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Configuring the Maximum Number of MPLS Labels

For interfaces that you configure for MPLS applications, you can set the maximum number of labels upon which MPLS can operate.

By default, the maximum number of labels is three. You can change the maximum to four labels or five labels for applications that require four or five labels. For example, suppose you configure a two-tier carrier-of-carriers VPN service for customers who provide VPN service. A carrier-of-carrier VPN is a two-tiered relationship between a provider carrier (Tier 1 ISP) and a customer carrier (Tier 2 ISP). In a carrier-of-carrier VPN, the provider carrier provides a VPN backbone network for the customer carrier. The customer carrier in turn provides Layer 3 VPN service to its end customers. The customer carrier sends labeled traffic to the provider carrier to deliver it to the next hop on the other side of the provider carrier’s network. This scenario requires a three-label stack: one label for the provider carrier VPN, another label for the customer carrier VPN, and a third label for the transport route.

If you add fast reroute service, the PE routers in the provider carrier’s network must be configured to support a fourth label (the reroute label). If the customer carrier is using LDP as its signaling protocol and the provider carrier is using RSVP, the provider carrier must support LDP over RSVP tunnel service. This additional service requires an additional label, for a total of five labels.

To the customer carrier, the router it uses to connect to the provider carrier’s VPN is a PE router. However, the provider carrier views this device as a CE router.

Table 1 summarizes the label requirements.

Table 1: Sample Scenarios for Using 3, 4, or 5 MPLS Labels

Number of Labels Required



Carrier-of-carriers VPN or a VPN with two labels and fast reroute


Combination of carrier-of-carriers and fast reroute


Carrier-of-carriers with fast reroute and the customer carrier running LDP, with the provider carrier running RSVP

The system reserves label space when you configure the maximum number of labels on the interface. When you configure features that require MPLS labels, the label push is automatic. You do not need to explicitly push the labels. The transport route can be a static, LDP-signaled, or RSVP-signaled LSP.

To configure and monitor the maximum number of labels:

  1. Specify the maximum on the logical interface. Apply this configuration to the carrier’s PE routers.

    [edit interfaces ge-0/1/3 unit 0 family mpls]
    user@switch# set maximum-labels 5
  2. Verify the configuration.

    [edit system]
    user@switch# show interfaces ge-0/1/3.0
    Logical interface ge-0/1/3.0 (Index 77) (SNMP ifIndex 507) 
        Flags: SNMP-Traps Encapsulation: ENET2
        Input packets : 0 
        Output packets: 0
        Protocol mpls, MTU: 1480, Maximum labels: 5
          Flags: Is-Primary

    The command output includes the Maximum labels: 5 field under the logical interface unit 0.

Published: 2010-04-28

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