Diff-Serv TE LSP Class Type Change under Failure
Page 122 7450 ESS MPLS Guide
Upgrading the CT of Bandwidth Sharing LSP Paths
When upgrading the CT the following issue can be apparent. Assume an LSP path X exists with
CT0. An attempt is made to upgrade this path to a new path Z with CT1 using an MBB.
Figure 29: Sharing Bandwidth When an LSP Primary Path is Upgraded to Main CT
In Figure 29 (a), if the path X occupies the bandwidth as shown it can not share the bandwidth
with the new path Z being setup. If a condition exists, as shown in Figure 29, (b) the path Z can
never be setup on this particular link.
Consider Figure 29 (c). The CT0 has a region that overlaps with CT1 as CT0 has incursion into
CT1. This overlap can be shared. However, in order to find whether such an incursion has
occurred and how large the region is, it is required to know the reserved bandwidths in each class.
Currently, IGP-TE advertises only the unreserved bandwidths. Hence, it is not possible to compute
these overlap regions at the head end during CSPF. Moreover, the head end needs to then try and
mimic each of the traversed links exactly which increases the complexity.
CSPF at the head-end node will only attempt to signal the LSP path with an upgraded CT if the
advertised bandwidth for that CT can accommodate the bandwidth. In other words, it will assume
that in the worst case this path will not share bandwidth with another path of the same LSP using a
lower CT.
al_0211
A
C
CT1
BC0 = CT0 + CT1
BC1 = CT1
B
BC0 = CT0 + CT1
BC1 = CT1
BC0 = CT0 + CT1
BC1 = CT1
Shareable region due to CTO occupying CT1bandwidth
No BW
available
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