Juniper JUNOSE 11.2.X BGP AND MPLS, Configuration Manual

Page: 297 / 742

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)
261 Copyright © 2010, Juniper Networks, Inc.
Chapter 3: MPLS Overview