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# Determining Peer Reachability with RSVP-TE Hello Messages

RSVP-TE hello messages enable the router to detect when an RSVP-TE peer is no longer reachable. When the router makes this determination, all LSPs that traverse that neighbor are torn down. Hello messages are optional and can be ignored safely by peers that are not configured to use the feature.

When you enable the hello feature on a virtual router or interface configured with RSVP-TE, that RSVP-TE node periodically sends a unicast hello message to each neighbor with which the node has established an LSP. The exchange of hello messages between the peers establishes a hello adjacency. You can configure the hello interval to establish how frequently the node sends hello messages. Hello messages are exchanged when an LSP is set up and are stopped when the last LSP between the two peers goes away.

You can use the hello feature to reduce the impact of RSVP-TE on system resources. Because a hello timeout is treated as a link failure, RSVP-TE can use the hello timeout instead of path and resv timeouts to determine when to bring down an LSP. High RSVP-TE refresh values reduce the amount of control traffic (and CPU cycles) needed by RSVP-TE to refresh LSP state across the network, thus reducing the impact of RSVP-TE on system resources.

## Hello Message Objects

Hello messages can contain a hello request object or a hello ack object. These objects provide a way to request an instance value from a peer and to provide an instance value to a peer. Hello requests are sent to establish and confirm an adjacency with a peer. Hello acks are sent in response to hello requests. However, a hello adjacency peer can treat a hello request as an implicit response to its own request, thus reducing the amount of polling that needs to be done for efficient failure detection.

## Hello Message Instances

Each object includes a source instance and a destination instance. The sender generates the source instance for its relationship with the receiver. The value of the source instance is unique for each peer. A given source instance value does not change while the two peers are successfully exchanging hello messages.

The destination instance is simply the source instance value that an RSVP-TE node has most recently received from its peer. If the node has never received a hello message from that peer, then it sets the destination instance value to zero.

Hello adjacency peers monitor the source instance sent by their neighbors. When a peer detects that the value has changed or that its neighbor does not properly report the source instance that the peer transmitted, then the peer treats that neighbor as if it has reset. In these cases, the local peer changes the instance value that it advertises to the neighbor.

## Sequence of Hello Message Exchange

When a peer receives a hello message with a hello request object, the receiver generates a hello message with a hello ack object. If the receiver has never received a hello from the sender and the source instance is nonzero, then the receiver updates the destination instance that it sends in response with this new value. When the original sender first receives a hello ack from the peer in response to the hello request, the sender updates the destination instance that it sends in the subsequent hello request with the nonzero source instance it receives in the hello ack.

Consider the following example. An LSP has been established between peers A and B. These adjacent peers have not yet exchanged hello messages.

1. Peer A sends a hello request to Peer B. The request object contains the following:
   - Source instance = 5 (generated by Peer A for this adjacency)
   - Destination instance = 0 (because it has never exchanged messages with Peer B)
2. Peer B receives the hello request and sends a hello ack to Peer A. The ack object contains the following:
   - Source instance = 8 (generated by Peer B for this adjacency)
   - Destination instance = 5 (because that is what Peer B detected in the source instance from peer A)
3. Peer A receives the hello ack and sends another hello request to peer B. The request object contains the following:
   - Source instance = 5 (generated by Peer A for this adjacency)
   - Destination instance = 8 (the source instance generated by Peer B for this adjacency)

The two peers continue exchanging hello messages until the LSP is torn down. The following is true for these message exchanges unless a peer resets:

- Peer A always sends source instance= 5 and destination instance= 8 to Peer B.
- Peer B always sends instance= 8 and destination instance= 5 to Peer A.

## Determination That a Peer Has Reset

After the initial exchange of hello messages, both peers perform checks on the messages they receive to determine whether the peer has reset.

### Behavior of the Requesting Peer

The requesting peer examines the ack messages it receives. It compares the source instance in each subsequent ack message with the previous value. If the value differs or is set to zero, then the requesting peer treats the acknowledging peer as if communication has been lost.

The requesting peer also determines whether the acknowledging peer is reflecting back the requesting peer’s source instance. If the acknowledging peer advertises a wrong value in the destination instance field of the ack message, then the requesting peer treats the acknowledging peer as if communication has been lost.

### Behavior of the Acknowledging Peer

The acknowledging peer examines the request messages it receives. It compares the source instance in each subsequent request message with the previous value. If the value differs or is set to zero, then the acknowledging peer treats the requesting peer as if communication has been lost.

The acknowledging peer also determines whether the requesting peer is reflecting back the acknowledging peer’s source instance. It compares the destination instance value in each request message with the source instance value that it most recently advertised to the requesting peer. If the requesting peer advertises a wrong value in the destination instance field of the request message, then the acknowledging peer treats the requesting peer as if communication has been lost.

### Behavior of Both Peers

When no hello messages are received from a peer within the configured hello interval, the peer is treated as if communication has been lost.

When a peer determines that communication has been lost, it can reinitiate the sending of hello messages. In this case, the peer generates a new source instance different than the one it previously used for communication with its peer.

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