45
Each LSP set up using RSVP-TE is assigned a resource reservation style. During an RSVP session, the
receiver decides which reservation style can be used for this session and which LSPs can be used.
The following reservation styles are available:
{
Fixed-filter style (FF) where resources are reserved for individual senders and cannot be shared
among senders on the same session.
{
Shared-explicit style (SE) where resources are reserved for senders on the same session and
shared among them.
NOTE:
SE is only used for make-before-break because multiple LSPs cannot be present on the same session.
Make-before-break
Make-before-break is a mechanism to change MPLS TE tunnel attributes with minimum data loss and
without extra bandwidth.
Figure 15
Diagram for make-before-break
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 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. This helps avoid traffic interruption
effectively.
RSVP-TE messages
RSVP-TE uses RSVP messages with extensions. The following are RSVP 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 immediately once created to remove the path state and
related reservation state on each node along the path.
•
ResvTear messages: sent upstream immediately once created to remove the reservation state on
each node along the path.
