Bandwidth Reservation

As mentioned previously, RSVP reservations are unidirectional in nature.

The source initiates the reservation procedure by transmitting a path message containing a sender Tspec object. The Tspec describes the source traffic characteristics in terms of peak data rate, average data rate, burst size, and minimum/maximum packet sizes. The path message can also contain an optional AdSpec object that is updated by network elements along the path to indicate information such as the availability of particular QoS services, the maximum bandwidth available along the path, the minimum path latency, and the path maximum transmission unit (MTU).

The ExtremeXOS software supports LSR bandwidth reservation requests per LSP. Only the Int-Serv Controlled-Load service request is supported. Bandwidth is always reserved on the physical ports that the LSP traverses. Depending on the platform, the bandwidth reservation may also be policed. The network administrator can verify that the requested bandwidth was actually reserved. In those cases when the bandwidth reserved is less than the requested bandwidth, the LSP can be manually torn down, re-signaled using a different path, or accepted. The LSR automatically attempts to find a path that best satisfies the bandwidth request. Constrained path selections are supported using OSPF (Open Shortest Path First)-TE. Best effort LSPs are provisioned by specifying a reserved bandwidth as best-effort. The reserved LSP bandwidth is configured by setting the bps rate for a specific RSVP-TE profile, using the configure mpls rsvp-te lsp profile lsp_profile_name bandwidth command and associating the profile to an LSP.

Accounting of bandwidth reserved through an Extreme LSR RSVP-TE enabled VLAN is supported. The maximum available bandwidth per physical port or trunk group is enforced. Thus, the available bandwidth specified in the Adspec object is not modified as the path message is forwarded to the LSP endpoint. As reserve messages are processed, the reserved bandwidth specified in the Flowspec is added to the total reserved bandwidth allocated for the physical ports.

Because LSP bandwidth is dynamically allocated, a configuration command is provided to reserve port bandwidth for use by MPLS (Multiprotocol Label Switching). The command configure mpls rsvp-te bandwidth committed-rate pre-reserves bandwidth from the specified MPLS enabled VLAN for RSVP-TE traffic only. This pre-allocation of bandwidth is useful since other applications may compete with MPLS for available bandwidth. By pre-reserving a portion of the MPLS interface's bandwidth capacity, MPLS is guaranteed to have that amount of the MPLS interface's bandwidth to meet RSVP-TE LSP reservation requests.

CIR bandwidth for the receive direction is not tracked by TE IGPs, such as OSPF-TE, and configuring it is not required. Configuring CIR bandwidth for the receive direction does not prevent an LSP from going operational due to lack of receive bandwidth; however, it can be useful for tracking and informational purposes. An Info level log (MPLS.RSVPTE.IfRxBwdthExcd) is generated if the setup of a TE LSP requires receive bandwidth greater than that which is currently available for the receive direction on a particular interface. This generally happens only when TE LSPs with different previous hops ingress the switch on the same interface (for example, from a multi-access link) and egress the switch on different interfaces.