Overview of the Operation of Mpls Te

Following is what MPLS TE needs to make it work. These are the building blocks of MPLS TE:

■ Link constraints (how much traffic each link can support and which TE tunnel can use the link)

Overview of the Operation of MPLS TE 253

■ TE information distribution (by the MPLS TE-enabled link-state routing protocol)

■ An algorithm (path calculation [PCALC]) to calculate the best path from the head end LSR to the tail end LSR

■ A signaling protocol (Resource Reservation Protocol [RSVP]) to signal the TE tunnel across the network

■ A way to forward traffic onto the TE tunnel

Figure 8-3 has the TE building blocks in the network from Figure 8-2. One TE tunnel or LSP extends from R6 to R5.

Figure 8-3 MPLS TE Building Blocks

Figure 8-3 MPLS TE Building Blocks

OSPF or IS-IS Distributing TE Information

The first name used at Cisco for MPLS TE was Routing with Resource Reservation, also known

as RRR or R3 (read as R cubed). This name indicates that an important reason to have MPLS TE is the routing or steering of traffic according to resources or constraints. These resources are the bandwidth of the links and some attributes of the links that the operator specifies. These attributes are configured on the links and advertised by the link state protocol. (This means OSPF or IS-IS.) Instead of creating a new protocol to carry this information and advertise it to all the LSRs, OSPF and IS-IS were extended to piggyback this information. When you configure a TE tunnel on an LSR, it becomes the head end LSR of that TE tunnel or TE LSP. You then specify the destination LSR of the TE tunnel and the constraints it must adhere to. For example, you can specify the bandwidth requirement of the tunnel.

Inside Cisco IOS, a TE database is built from the TE information that the link state protocol sends. This dataset contains all the links that are enabled for MPLS TE and their characteristics or attributes. From this MPLS TE database, path calculation (PCALC) or constrained SPF (CSPF) calculates the shortest route that still adheres to all the constraints (most importantly the bandwidth) from the head end LSR to the tail end LSR. PCALC or CSPF is a shortest path first (SPF) algorithm modified for MPLS TE, so that constraints can be taken into account. The bandwidth available to TE and the attributes are configurable on all links of the networks. You configure the bandwidth requirement and attributes of the TE tunnel on the tunnel configuration of the head end LSR. PCALC matches the bandwidth requirement and attributes of the TE tunnel with the ones on the links, and from all possible paths, it takes the shortest one. The calculation is done on the head end LSR.

The intermediate LSRs on the LSP need to know what the incoming and outgoing labels are for the particular LSP for that TE tunnel. The intermediate LSRs can only learn the labels if the head end router and intermediate LSRs signal the labels by a signaling protocol. In the past, two signaling protocols were proposed: RSVP with extensions for TE (RSVP-TE) and constraint-based LDP (CR-LDP). Cisco IOS has RSVP with extensions for signaling MPLS TE tunnels and never had an implementation of CR-LDP. At the Internet Engineering Task Force (IETF), consensus was reached to carry on with developing RSVP as the signaling protocol for MPLS TE and to stop further development on CR-LDP. This was documented in RFC 3468, "The Multiprotocol Label Switching (MPLS) Working Group Decision on MPLS Signaling Protocols." The following is a quotation from the abstract of that RFC:

This document documents the consensus reached by the Multiprotocol Label Switching (MPLS) Working Group within the IETF to focus its efforts on "Resource Reservation Protocol (RSVP)-TE: Extensions to RSVP for Label-Switched Paths (LSP) Tunnels" (RFC 3209) as the MPLS signalling protocol for traffic engineering applications and to undertake no new efforts relating to "Constraint-Based LSP Setup using Label Distribution Protocol (LDP)" (RFC 3212).

Extensions were made to RSVP to allow it to carry the MPLS label information and other TE specifics, such as the Explicit Route and Record Route objects. In essence, RSVP tries to signal the TE tunnel along the path—from head end LSR to tail end LSR—which is the result from the calculation based on the TE database on the head end LSR. RSVP needs to signal it to get the label information set up at each LSR. The RSVP PATH message is sent from head end LSR to tail end LSR and carries a request for an MPLS label. The RSVP RESV message sent back from the tail end LSR to the head end LSR carries the MPLS label that each LSR along the TE tunnel LSP can use to forward the TE traffic. RSVP also verifies that the TE tunnel with the constraints can be set up at each node. The latter part should not be a problem, because OSPF or IS-IS has advertised this information. Thus, the head end should have calculated a feasible path through the network for the TE LSP. However, it might be that another TE tunnel just reserved an amount of bandwidth on a link of an intermediate LSR, and OSPF or IS-IS has not advertised this yet. Therefore, it might be that the remainder of the bandwidth at that link is no longer sufficient for this TE tunnel to be set up—hence the need for a signaling protocol to make sure the bandwidth is reserved at each hop.

How is the RSVP PATH message routed through the network? The Explicit Route object (ERO) details the hops that the RSVP PATH message must follow to signal the TE tunnel. The series of hops or path is the result of the path calculation on the head end router. At each hop, this PATH message temporarily reserves the bandwidth and requests a label. Eventually, the PATH message gets to the tail end of the LSP, which returns a RESV message to the head end of the LSP. This RESV message then returns a label that the MPLS data plane can use to forward the packets of this MPLS TE tunnel along the LSP. The RESV message also tells the intermediate LSR to reserve the resources for the links that the TE LSP uses.

The most important task is for you to ensure that traffic is forwarded on to the TE tunnels. A later section "Forwarding Traffic onto MPLS TE Tunnels" of this chapter explains the different methods to achieve this.

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