR Overload Status TLV to signal or clear the overload condition.
The second one is the Overload Protection Capability Parameter, which allows LDP peers to negotiate the use of the overload notification feature and therefore the enhanced graceful handling mechanism.
When interoperating with an LDP peer which does not support the enhanced resource handling mechanism, the router reverts automatically to the default base resource handling mechanism.
The following are the details of the mechanism.