Understanding the RSVP Signaling Protocol

RSVP is a signaling protocol that handles bandwidth allocation and true traffic engineering across an MPLS network. Like LDP, RSVP uses discovery messages and advertisements to exchange LSP path information between all hosts. However, RSVP also includes a set of features that control the flow of traffic through an MPLS network. Whereas LDP is restricted to using the configured IGP's shortest path as the transit path through the network, RSVP uses a combination of the Constrained Shortest Path First (CSPF) algorithm and Explicit Route Objects (EROs) to determine how traffic is routed through the network.

Basic RSVP sessions are established in exactly the same way that LDP sessions are established. By configuring both MPLS and RSVP on the appropriate transit interfaces, you enable the exchange of RSVP packets and the establishment of LSPs. However, RSVP also lets you configure link authentication, explicit LSP paths, and link coloring.

This topic contains the following sections:

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 2 shows a typical RSVP-signaled LSP that uses EROs.

Figure 2: Typical RSVP-Signaled LSP with EROs

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In the topology shown in Figure 2, 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:

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:

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:

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

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