LDP Hello interval and Hello hold timeout timers
The LDP Hello interval and Hello hold timeout timers are used to establish Hello adjacency between peers. The Hello interval is the time period between which the LSR sends out Hello messages and the Hello hold timeout is the amount of time that the sending LSR maintains its record of Hellos from the receiving LSR without receipt of another Hello message.
The Hello interval and Hello hold timeout timer values can be obtained from the global default values or configured globally on a router. The Hello hold timeout timer value can also be configured through an interface. When configuring these values the following constraints must be followed:
- The Hello interval value must be less than 1200
- The Hello hold timeout value must be less than 3600
- The Hello hold timeout value must be greater than or equal to 3 times the Hello interval value
The values can be set that determine the values used on the configured router and values sent to adjacent peers for their configuration. The following sections describe how to set the values:
- Setting the LDP Hello interval values
- Setting the LDP Hello hold time sent to adjacent LSRs
- Determining the LDP Hello hold time on an MPLS interface
Precedence of the Hello hold time settings
The precedence of the Hello hold time value settings is as follows:
Determining the LDP hold time on an MPLS interface:
An MPLS interface uses the LDP Hello hold time to determine how long it waits for its LDP peers to send a Hello message.
∙ For link LDP sessions - In this case, the wait time is determined by any one of the below criteria.
- When the Hello hold time is set per-interface, that value is used.
- When the Hello hold time is not set per-interface, the hold time in the received message is used.
- When the Hello hold time in the received message is zero (0), the default value of 15 seconds is used.
Precedence of the Hello interval settings
The precedence of the Hello interval value settings is as follows:
- For link LDP sessions - the LDP Hello interval can be set globally which applies to all LDP interfaces or on a per-interface basis. The LDP Hello interval values in LDP link sessions are determined by the following procedure in the order described below.
- When the Hello interval is set per-interface, that value is used.
- When the Hello interval is not set per-interface, then the value set for LDPs globally is used.
- When the Hello interval is not set either globally or per-interface, the global default value is used. When a Hello adjacency already exists, the adjacency remains up and any new configured interval takes effect upon the expiration of the current Hello interval timer. Consequently, the next and subsequent Hello messages are sent at the new interval.