3Com MSR 50 Series Configuration Manual page 1351

Make-before-break
Make-before-break is a mechanism to change MPLS TE tunnel attributes with
minimum data loss and without extra bandwidth.
Figure 385 Diagram for make-before-break
Router A Router B
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Router E
presents a scenario where a path Router A → Router B → Router C →
Figure 385
Router D is established with 30 Mbps reserved bandwidth between Router A and
Router B. 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 will be 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 may 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.
RSVP-TE messages
RSVP-TE use RSVP messages with extensions. The following are RSVP messages:
Path messages: transmitted along the path of data transmission downstream
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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
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resource reservation and to create and maintain reservation state on each node
along the reverse of data transmission path.
PathTear messages: sent downstream immediately once created to remove the
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path state and related reservation state on each node along the path.
ResvTear messages: sent upstream immediately once created to remove the
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reservation state on each node along the path.
PathErr messages: sent upstream to report Path message processing errors to
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senders. They do not affect the state of the nodes along the path.
ResvErr messages: sent downstream to notify the downstream nodes that error
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occurs during Resv message processing or reservation error occurs as the result
of preemption.
ResvConf messages: sent to receivers to confirm Resv messages.
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Router C
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MPLS TE Overview
Router D
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