# Selecting Service Modules for LNS Sessions Using MLPPP

You can install multiple service modules in an E Series router deployed as an LNS where the tunnel sessions carry MLPPP. To use an LNS, at least one Service line module (SM), ES2-S1 Service IOA, or a module that supports the use of shared tunnel-server ports must be installed in the E Series router.

The router selects service modules based on the LNS sessions that underlie the PPP link interfaces of an MLPPP bundle, also known as bundled sessions. To determine the appropriate SM where it places the first bundled session for an MLPPP bundle, the router uses a load-balancing mechanism. After the router determines the appropriate SM, it places all sessions for the same bundle on the same SM. By default, the router determines bundled membership based on the endpoint discriminator that the LNS receives from the LAC in the proxy LCP information.

For example, an ERX1440 Broadband Services Router has service modules installed in slots 4, 9, and 12. Using the load-balancing mechanism, the router determines that the SM in slot 4 can accommodate the first bundled session for MLPPP bundle A, and places it there. The first bundled session for bundle A has an endpoint discriminator of 5. The router subsequently places all bundled sessions for bundle A (which have an endpoint discriminator of 5) on the SM in slot 4.

When the SM on which the bundled sessions reside has no more space for additional sessions, the router refuses the L2TP session. This can happen even when other service modules installed in the router have available space.

For more information about endpoint discriminators, see the Configuring Multilink PPP chapter in JunosE Link Layer Configuration Guide.

## Assigning Bundled Group Identifiers

In some cases, an endpoint discriminator is not available for the LNS to use to identify the links in a bundled session.

This situation might occur when:

- PPP clients provide endpoint discriminators with null values.
- PPP clients do not provide an endpoint discriminator option when negotiating LCP with the LAC.
- The LAC does not include a endpoint discriminator option in the LCP proxy AVPs.

The router places all bundled sessions without endpoint discriminators on the same SM. However, if there are many such bundled sessions, the load-balanced distribution of LNS sessions across the service modules can deteriorate because the router places all bundled sessions on the same SM without evenly distributing the load.

The bundled-group-id command enables you to correct this situation by assigning a numeric bundled group identifier for the router to use when the endpoint discriminator is unavailable to identify the bundled membership. The router places bundled sessions with the same bundled group identifier on the same SM in the same way that it does with endpoint discriminators.

The bundled group identifier applies to the entire router; therefore, if you assign the same bundled group identifier for different L2TP destination host profiles, the router places all of the bundled sessions with the same bundled group identifier on the same SM.

| Note: We recommend that you assign bundled group identifiers only when you are certain that endpoint discriminators are unavailable to identify bundle membership.

- To assign a numeric bundled group identifier:
  host1:boston(config-l2tp-dest-profile-host)#bundled-group-id 4

## Overriding All Endpoint Discriminators

| Note: We strongly recommend that you use this feature only with the support of JTAC.

You can also configure the router to ignore the value of all endpoint discriminators when it selects a SM and to use only the bundled group identifier that you assigned by issuing the bundled-group-overrides-mlppp-ed command.

Issuing the bundled-group-id and bundled-group-id-overrides-mlppp-ed commands together forces the router to place the bundled sessions on the same SM when a PPP client incorrectly specifies different endpoint discriminators for links in the same bundle.

- To configure the router to ignore the value of all endpoint discriminators:
  host1:boston(config-l2tp-dest-profile-host)#bundled-group-id-overrides-mlppp-ed

#### Related Documentation

- bundled-group-id
- bundled-group-id-overrides-mlppp-ed