MPLS and RSVP
7450 ESS MPLS Guide Page 121
It is required that multiple paths of the same LSP share common link bandwidth since they are
signaled using the Shared Explicit (SE) style. Specifically, two instances of a primary path, one
with the main CT and the other with the backup CT, must temporarily share bandwidth while
MBB is in progress. Also, a primary path and one or many secondary paths of the same LSP must
share bandwidth whether they are configured with the same or different CTs.
Downgrading the CT of Bandwidth Sharing LSP Paths
Consider a link configured with two class types CT0 and CT1 and making use of the RDM
admission control model as shown in Figure 28.
Figure 28: Sharing bandwidth when an LSP primary path is downgraded to backup CT
Consider an LSP path Z occupying bandwidth B at CT1. BC0 being the sum of all CTs below it,
the bandwidth occupied in CT1 is guaranteed to be available in CT0. Thus when new path X of the
same LSP for CT0 is setup, it will use the same bandwidth B as used by path Z as shown in
Figure 28 (a). When path Z is torn down the same bandwidth now occupies CT0 as shown in
Figure 28 (b). Even if there were no new BW available in CT0 as can be seen in Figure 28 (c),
path X can always share the bandwidth with path Z.
CSPF at the head-end node and CAC at the transit LSR node will share bandwidth of an existing
path when its CT is downgraded in the new path of the same LSP.
A
B
C
LSP Z and X share the same BW
When LSP is torn down,
the same BW moves to CT0
CT0
CT1
CT1
BC0 = CT0 + CT1
BC1 = CT1
BC0 = CT0 + CT1
BC1 = CT1
BC0 = CT0 + CT1
BC1 = CT1
In case all BW in CT0 was used, LSP
X BW is still available for sharing
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