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Pop-and-Forward LSP Configuration

Pop-and-forward LSPs introduces the notion of pre-installed per traffic engineering link pop labels that are shared by RSVP-TE LSPs that traverse these links and significantly reducing the required forwarding plane state . A transit label-switching router (LSR) allocates a unique pop label per traffic engineering link with a forwarding action to pop the label and forward the packet over that traffic engineering link should the label appear at the top of the packet. These pop labels are sent back in the RESV message of the LSP at each LSR and further recorded in the record route object (RRO). The label stack is constructed from the recorded labels in the RRO and pushed by the ingress label edge router (LER), as each transit hop performs a pop-and-forward action on its label. The pop-and-forward tunnels enhances the RSVP-TE control plane feature benefits with the simplicity of the shared MPLS forwarding plane.

Benefits of RSVP-TE Pop-and-Forward LSP Tunnels

  • Scaling advantage of RSVP-TE—Any platform-specific label space limit on an LSR is prevented from being a constraint to the control plane scaling on that interface.

  • Reduced forwarding plane state—The transit labels on a traffic engineering link are shared across RSVP-TE tunnels traversing the link, and are used independent of the ingress and egress devices of the LSPs, thereby significantly reducing the required forwarding plane state.

  • Reduced transit data plane state—Because the pop labels are allocated per traffic engineering link and shared across LSPs, the total label state in the forwarding plane is reduced to a function of the number of RSVP neighbors on that interface.

  • Faster LSP setup time—The forwarding plane state is not programmed during the LSP setup and teardown. As a result, the control plane need not wait sequentially at each hop for the forwarding plane to be programmed prior to sending the label upstream in the RESV message, resulting in reduced LSP setup time.

  • Backward compatibility—This allows backward compatibility with transit LSRs that provide regular labels in RESV messages. Labels can be mixed across transit hops in a single MPLS RSVP-TE LSP. Certain LSRs can use traffic engineering link labels and others can use regular labels. The ingress can construct a label stack appropriately based on what type of label is recorded from every transit LSR.

Pop-and-Forward LSP Tunnel Terminology

The following terminology is used in the implementation of RSVP-TE pop-and-forward LSP tunnels:

  • Pop label—An incoming label at an LSR that is popped and forwarded over a specific traffic-engineering link to a neighbor.

  • Swap label—An incoming label at an LSR that is swapped to an outgoing label and forwarded over a specific downstream traffic engineering link.

  • Delegation label—An incoming label at an LSR that is popped. A new set of labels is pushed before the packet is forwarded.

  • Delegation hop— A transit hop that allocates a delegation label.

  • Application label depth (AppLD)—Maximum number of application or service labels (for example, VPN, LDP, or IPv6 explicit-null labels) that can be beneath the RSVP transport labels. It is configured on a per-node basis, and is equally applicable for all LSPs, and is neither signaled nor advertised.

  • Outbound label depth (OutLD)—Maximum number of labels that can be pushed before a packet is forwarded. This is local to the node, and is neither signaled nor advertised.

  • Additional transport label depth (AddTLD)—Maximum number of other transport labels that can be added (for example, bypass label). This is a per-LSP parameter that is neither signaled or advertised. The value is discerned by checking if the LSP has been signaled with link protection (AddTLD=1) or without link protection (AddTLD=0).

  • Effective transport label depth (ETLD)— Number of transport labels that the LSP hop can potentially send to its downstream hop. This value is signaled per LSP in the hop attributes subobject. The hop attributes subobject is added to the record route object (RRO) in the path message.

Pop-and-Forward LSP Tunnel Label and Signaling

Every traffic engineering link is allocated a pop label that is installed in the mpls.0 routing table with a forwarding action to pop the label and forward the packet over the traffic engineering link to the downstream neighbor of the RSVP-TE tunnel.

For pop-and-forward LSP tunnels, the pop label for the traffic engineering link is allocated when the first RESV message for a pop-and-forward transit LSP arrives over that traffic engineering link. This is done to avoid preallocating pop labels and installing them in networks where pop-and-forward LSPs are not configured.


