There are a couple of admission control decisions made when an LSP with a specified bandwidth is to be signaled. The first is in the head-end node. CSPF only considers network links that have sufficient bandwidth. Link bandwidth information is provided by IGP TE advertisement by all nodes in that network.
Another decision made is local CAC and is performed when the RESV message for the LSP path is received in the reverse direction by a SR OS in that path. The bandwidth value selected by the egress LER is checked against link bandwidth, otherwise the reservation is rejected. If accepted, the new value for the remaining link bandwidth is advertised by IGP at the next advertisement event.
Both of these admission decisions are enhanced to be performed at the TE class level when DiffServ TE is enabled. In other words, CSPF in the head-end node must check the LSP bandwidth against the ‛unreserved bandwidth’ advertised for all links in the path of the LSP for that TE class which consists of a combination of a CT and a priority. Same for the admission control at SR OS receiving the Resv message.