The hello-interval command at a mpls-interface level is used to configure the interval time for sending RSVP-TE Hello request messages. Configuring the Hello-interval allows the interface to initiate Hello request messages. When both ends of the link are configured to respond to RSVP-TE Hello messages, the neighbor receiving the request message generates an ACK message.
The hello-interval command at a mpls-interface level is used to configure the interval time for sending RSVP-TE Hello request messages. If the neighbor does not wish to participate in RSVP-TE Hello message communication, it can ignore the Hello request messages. The neighbor may send out the ACKs only if it chooses to participate in the RSVP-TE Hello messages. If a particular peer never responds to Hello messages, do not assume that the peer is dead, but simply assume that it does not support Hello messages.
Consider the case when both ends of the link supported RSVP-TE Hello messages, and the exchange of messages was normal as both links were up. Remove the support for Hello from one side of the link. The other side keeps sending Hello Request messages, but the neighbor starts ignoring these requests as it no longer wishes to participate in Hello messages exchange. In this case, because the neighbor stops sending ACKs, the router considers this as a neighbor down case and brings down all the RSVP sessions going over that interface. After a neighbor down event, Hello message exchange starts off from scratch (re-initiates). If the neighbor does not respond to Hello requests, the router assumes that the neighbor does not support Hello because no ACK was ever received after re-initiating Hello.
Also, when RSVP Hello is supported only on one end of the link, the end that supports Hello sends Hello request messages until it hits the hello_tolerance limit, then stops sending any further Hellos messages. It restarts sending Hellos when it receives a Hello message from the neighbor and then again continues the two-way communication as before.
Note
Caution: When disabling RSVP Hello, disable it on both sides of the link at the same time to avoid bringing down all the RSVP sessions going over that link.Hello-tolerance can be individually configured on both ends of the interface. Considering both sides of the link are participating in Hello communication, if no Instance values are received, through either of the REQUEST or ACK objects, from a neighbor within this configured hello-tolerance number of hello-intervals, then this node presumes that it cannot communicate with the neighbor.
Configuring hello-acknowledgments command (on the global MPLS RSVP Hello level) enables the router to respond back by sending Hello ACKs on neighbors not carrying any RSVP sessions. By default, HelloACKs are sent only to neighbors carrying RSVP sessions.