Configuring Ultimate-Hop Popping for LSPs

 

By default, RSVP-signaled LSPs use penultimate-hop popping (PHP). Figure 1 illustrates a penultimate-hop popping LSP between Router PE1 and Router PE2. Router CE1 forwards a packet to its next hop (Router PE1), which is also the LSP ingress. Router PE1 pushes label 1 on the packet and forwards the labeled packet to Router P1. Router P1 completes the standard MPLS label swapping operation, swapping label 1 for label 2, and forwards the packet to Router P2. Since Router P2 is the penultimate-hop router for the LSP to Router PE2, it first pops the label and then forwards the packet to Router PE2. When Router PE2 receives it, the packet can have a service label, an explicit-null label, or just be a plain IP or VPLS packet. Router PE2 forwards the unlabeled packet to Router CE2.

Figure 1: Penultimate-Hop Popping for an LSP
Penultimate-Hop Popping for an LSP

You can also configure ultimate-hop popping (UHP) (as shown in Figure 2) for RSVP-signaled LSPs. Some network applications can require that packets arrive at the egress router (Router PE2) with a non-null outer label. For an ultimate- hop popping LSP, the penultimate router (Router P2 in Figure 2) performs the standard MPLS label swapping operation (in this example, label 2 for label 3 ) before forwarding the packet to egress Router PE2. Router PE2 pops the outer label and performs a second lookup of the packet address to determine the end destination. It then forwards the packet to the appropriate destination (either Router CE2 or Router CE4).

Figure 2: Ultimate-Hop Popping for an LSP
Ultimate-Hop Popping for an LSP

The following network applications require that you configure UHP LSPs:

  • MPLS-TP for performance monitoring and in-band OAM

  • Edge protection virtual circuits

The following features do not support the UHP behavior:

  • LDP-signaled LSPs

  • Static LSPs

  • Point-to-multipoint LSPs

  • CCC

  • traceroute command

For more information about UHP behavior, see Internet draft draft-ietf-mpls-rsvp-te-no-php-oob-mapping-01.txt, Non PHP behavior and Out-of-Band Mapping for RSVP-TE LSPs.

For point-to-point RSVP-signaled LSPs, UHP behavior is signaled from the LSP ingress. Based on the ingress router configuration, RSVP can signal the UHP LSP with the non-PHP flag set. RSVP PATH messages carry the two flags in the LSP-ATTRIBUTES object. When the egress router receives the PATH message, it assigns a non-null label to the LSP. RSVP also creates and installs two routes in the mpls.0 routing table. S refers to the S bit of the MPLS label, which indicates whether or not the bottom of the label stack has been reached.

  • Route S=0—Indicates that there are more labels in the stack. The next hop for this route points to the mpls.0 routing table, triggering a chained MPLS label lookup to discover the remaining MPLS labels in the stack.

  • Route S=1—Indicates that there are no more labels. The next hop points to the inet.0 routing table if the platform supports chained and multi-family lookup. Alternatively, the label route can point to a VT interface to initiate IP forwarding.

If you enable UHP LSPs, MPLS applications such as Layer 3 VPNs, VPLS, Layer 2 VPNs, and Layer 2 circuits can use the UHP LSPs. The following explains how UHP LSPs affect the different types of MPLS applications:

  • Layer 2 VPNs and Layer 2 circuits—A packet arrives at the PE router (egress of the UHP LSP) with two labels. The outer label (S=0) is the UHP label, and the inner label (S=1) is the VC label. A lookup based on the transport label results in a table handle for the mpls.0 routing table. There is an additional route in the mpls.0 routing table corresponding to the inner label. A lookup based on the inner label results in the CE router next hop.

