The RSVP-TE extensions enable MPLS to support the creation of explicitly routed LSPs, with or without resource reservation. Several of the features enabled by these extensions were implemented to meet the requirements for traffic engineering over MPLS, which enables the creation of traffic trunks with specific characteristics. None of the TE extensions result in backward compatibility problems with traditional RSVP implementations.
To run properly, the traffic engineering capabilities of RSVP-TE require an underlying TE-enabled IGP routing protocol. The 7705 SAR supports OSPF and IS-IS with TE extensions.
Routing protocols make it possible to advertise the constraints imposed over various links in the network. For example, in order for the nodes in a network to choose the best link for signaling a tunnel, the capacity of a particular link and the amount of reservable capacity must be advertised by the IGP. RSVP-TE makes use of these constraints to request the setup of a path or LSP that traverses only those links that are part of an administrative group (admin groups are described in the following list). Therefore, both RSVP-TE and the IGP-TE (that is, OSPF-TE or IS-IS-TE for the 7705 SAR) must be enabled and running simultaneously.
The following TE capabilities are supported:
hop limit — the hop limit is the maximum number of LSRs that a given LSP can traverse, including the ingress and the egress LERs. Typically, the hop limit is used to control the maximum delay time for mission-critical traffic such as voice traffic.
The hop limit applies to the primary LSP, any backup LSPs, and LSPs configured to be used in Fast Reroute (FRR) situations.
admin groups — administrative groups provide a way to define which LSR nodes should be included or excluded while signaling an LSP. For example, it might be desirable to avoid some nodes or links that are known to be used heavily from being included in the path of an LSP, or to include a specific LSR node to ensure that a newly signaled RSVP-TE tunnel traverses that LSR node.
Administrative groups apply to both primary and secondary LSPs. They are defined under the config>router>if-attribute context, and are applied at the MPLS interface level, as well as at the LSP and the primary and secondary LSP levels through include and exclude commands.
bandwidth — the bandwidth capability (supported by RSVP-TE), is similar to the Connection Admission Control (CAC) function in ATM. During the establishment phase of RSVP-TE, the LSP PATH message contains the bandwidth reservation request. If the requested capacity is available, the RESV message confirms the reservation request. The amount of reserved bandwidth stated in the request is deducted from the amount of reservable bandwidth for each link over which the LSP traverses.
The bandwidth capability applies to both primary and secondary LSPs, and LSPs configured to be used in Fast Reroute (FRR) situations.