How to configure MPLS basic traffic engineering using OSPF

[TCC_2]

Core Issue

Multiprotocol Label Switching - Traffic Engineering (MPLS-TE) provides an integrated approach to traffic engineering by combining 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. Open Shortest Path First (OSPF) protocol uses Type-10 Link-State Advertisements (LSAs) known as Opaque LSAs to carry information associated with TE. OSPF-TE extensions define various Type Length Value (TLV) fields inside the Type-10 LSA.

Resolution

MPLS-TE builds uni-directional 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 and determine the path using constraint-based Shortest Path First (SPF) to find the shortest path that meets the resource requirements and setting up the path using Resource Reservation Protocol (RSVP) as the signaling protocol.

To configure MPLS-TE using OSPF, perform these steps:

1. To configure basic OSPF as the Interior Gateway Protocol (IGP), perform these steps:
   1. Issue the router ospf x command in global configuration mode, where x is the process ID local to the router.
   2. Include the interfaces on the router to run OSPF as part of a specific area by issuing the network command under router configuration mode.
2. To configure OSPF for MPLS-TE from router configuration mode, perform these steps:
   1. 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 TE router ID by issuing the mpls traffic-eng router-id command with the corresponding loopback interface; it should be the same as the regular OSPF router ID.
   2. TE is generally enabled within a single OSPF area. Configure the area for which TE is enabled by issuing the mpls traffic-eng area x command, where x refers to the area number.
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 under global configuration mode as well as under the specific interfaces.
4. To configure MPLS-TE, perform these steps:
   1. Enable 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 which will be part of the tunnel.
   2. Configure an MPLS-TE tunnel interface by issuing the interface tunnel command from global configuration mode and then issuing the tunnel mode mpls traffic-eng command under the tunnel interface.
   3. Configure an IP address on the tunnel interface to forward IP packets. The IP address is configured as an unnumbered interface by using the address of a loopback interface. This is done by issuing the ip unnumbered command.
   4. Configure the tunnel destination to which an LSP is built by issuing 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 from 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. Configure the tunnel setup and hold priority by issuing the tunnel mpls traffic-eng priority command. This is useful when multiple tunnels compete for the resources available. Least priority tunnels are teared down to accommodate higher priority tunnels.
5. To configure RSVP, enable RSVP on the physical interfaces and configure the available bandwidth by issuing the ip rsvp bandwidth command under interface configuration mode. RSVP with TE extensions is used for reserving the resources based on the path determined by constraint-based SPF, and to exchange labels and setup the LSP for the tunnels.

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

For an example of MPLS-TE configuration using OSPF, refer to MPLS Basic Traffic Engineering Using OSPF Configuration Example.