HP HSR6800 Configuration Manual page 50

Figure 15 Diagram for make-before-break
Figure 15 presents a scenario where a path Router A — Router B — Router C — Router D is
established with 30 Mbps reserved bandwidth between Router A and Router D. The remaining
bandwidth is then 30 Mbps.
If 40 Mbps path bandwidth is requested, the remaining bandwidth of the Router A—Router
B—Router C—Router D path is inadequate. The problem cannot be addressed by selecting another
path, Router A—Router E—Router C—Router D, because the bandwidth of the Router C—Router D
link is inadequate.
To address the problem, you can use the make-before-break mechanism. It allows the new path to
share the bandwidth of the original path at the Router C—Router D link. Upon creation of the new
path, traffic is switched to the new path and the previous path is torn down. This helps avoid traffic
interruption effectively.
RSVP-TE messages
RSVP-TE uses RSVP messages with extensions. RSVP uses the following types of messages:
•
Path messages—Transmitted along the path of data transmission downstream by each RSVP
sender to save path state information on each node along the path.
•
Resv messages—Sent by each receiver upstream towards senders to request resource
reservation and to create and maintain reservation state on each node along the reverse of data
transmission path.
•
PathTear messages—Sent downstream to remove the path state and related reservation state
on each node along the path.
•
ResvTear messages—Sent upstream to remove the reservation state on each node along the
path.
•
PathErr messages—Sent upstream to report Path message processing errors to senders.
They do not affect the state of the nodes along the path.
•
ResvErr messages—Sent downstream to notify the downstream nodes that an error occurs
during Resv message processing or that a reservation error occurs because of preemption.
•
ResvConf messages—Sent to receivers to confirm Resv messages.
•
Hello messages—Sent between any two directly connected RSVP neighbors to set up and
maintain the neighbor relationship that has local significance on the link.
The TE extension to RSVP adds new objects to the Path message and the Resv message. These
objects carry not only label bindings but also routing constraints, supporting CR-LSP and FRR.
•
New objects added to the Path message include LABEL_REQUEST, EXPLICIT_ROUTE,
RECORD_ROUTE, and SESSION_ATTRIBUTE.
•
New objects added to the Resv message include LABEL and RECORD_ROUTE
The LABEL_REQUEST object in the Path message requests the label bindings for an LSP. It is also
saved in the path state block. The node receiving LABEL_REQUEST advertises the label binding
using the LABEL object in the Resv message to the upstream node, accomplishing label
advertisement and transmission.
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