For the pop-and-forward LSP tunnels to function effectively, we recommend that you configure the maximum-labels statement on all the interfaces in the RSVP-TE network.

Figure 1 displays pop labels at all interfaces for neighboring devices.

Figure 1: Pop-and-Forward LSP Tunnel LabelsPop-and-Forward LSP Tunnel Labels

There are two pop-and-forward LSP tunnels—T1 and T2. Tunnel T1 is from Device A to Device E on path A-B-C-D-E. Tunnel T2 is from Device F to Device E on path F-B-C-D-E. Both the tunnels, T1 and T2, share the same traffic engineering links B-C, C-D, and D-E.

As RSVP-TE signals the setup of the pop-and-forward tunnel T1, the LSR D receives the RESV message from the egress E. Device D checks the next-hop traffic engineering link (D-E) and provides the pop label (250) in the RESV message for the tunnel. The label is sent in the label object and is also recorded in the label subobject (with the pop label bit set) carried in the RRO. Similarly, Device C provides the pop label (200) for the next- hop traffic engineering link C-D and Device B provides the pop label (150) for the next-hop traffic engineering link B-C. For the tunnel T2, the transit LSRs provide the same pop labels as described for tunnel T1.

Both the label edge routers (LERs), Device A and Device F, push the same stack of labels [150(top), 200, 250] for tunnels T1 and T2, respectively. The recorded labels in the RRO are used by the ingress LER to construct a stack of labels.

The pop-and-forward LSP tunnel labels are compatible with transit interfaces that use swap labels. Labels can be mixed across transit hops in a single MPLS RSVP-TE LSP, where certain LSRs can use pop labels and others can use swap labels. The ingress device constructs the appropriate label stack based on the label type recorded from every transit LSR.

Pop-and-Forward LSP Tunnel Label Stacking

Construction of Label Stack at the Ingress

The ingress LER checks the type of label received from each transit hop as recorded in the RRO in the RESV message and generates the appropriate label stack to use for the pop-and-forward tunnel.

The following logic is used by the ingress LER while constructing the label stack:

  • Each RRO label subobject is processed starting with the label subobject from the first downstream hop.

  • Any label provided by the first downstream hop is always pushed on the label stack. If the label type is a pop label, then any label from the succeeding downstream hop is also pushed on the constructed label stack.

  • If the label type is a swap label, then any label from the succeeding downstream hop is not pushed on the constructed label stack.

Auto-Delegation of Label Stack

The ingress device runs the Constrained Shortest Path First (CSPF) to compute the path, and if the hop length is greater than the OutLD-AppLD-AddTLD, the ingress device cannot impose the entire label stack to reach the egress device.

When requesting RSVP-TE to signal the path, the ingress device always requests autodelegation for the LSP, where one or more transit hops automatically select themselves as delegation hops to push the label stack to reach the next delegation hop. Junos OS uses an algorithm based on the received Effective Transport Label-Stack Depth (ETLD), that each transit executes to decide whether it should autoselect itself as a delegation hop. This algorithm is based on the section on ETLD in the Internet draft draft-ietf-mpls-rsvp-shared-labels-00.txt (expires September 11 2017), Signaling RSVP-TE Tunnels on a Shared MPLS Forwarding Plane.

The label stack imposed by the ingress device delivers the packet up to the first delegation hop. Each delegation hop's label stack also includes the delegation label of the next delegation hop at the bottom of the stack.

Figure 2 displays labels at every device interface, where Device D and Device I are delegation hops, [Label] P is the pop label, and [Label] D is the delegation label. The RSVP-TE pop-and-forward LSP tunnel is A-B-C-D-E-F-G-H-I-J-K-L. Delegation label 1250 represents (300, 350, 400, 450, 1500); Delegation label 1500 represents (550, 600).

