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How to configure MPLS basic Traffic Engineering using IS-IS

Core Issue

Multiprotocol Label Switching - Traffic Engineering (MPLS-TE) provides an integrated approach to traffic engineering. MPLS-TE combines the traffic engineering capabilities of ATM with the flexibility and Class of Service (CoS) differentiation of IP. MPLS-TE maps traffic flows to a particular path based on the resources the traffic flow requires and the available resources.

Since the router has to have the complete information about the topology and resources available in a network, a link-state routing protocol is necessary for use with MPLS-TE. Integrated Intermediate System-to-Intermediate System (IS-IS) has been extended with new Type Length Values (TLVs) to carry information pertaining to TE. Since the original IS-IS protocol had a limit of six bits for the metric value, this also has been increased to 24 bits as part of this extension. Support for these new TLVs have to be enabled when IS-IS is used for MPLS-TE.

Resolution

MPLS-TE builds unidirectional tunnels from a source to the destination in the form of Label Switched Paths (LSPs), which is then used for forwarding traffic. The point where the tunnel begins is called the tunnel headend or tunnel source, and the node where the tunnel ends is called the tunnel tail-end or tunnel destination. The tunnels are built based on the information learned through the link-state routing protocols. They determine the path using constraint-based Shortest Path First (SPF) to find the shortest path that meets the resource requirements and set up the path using Resource Reservation Protocol (RSVP) as the signaling protocol. 

Configuring MPLS-TE using IS-IS can be broken into these sub-tasks:

  1. To configure basic IS-IS as Interior Gateway Protocol (IGP), perform these steps:
    1. Issue the router isis command in global configuration mode.
    2. To assign the area and system ID, configure the IS-IS Network Entity Title (NET) and issue the net command in router configuration mode.
    3. To include the interfaces on the router to run IS-IS for IP, issue the ip router isis commands under the specific interfaces.
  2. To configure IS-IS for MPLS-TE from router configuration mode, perform these steps:

    1. Enable the support for new TLVs by issuing the metric-style command with the wide, transition, or wide transition option.
    2. A loopback interface with a /32 address has to be used as the router ID for establishing TE tunnels and is necessary for proper operation. Configure the router ID by issuing the mpls traffic-eng router-id command with the corresponding loopback interface.
    3. TE is generally enabled within a single level for IS-IS, Level 1 (L1) or Level 2 (L2). Configure the level for which TE is enabled by issuing the mpls traffic-eng command.
  3. To configure MPLS for forwarding labeled packets, perform these steps:

    1. Since Cisco Express Forwarding (CEF) is a prerequisite for configuring MPLS on Cisco devices, configure CEF by issuing the ip cef command in global configuration mode.
    2. To configure MPLS, issue the tag-switching ip command or the mpls ip command in global configuration mode as well as under the specific interfaces.
  4. To configure MPLS-TE, perform these steps:

    1. Enable the MPLS-TE feature on the router by issuing the mpls traffic-eng tunnels command from global configuration mode as well as under the specific physical interfaces that will be part of the tunnel.
    2. To configure a MPLS-TE tunnel interface, issue the interface tunnel command from global configuration mode and then the tunnel mode mpls traffic-eng command under the tunnel interface.
    3. Configure an IP address on the tunnel interface to forward IP packets. Generally, the IP address is configured as an unnumbered interface using the address of a loopback interface. This is done by issuing the ip unnumbered command.
    4. To configure the tunnel destination to which an LSP is built, issue the tunnel destination command. This must be the MPLS-TE router ID configured on the tunnel interface at the other end.
    5. To configure how much of the available bandwidth will be used, issue the tunnel mpls traffic-eng bandwidth command under the tunnel interface.
    6. The MPLS-TE tunnel can be configured to use multiple path options from the source to the destination so that it can choose the path it needs in the order of preference. Paths can be explicitly configured or dynamically established using the information available in the topology table and the constraints imposed. Configure the path options by issuing the tunnel mpls traffic-eng path-option command under the tunnel interface. If there is any explicit path configuration involved, define the path to be used by issuing the ip explicit-path command in global configuration mode.
    7. To forward packets through the MPLS-TE tunnel, use policy routing or configure the static route for the tunnel destination address pointing to the tunnel interface. Alternatively, you can issue the tunnel mpls traffic-eng autoroute announce command to automatically install a route in the routing table for the tunnel destination and other networks that are available behind the tunnel destination.
    8. To configure the tunnel setup and hold priority, issue the tunnel mpls traffic-eng priority command. This is useful when multiple tunnels compete for the resources available. The least priority tunnels are teared down to accommodate higher priority tunnels.
  5. To configure RSVP, proceed with this option:

    • RSVP with TE extensions is used for reserving the resources based on the path determined by constraint-based SPF and to exchange labels and set up the LSP for the tunnels. Enable RSVP on the physical interfaces and configure the available bandwidth by issuing the ip rsvp bandwidth command in interface configuration mode.

For more information on configuring MPLS-TE, refer to MPLS Traffic Engineering.

For an example of MPLS-TE configuration using IS-IS, refer to Configuring MPLS Basic Traffic Engineering Using IS-IS.


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