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Understanding the RSVP Signaling Protocol

RSVP Fundamentals

RSVP uses unidirectional and simplex flows through the network to perform its function. The inbound router initiates an RSVP path message and sends it downstream to the outbound router. The path message contains information about the resources needed for the connection. Each router along the path begins to maintain information about a reservation.

When the path message reaches the outbound router, resource reservation begins. The outbound router sends a reservation message upstream to the inbound router. Each router along the path receives the reservation message and sends it upstream, following the path of the original path message. When the inbound router receives the reservation message, the unidirectional network path is established.

The established path remains open as long as the RSVP session is active. The session is maintained by the transmission of additional path and reservation messages that report the session state every 30 seconds. If a router does not receive the maintenance messages for 3 minutes, it terminates the RSVP session and reroutes the LSP through another active router.

Bandwidth Reservation Requirement

When a bandwidth reservation is configured, reservation messages propagate the bandwidth value throughout the LSP. Routers must reserve the bandwidth specified across the link for the particular LSP. If the total bandwidth reservation exceeds the available bandwidth for a particular LSP segment, the LSP is rerouted through another LSR. If no segments can support the bandwidth reservation, LSP setup fails and the RSVP session is not established.

Explicit Route Objects

Explicit Route Objects (EROs) limit LSP routing to a specified list of LSRs. By default, RSVP messages follow a path that is determined by the network IGP's shortest path. However, in the presence of a configured ERO, the RSVP messages follow the path specified.

EROs consist of two types of instructions: loose hops and strict hops. When a loose hop is configured, it identifies one or more transit LSRs through which the LSP must be routed. The network IGP determines the exact route from the inbound router to the first loose hop, or from one loose hop to the next. The loose hop specifies only that a particular LSR be included in the LSP.

When a strict hop is configured, it identifies an exact path through which the LSP must be routed. Strict-hop EROs specify the exact order of the routers through which the RSVP messages are sent.

You can configure loose-hop and strict-hop EROs simultaneously. In this case, the IGP determines the route between loose hops, and the strict-hop configuration specifies the exact path for particular LSP path segments.

Figure 1 shows a typical RSVP-signaled LSP that uses EROs.

Figure 1: Typical RSVP-Signaled LSP with EROs

In the topology shown in Figure 1, traffic is routed from Host C1 to Host C2. The LSP can pass through Routers R4 or Router R7. To force the LSP to use R4, you can set up either a loose-hop or strict-hop ERO that specifies R4 as a hop in the LSP. To force a specific path through Router R4, R3, and R6, configure a strict-hop ERO through the three LSRs.

Constrained Shortest Path First

Whereas IGPs use the Shortest Path First (SPF) algorithm to determine how traffic is routed within a network, RSVP uses the Constrained Shortest Path First (CSPF) algorithm to calculate traffic paths that are subject to the following constraints:

• LSP attributes—Administrative groups such as link coloring, bandwidth requirements, and EROs
• Link attributes—Colors on a particular link and available bandwidth

These constraints are maintained in the traffic engineering database (TED). The database provides CSPF with up-to-date topology information, the current reservable bandwidth of links, and the link colors.

In determining which path to select, CSPF follows these rules:

• Computes LSPs one at a time, beginning with the highest priority LSP—the one with the lowest setup priority value. Among LSPs of equal priority, CSPF starts with those that have the highest bandwidth requirement.
• Prunes the traffic engineering database of links that are not full duplex and do not have sufficient reservable bandwidth.
• If the LSP configuration includes the include statement, prunes all links that do not share any included colors.
• If the LSP configuration includes the exclude statement, prunes all links that contain excluded colors. If a link does not have a color, it is accepted.
• Finds the shortest path toward the LSP's outbound router, taking into account any EROs. For example, if the path must pass through Router A, two separate SPF algorithms are computed: one from the inbound router to Router A and one from Router A to the outbound router.
• If several paths have equal cost, chooses the one with a last-hop address the same as the LSP's destination.
• If several equal-cost paths remain, selects the path with the least number of hops.
• If several equal-cost paths remain, applies CSPF load-balancing rules configured on the LSP.

Link Coloring

RSVP allows you to configure administrative groups for CSPF path selection. An administrative group is typically named with a color, assigned a numeric value, and applied to the RSVP interface for the appropriate link. Lower numbers indicate higher priority.

After configuring the administrative group, you can either exclude, include, or ignore links of that color in the TED:

• If you exclude a particular color, all segments with an administrative group of that color are excluded from CSPF path selection.
• If you include a particular color, only those segments with the appropriate color are selected.
• If you neither exclude nor include the color, the metrics associated with the administrative groups and applied on the particular segments determine the path cost for that segment.

The LSP with the lowest total path cost is selected into the TED.