Figure 2: Pop-and-Forward LSP Tunnel Pop and Delegation LabelsPop-and-Forward LSP Tunnel Pop and Delegation Labels

In this approach, for the tunnel, the ingress LER Device A pushes (150, 200, 1250). At LSR Device D, the delegation label 1250 gets popped and labels 300, 350, 400, 450, and 1500 get pushed. At LSR Device I, the delegation label 1500 gets popped and the remaining set of labels (550, 600) get pushed. In Junos OS, the pop and push action occurs as a swap to the bottom label of the outgoing stack and push the remaining labels.

A delegation label and the LSP segment that it covers can be shared by multiple pop-and-forward LSPs. A LSP delegation segment consist of an ordered set of hops (IP addresses and labels) as seen in the RESV RRO. The delegation label (and the segment that it covers) is not owned by a particular LSP, but can be shared. When all LSPs using a delegation label are deleted, the delegation label (and route) is deleted.

Pop-and-Forward LSP Tunnel Link Protection

To provide link protection at a point of local repair (PLR) with a pop-and-forward data plane, the LSR allocates a separate pop label for the traffic engineering link that is used for the RSVP-TE tunnels that request link protection from the ingress device. No signaling extensions are required to support link protection for the RSVP-TE tunnels over the pop-and-forward data plane.

Figure 3 displays pop labels at every device interface; labels marked with P are pop labels that offer link protection for the traffic-engineering link.

Figure 3: Pop-and Forward LSP Tunnel Link ProtectionPop-and Forward LSP Tunnel Link Protection

At each LSR, link-protected pop labels can be allocated for each traffic engineering link, and a link-protecting facility bypass LSP (which is not a pop-and-forward LSP, but rather a normal bypass LSP) can be created to protect the traffic engineering link. These labels can be sent in the RESV message by the LSR for LSPs requesting link protection over the specific traffic engineering link. Because the facility bypass terminates at the next hop (merge point), the incoming pop label on the packet at the PLR is what the merge point expects.

For example, LSR Device B can install a facility bypass LSP for the link-protected pop label 151. When the traffic engineering link B-C is up, LSR Device B pops 151 and sends the packet to C. If the traffic engineering link B-C is down, the LSR can pop 151 and send the packet through the facility backup to Device C.

RSVP-TE Pop-and-Forward LSP Tunnel Supported and Unsupported Features

Junos OS supports the following features with RSVP-TE pop-and-forward LSP tunnels:

  • Pop labels per RSVP neighbor for unprotected LSP.

  • Pop labels per RSVP neighbor for LSPs requesting link protection using facility bypass

  • Autodelegation of LSP segment.

  • Mixed label mode, where certain transit LSRs do not support pop-and-forward LSP tunnels

  • LSP ping and traceroute

  • All existing CSPF constraint.

  • Load balancing of traffic between pop-and-forward LSPs and regular point-to-point RSVP-TE LSP.

  • Autobandwidth, LDP tunneling, and TE++ container LSP.

  • Aggregated Ethernet interface.

  • Virtual platforms support, such as Juniper Networks vMX Virtual Router.

  • 64-bit support

  • Logical systems

Junos OS does not support the following functionality for RSVP-TE pop-and-forward LSP tunnels:

  • Node link protection

  • Detour protection for MPLS fast reroute

  • Point-to-multipoint LSPs.

  • Switch-away LSP.

  • Generalized MPLS (GMPLS) LSPs (including bidirectional LSPs, associated LSPs, VLAN user-to-network interface [UNI] and so on)

  • IP Flow Information Export (protocol) (IPFIX) inline flow sampling for MPLS template

  • RFC 3813, Multiprotocol Label Switching (MPLS) Label Switching Router (LSR) Management Information Base (MIB)

  • IPv4 Explicit-null (Inserting label 0 at the bottom of the label stack is not supported. If there are service labels beneath the RSVP-TE pop-and-forward label stack, because the penultimate hop for the LSP copies the EXP value to the service label, this can allow continuity of class of service (CoS) across the MPLS forwarding plane).

  • Ultimate-hop popping (UHP)

  • Graceful Routing Engine switchover (GRES)

  • Nonstop active routing (NSR)