Primary paths are optional and when configured, limit the RSVP calculation of the complete path to the routers specified in the primary Explicit Route Object (ERO) list, which determines the physical path for the LSP. When primary paths are not configured, the ingress router determines the path to the egress router. Only one primary path is permitted per LSP, as shown in Figure 2.
Within the configuration of the primary physical path, you can specify strict or loose ERO values and parameters that affect only the primary physical path, such as bandwidth or priority. The ERO list for the primary path includes an address for each transit router. Specifying the ingress and/or egress routers is optional. For each router address, you can specify the type, which can be one of the following:
- Strict—The route taken from the previous router to this router is a direct path and cannot include any other routers. This is the default. If the address is an interface address, this router also ensures that the incoming interface is the one specified. Specifying the incoming interface is important when there are parallel links between the previous router and this router, and because it ensures that routing can be enforced on a per-link basis.
For strict addresses, you must ensure that the router immediately preceding the router you are configuring has a direct connection to that router. The address can be a loopback interface address, in which case the incoming interface is not checked.
- Loose—The route taken from the previous router to this router need not be a direct path, can include other routers, and can be received on any interface. The address can be any interface address or the address of the loopback interface.
If you are listing more than one address, specify the addresses in order, starting with the ingress router (optional) or the first transit router, and continuing sequentially along the path up to the egress router (optional) or the router immediately before the egress router. You need to specify only one address per router hop. If you specify more than one address for the same router, only the first address is used; the additional addresses are ignored and truncated.
When configuring a primary path, you can specify the bandwidth and priority values associated with that primary path.
The bandwidth value is included in the sender's Tspec field in RSVP path setup messages. You specify the bandwidth value in bits per second, with a higher value implying a greater user traffic volume. The default bandwidth is 0 bits per second. A nonzero bandwidth requires transit routers to reserve capacity along the outbound links for the path. The RSVP reservation scheme is used to reserve this capacity. Any failure in bandwidth reservation (such as failures at RSVP policy control or admission control) might cause the LSP setup to fail.
The priority value is composed of two distinct values: a setup and a hold priority. The setup priority value is used to determine if there is enough bandwidth available at that priority level to establish the primary path. The priority level is between 0 (best) and 7 (worst).The hold priority value is used by an established primary path to retain its bandwidth reservations in the network. If insufficient link bandwidth is available during session establishment, the setup priority is compared to the hold priorities of other established sessions to determine whether some of them should be preempted to accommodate the new session. Sessions with worse hold priorities are preempted.