MPLS TE Tunnel Attributes

MPLS TE Tunnel attributes https://tutorzine.com

The MPLS TE tunnel attributes are as follows:

  • Tunnel destination
  • Desired bandwidth
  • Affinity
  • Setup and holding priorities

The tunnel destination is the MPLS TE router ID of the tail end LSR that the tunnel LSP should be routed to. The desired bandwidth of the TE tunnel is the bandwidth requirement of the TE tunnel.

Affinity

A link can have attribute flags associated with it by configuring mpls traffic-eng attribute-flags attributes on it. These attribute flags indicate the resources of the link, the capabilities of the link (for example, whether it is encrypted), or administrative policies. They indicate whether a tunnel that has specific resource needs can cross that link.

The attribute flags are 32 bits with no syntax associated with them. Each bit can be set or left cleared and can have any meaning that the operator wants to associate with it. For example, one bit can indicate a link delay of less than 100 ms; another bit can indicate that the link is transatlantic; and so on.

On the tunnel configuration on the head end router, you can configure affinity bits and a mask to control whether the tunnel is allowed to cross the link with those attribute flags. The affinity bits are also 32 bits in total and are matched one by one with the attribute bits of the links. The mask of the affinity bits indicates whether each specific bit needs to be checked with the corresponding bit in the attribute field of the link. If the nth bit in the mask is set, the nth bit in the Attribute Flags field must match the nth bit in the affinity field of the TE tunnel.

If the bit in the mask is not set, it does not matter whether the two bits in the same position in the affinity field and attribute flags match. The tunnel affinity bits are configured with the tunnel mpls traffic-eng affinity properties [mask mask-value] command on the tunnel interface on the head end router. The properties and mask are from the range 0x0 to 0xFFFFFFFF. The default value of the properties is 0X00000000; the default value for the mask is 0X0000FFFF.


Setup and Holding Priority

MPLS TE tunnels can have different importance in the network. For example, you can think of tunnels that are longer—in terms of router hops—as being more important than the shorter TE tunnels. Alternatively, you can consider MPLS TE tunnels with a greater bandwidth need as more important than tunnels with a smaller bandwidth need. These more important TE tunnels, however, might be signaled later than the less important ones, or they might be configured later.

That can lead to a situation in which the more important TE tunnels are not routed optimally, or they might not find a path that has enough bandwidth. TE tunnels have priorities to avoid such situations and to make sure that the more important TE tunnels can still be routed optimally by preempting the less important TE tunnels.

You can configure two priorities for each TE tunnel: setup and holding. Both the setup priority and holding priority indicate with their relative values whether a TE tunnel can preempt another tunnel. The lower the priority value, the higher the importance. The setup priority indicates how important the tunnel is to preempt the other tunnels, whereas the holding priority indicates how much the weight of that tunnel is to hold on to its reservations on the links.

A tunnel that has a lower setup priority than the holding priority of a second tunnel can preempt that second tunnel. This means that the newly signaled tunnel preempts the existing tunnel. The setup and holding priorities are configured on the TE tunnel with the following command in Cisco devices:

tunnel mpls traffic-eng priority setup-priority [hold-priority]

Important TE tunnels have a low setup priority (so that they can preempt other tunnels) and a low holding priority (so that other tunnels do not preempt them). The values that can be configured for both priorities are 0 to 7, with 0 indicating the highest priority.

Path Setup Option

You can configure the path option on the tunnel configuration on the head end router. You can set up a tunnel in two ways:

  • Explicitly
  • Dynamically

In the explicit way, you must specify every router that the TE tunnel must be routed on, up to and including the tail end router. You can either specify the TE router ID or the link IP address of the intermediate routers.

In the dynamic way, you let the tunnel head end figure out how the TE tunnel should best be routed throughout the network toward the tail end router. In the dynamic way, you only need to configure the destination of the TE tunnel (this means the tail end router). The path from the head end router to the tail end router is calculated on the head end router. This router figures out how to route the TE tunnel by looking at the MPLS TE database learned from either OSPF or IS-IS.

More than one dynamic and explicit path option can be configured, as long as they have a different preference. This preference is a number from 1 to 1000. The lower the number, the more preferred the path option is. Only if the path is not available is the next preferred path option tried. If all of the path options on the tunnel fail, the TE tunnel LSP fails and the tunnel interface on the head end router remains in a down state.

The example 8-7 shows how to use path options. you can use the command show mpls traffic eng tunnels tunnel tunnel-num to verify the state of the TE tunnel.

Explicit Path configuration

Based on the above configuration example The explicit path option has a lower value and is tried first. If that explicit path is unavailable for the TE tunnel 1, the second best path option (here a dynamic path option) is tried.


IP Explicit Address Exclusion

Instead of including an IP address in the explicit path option, you can exclude an IP address from the path that the TE LSP will take. This IP address is the address of the link that should not be included when the path calculation takes place for the TE tunnel. The head end LSR must never use this link to route the TE tunnel LSP through the network.

Remember that, Instead of the IP address of a link, the IP address can also be the MPLS TE router ID of a node. In that case, the LSR with that MPLS TE router ID is excluded when CSPF runs to calculate the path of a TE LSP, thus avoiding a particular router.

Dual TE Metrics

By default, MPLS TE uses the TE metrics of the links to route the TE tunnels; however, by default the TE link metrics are the same as the IGP link metrics. However, you can override this option when you set the TE metrics. You do this with the traffic-eng path-selection metric {igp | te} command on the tunnel interface on the head end router.

MPLS TE dual metric configuration

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