MPLS Traffic Engineering

MPLS Traffic Engineering

Traffic engineering (TE), or the ability to steer traffic through a network, has been around for a while, but it was mainly present in ATM or Frame Relay networks. The role of TE is to get the traffic from edge to edge in the network in the most optimal way.

Since early networks rely on a pure IP solution or IP running over an MPLS, therefore, MPLS TE solution can run for IP networks. In short, MPLS TE is a solution to cover following:

  • MPLS TE provides efficient spreading of traffic throughout the network, avoiding underutilized and overutilized links.
  • MPLS TE takes into account the configured (static) bandwidth of links.
  • MPLS TE takes link attributes into account (for instance, delay, jitter).
  • MPLS TE adapts automatically to changing bandwidth and link attributes.
  • Source-based routing is applied to the traffic-engineered load as opposed to IP destination based routing.

MPLS TE Head End Router

MPLS TE allows for a TE scheme where the head end router of a label switched path (LSP) can calculate the most efficient route through the network toward the tail end router of the LSP. The head end router can do that if it has the topology of the network. Furthermore, the head end router needs to know the remaining bandwidth on all the links of the network. Finally, you need to enable MPLS on the routers so that you can establish LSPs end to end. 

The fact that label switching is used and not IP forwarding allows for source-based routing instead of IP destination-based routing. That is because MPLS does forwarding in the data plane by matching an incoming label in the label forwarding information base (LFIB) and swapping it with an outgoing label.

Therefore, it is the head end label switching router (LSR) of the LSP that can determine the routing of the labeled packet, after all LSRs agree which labels to use for which LSP. The figure below shows an example of this source-based routing ability of MPLS TE.

MPLS TE Head End Router
Figure 1

To illustrate this concept, routers R6 and R7 have been added in front of router R1. Assume that routers R6 and R7 want to send traffic to R5. If this network is running IP forwarding only, this traffic follows the path R1-R2-R5 only, no matter what you configured on routers R6 and R7. That is because the forwarding of IP packets is done independently on every hop in the network.

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)
  • 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 
The figure below has the TE building blocks in the network from Figure 1. One TE tunnel or LSP extends from R6 to R5.
MPLS TE Building Blocks

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.

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).

Distribution of TE Information

A link state routing protocol needs to flood the constraints of the links in the network to all routers that are running TE. In the next sections, you can see what link information the routing protocol needs to flood and how OSPF and IS-IS have been extended to carry this TE information.

Requirements for the IGP

The Interior Gateway Protocol (IGP) needs to be capable of sending all the topology information (the state of the links) to all routers in the area in which TE has been enabled. Only a link state protocol can perform this task because it floods the state of all links of a router to all the routers in one area. Therefore, every router in the area knows all alternative paths to get to the destination.

The head end of the TE tunnel must have all topology information to see all the possible paths, but it must also have all the constraints information of the links available to it. This constraint information is the collection of resource information of the links that are associated with TE. The link state routing protocol must be extended to carry this extra resource information. The TE resources of a link are as follows:

  • TE metric
  • Maximum bandwidth
  • Maximum reservable bandwidth
  • Unreserved bandwidth
  • Administrative group

The TE metric is a parameter that you can use to construct a TE topology that is different from the IP topology. As such, the TE metric of a link can be different from the OSPF cost or IS-IS metric of the link. The maximum bandwidth is the total bandwidth of the link.

The maximum reservable bandwidth is obviously the bandwidth available to TE on the link. You set this by using the ip rsvp bandwidth command.

The unreserved bandwidth is the remainder of the bandwidth that is available to TE. It is the maximum reservable bandwidth minus whatever bandwidth is currently reserved by TE tunnels crossing this link.

The administrative group is a 32-bit field with no further syntax. The operator of the network can individually set each bit of this 32-bit field and can have a meaning chosen by him. For example, one bit might mean that the link is a pos link with a speed greater than OC 48, or a link that is intercontinental, or a link that has a delay smaller than 100 ms.

One link can have multiple resources associated with it, with a maximum of 32. These resources are flooded throughout the area whenever they change in value or at regular intervals. The flooding of these IGP changes is backward compatible. This means that not all routers must support these changes before you can run MPLS TE. Routers that do not understand the IGP changes for TE just ignore them.

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