Configuring the Delay Before LDP Neighbors Are Considered Down
The hold time determines how long an LDP node should wait for a hello message before declaring a neighbor to be down. This value is sent as part of a hello message so that each LDP node tells its neighbors how long to wait. The values sent by each neighbor do not have to match.
The hold time should normally be at least three times the hello interval. The default is 15 seconds for link hello messages and 45 seconds for targeted hello messages. However, it is possible to configure an LDP hold time that is close to the value for the hello interval.
 | Note: By configuring an LDP hold time close to the hello interval (less than three times the hello interval), LDP neighbor failures might be detected more quickly. However, this also increases the possibility that the router might declare an LDP neighbor down that is still functioning normally. For more information, see Configuring the LDP Timer for Hello Messages. |
The LDP hold time is also negotiated automatically between LDP peers. When two LDP peers advertise different LDP hold times to one another, the smaller value is used. If an LDP peer router advertises a shorter hold time than the value you have configured, the peer router’s advertised hold time is used. This negotiation can affect the LDP keepalive interval as well.
If the local LDP hold time is not shortened during LDP peer negotiation, the user-configured keepalive interval is left unchanged. However, if the local hold time is reduced during peer negotiation, the keepalive interval is recalculated. If the LDP hold time has been reduced during peer negotiation, the keepalive interval is reduced to one-third of the new hold time value. For example, if the new hold-time value is 45 seconds, the keepalive interval is set to 15 seconds.
This automated keepalive interval calculation can cause different keepalive intervals to be configured on each peer router. This enables the routers to be flexible in how often they send keepalive messages, because the LDP peer negotiation ensures they are sent more frequently than the LDP hold time.
When you reconfigure the hold-time interval, changes do not take effect until after the session is reset. The hold time is negotiated when the LDP peering session is initiated and cannot be renegotiated as long as the session is up (required by RFC 5036, LDP Specification). To manually force the LDP session to reset, issue the clear ldp session command.
Configuring the LDP Hold Time for Link Hello Messages
To modify how long an LDP node should wait for a link hello message before declaring the neighbor down, specify a new time in seconds using the hold-time statement:
For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.
Configuring the LDP Hold Time for Targeted Hello Messages
To modify how long an LDP node should wait for a targeted hello message before declaring the neighbor down, specify a new time in seconds using the hold-time statement as an option for the targeted-hello statement:
For a list of hierarchy levels at which you can include these statements, see the statement summary sections for these statements.
