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    IP Data Packet Mapping onto MPLS LSPs Overview

    IP packets are mapped onto LSPs by one of the following methods:

    • RSVP-TE tunnels can be referenced directly by static routes that you configure. You can determine which routes (routes destined for which subnets) to direct through the LSP and issue the appropriate ip route commands, as shown in the following example:
      host1(config-if)#ip route 10.15.21.16 tunnel mpls:1

      You cannot create any static routes until the tunnel interface has been created. However, the tunnel does not have to be active before you create the static routes.

    • RSVP-TE tunnels are announced to IS-IS and OSPF; the IGP then uses the tunnels as next hop interfaces for its SPF calculations. For this method, you must issue the tunnel mpls autoroute announce command. When the LSP is established, the ingress LSR announces the LSP endpoint to the IGP. This is also referred to as registering the LSP. The IGP then recalculates the shortest path for all routes destined for or beyond that endpoint. You can choose to register endpoints with both IS-IS and OSPF. The following is an example registration command:
      host1(config-if)#tunnel mpls autoroute announce isis
    • For topology-driven LSPs, LDP can modify the IP routing table to use MPLS next hops in the routing table, replacing the regular IP next hops for the corresponding routes.
    • For labeled BGP routes, BGP adds routes with MPLS next hops to the appropriate VR or VRF routing table.

    When IP packets arrive at the ingress LER, they are looked up in the relevant IP forwarding table and then are forwarded into an LSP. Every IP route eventually points to an IP interface. The IP interface contains IP attributes that affect how the IP packet is forwarded. IPv4 routes point only to IPv4 interfaces and IPv6 routes point only to IPv6 interfaces.

    Because IP routes cannot point directly to MPLS major interfaces, MPLS automatically creates up to four dynamic IP shared interfaces that are stacked on each MPLS major interface. When you issue the mpls command in Interface Configuration mode, the interfaces are created dynamically and provide the interfaces an IP route needs to point to. You can specify a profile (created with the profile command) to configure attributes for these interfaces with the mpls create-dynamic-interfaces command. You can use the same command to enable or disable the creation of specific interface types or all types.

    Each dynamic interface is one of the following types:

    • By default, MPLS creates one dynamic IPv4 interface per MPLS major interface for non-VPN traffic. This interface is used by default for VPN traffic as well.
    • By default, but only if IPv6 is enabled in the virtual router, MPLS creates one dynamic IPv6 interface per MPLS major interface for non-VPN traffic. This interface is used by default for VPN traffic as well.
    • If you configure it to do so, MPLS creates one dynamic IPv4 interface per MPLS major interface for VPN traffic. If this interface is not created, then the VPN traffic uses the default IPv4 interface for non-VPN traffic.

      Typically, you request the creation of separate IPv4 interfaces for VPN traffic only when you want the IPv4 interface for VPN traffic to have different attributes, such as a different IP policy, from the IPv4 interface for non-VPN traffic. When it is acceptable for the VPN traffic and the non-VPN traffic to receive the same IP treatment, then you do not need to create separate IPv4 interfaces for the VPN traffic.

    • If you configure it to do so, but only if IPv6 is enabled in the virtual router, MPLS creates one dynamic IPv6 interface per MPLS major interface for VPN traffic. If this interface is not created, then the VPN traffic uses the default IPv6 interface for non-VPN traffic.

      Typically, you request the creation of separate IPv6 interfaces for VPN traffic only when you want the IPv6 interface for VPN traffic to have different attributes, such as a different IP policy, from the IPv6 interface for non-VPN traffic. When it is acceptable for the VPN traffic and the non-VPN traffic to receive the same IP treatment, then you do not need to create separate IPv6 interfaces for the VPN traffic.

    IPv6 must be enabled in the parent virtual router so that IPv6 dynamic interfaces can be created over MPLS interfaces. Otherwise, IPv6 VPNs do not work correctly,

    All VPN traffic sent onto or received from the same layer 2 interface uses the same IPv4 VPN or IPv6 VPN interface. Consequently, any policy attached to the interface applies to all that VPN traffic.

    Published: 2014-08-18