  • Layer 3 VPN—A packet arrives at the PE router (egress of the UHP LSP) with two labels. The outer label (S=0) is the UHP label, and the inner label is the VPN label (S=1). A lookup based on the transport label results in the table handle for the mpls.0 routing table. There are two cases in this scenario. By default, Layer 3 VPNs advertise the per-next hop label. A lookup based on the inner label results in the next hop toward the CE router. However, if you have configured the vrf-table-label statement for the Layer 3 VPN routing instance, the inner LSI label points to the VRF routing table. An IP lookup is also completed for the VRF routing table.

    Note

    UHP for Layer 3 VPNs configured with the vrf-table-label statement is supported on MX Series 5G Universal Routing Platforms only.

  • VPLS—A packet arrives at the PE router (egress of the UHP LSP) with two labels. The outer label is the transport label (S=0) and the inner label is the VPLS label (S=1). A lookup based on the transport label results in the table handle for the mpls.0 routing table. A lookup based on the inner label in mpls.0 routing table results in the LSI tunnel interface of the VPLS routing instance if tunnel-services is not configured (or a VT interface not available). MX 3D Series routers support chained lookup and multi-family lookup.

    Note

    UHP for VPLS configured with the no-tunnel-service statement is supported on MX 3D Series routers only.

  • IPv4 over MPLS—A packet arrives at the PE router (egress of the UHP LSP) with one label (S=1). A lookup based on this label returns a VT tunnel interface. Another IP lookup is completed on the VT interface to determine where to forward the packet. If the routing platform supports multi-family and chained lookups (for example, MX 3D routers and PTX Series Packet Transport Routers), lookup based on label route (S=1) points to the inet.0 routing table.

  • IPv6 over MPLS—For IPv6 tunneling over MPLS, PE routers advertise IPv6 routes to each other with a label value of 2. This is the explicit null label for IPv6. As a result, the forwarding next hops for IPv6 routes that are learned from remote PE routers normally push two labels. The inner label is 2 (it could be different if the advertising PE router is from another vendor), and the router label is the LSP label. Packets arrive at the PE router (egress of the UHP LSP) with two labels. The outer label is the transport label (S=0), and the inner label is the IPv6 explicit-null label (label 2). Lookup based on the inner label in the mpls.0 routing table redirects back to the mpls.0 routing table. On MX 3D Series routers, the inner label (label 2) is stripped off and an IPv6 lookup is done using the inet6.0 routing table.

  • Enabling both PHP and UHP LSPs—You can configure both PHP and UHP LSPs over the same network paths. You can separate PHP and UHP traffic by selecting forwarding LSP next hops using a regular expression with the install-nexthop statement. You can also separate traffic by simply naming the LSPs appropriately.

The following statements enable ultimate-hop popping for an LSP. You can enable this feature on a specific LSP or for all of the ingress LSPs configured on the router. Configure these statements on the router at the LSP ingress.

  1. To enable ultimate-hop popping, include the ultimate-hop-popping statement:

    Include this statement at the [edit protocols mpls label-switched-path label-switched-path-name] hierarchy level to enable ultimate-hop popping on a specific LSP. Include this statement at the [edit protocols mpls] hierarchy level to enable ultimate-hop popping on all of the ingress LSPs configured on the router. You can also configure the ultimate-hop-popping statement under the equivalent [edit logical-routers] hierarchy levels.Note

    When you enable ultimate-hop popping, RSVP attempts to resignal existing LSPs as ultimate-hop popping LSPs in a make-before-break fashion. If an egress router does not support ultimate-hop popping, the existing LSP is torn down (RSVP sends a PathTear message along an LSP’s path, removing the path state and dependent reservation state and releasing the associated networking resources).

    If you disable ultimate-hop popping, RSVP resignals existing LSPs as penultimate-hop popping LSPs in a make-before-break fashion.

  2. If you want to enable both ultimate-hop-popping and chained next hops on MX 3D Series routers only, you also need to configure the enhanced-ip option for the network-services statement:

    You configure this statement at the [edit chassis] hierarchy level. Once you have configured the network-services statement, you need to reboot the router to enable UHP behavior.