Hello Message Objects; Hello Message Instances; Sequence Of Hello Message Exchange - Juniper BGP - CONFIGURATION GUIDE V 11.1.X Configuration Manual

JUNOSe 11.1.x BGP and MPLS Configuration Guide

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.
2.
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Determining Peer Reachability with RSVP-TE Hello Messages
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)
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)