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Per-Subscriber Support of Maximum Transmission Unit for Dynamic Profiles

 

You can create per subscriber support of maximum transmission unit (MTU) for dynamic profiles. For more information, see the following topics:

Understanding Per-subscriber Support of Maximum Transmission Unit for Dynamic Profiles

Maximum transmission unit (MTU) is used to determine the maximum size of each packet in any TCP or IP transmission. MTU cannot be greater than the payload size that is the encapsulations at the assigned layer and any lower layers are excluded. You can specify the MTU for statically configured logical interfaces. Starting in Junos OS Release 18.2R1, you can configure an MTU value for a subscriber logical interface in a dynamic profile. This feature is required in customer applications requiring per-subscriber MTU for logical interfaces on the same underlying physical interface (from which the MTU is inherited by default). The use case is primarily dynamic VLANs for DHCP or DHCPv6 or IPoE or IPv6oE. The dynamic logical interface MTU must be no greater than the physical interface MTU minus the VLAN header size. The per-subscriber MTU feature is provided by extending dynamic-profiles to allow MTU to be configured, either with a static value or the predefined variable, $junos-interface-mtu, whose value is provided by RADIUS.

MTU Per-Subscriber for Dynamic Profiles

A dynamic profile is a set of characteristics, defined in a type of template, that you can use to provide dynamic subscriber access and services for broadband applications. These services are assigned dynamically to interfaces. You can identify subscribers statically or dynamically. To identify subscribers statically, you can reference a static VLAN interface in a dynamic profile. To identify subscribers dynamically, you need to create variables for demux interfaces that are dynamically created when the subscribers log in. Junos OS allows you to create MTU for each subscriber for dynamic profiles. The value can be static or can be represented through a new variable, $junos-interface-mtu. By default, the variable value is the MTU of the payload, that is, the MTU of the physical interface minus the VLAN header size. A specific value can be returned through the RADIUS authentication in the Framed-MTU attribute (12). The attribute includes a single value which is applied to both the inet and inet6 protocol families if both are configured with $junos-interface-mtu variable. While applying the MTU on the subscriber logical interface during dynamic profile instantiation, a check is made to ensure that the MTU of the logical interface does not exceed what is supported on its physical interface along with the family protocol overhead. The value of the static MTU should be within the acceptable MTU range. If RADIUS does not return a Framed MTU value for $junos-interface-mtu variable, the default value for interface-mtu is used. You configure this value at the [edit dynamic profiles dynamic-profiles predefined-variable-defaults] hierarchy level. If neither is provided, then the profile request is NACKed.

Benefits of Per Subscriber Support of MTU for Dynamic Profiles

  • Provides network scalability if each subscriber uses different dynamic profile or different subscriber name.

  • Allows each subscriber to send traffic with different traffic rate.

Limitations

The following are the limitations:

  • MTU for a dynamic logical interface is applied using the same rules as static logical interfaces.

  • Framed-MTU returned by RADIUS is applicable only to the authenticated session. In other words, an authenticated dynamic VLAN (DVLAN) profile affects only the MTU for the vlan logical interface, and an authenticated DHCP profile affects only the MTU for the DHCP subscriber logical interface.

  • If the RADIUS does not return a value in the Framed-MTU attribute (12), the profile request is NACKed.

  • A commit check ensures that the mtu is specified for inet and inet6 address family, they must both be configured as explicit values if not the same value, or both must be configured with the $junos-interface-mtu predefined variable (in which case they are set to the same value). Otherwise, the configuration is forbidden.

Configuring Per-subscriber Maximum Transmission Unit for Dynamic Profiles

The maximum transmission unit (MTU) can be configured per subscriber for dynamic profiles. The value of MTU can be static or represented through the $junos-interface-mtu predefined variable. By default, the variable value is the MTU of the payload, which must be less than or equal to the MTU of the physical interface minus the VLAN header size. A specific value can be returned through RADIUS authentication through the Framed-MTU attribute (12). If the RADIUS server fails to return a value in the Framed MTU attribute, then the default value configured with interface-mtu statement at the [edit dynamic-profiles profile-name predefined-variable-defaults] hierarchy level is used. You can configure the MTU value with the mtu statement at the [edit dynamic-profiles name interfaces name unit name family inet] hierarchy level or at the [edit dynamic-profiles name interfaces name unit name family inet6] hierarchy level.

Before you begin, configure the device interfaces.

To configure per-subscriber MTU for dynamic profiles:

  1. Configure per-subscriber MTU for dynamic profiles by hardcoding the mtu value for inet or inet 6 family.

    Configure pre-subscriber MTU for dynamic profiles by hardcoding an mtu value of 1450 for family inet of dynamic profile vlan-profile.

  2. Configure the value for default for junos-interface-mtu.

    Configure interface mtu value of 1450 for predefined variable defaults of dynamic profile vlan-profile.

Release History Table
Release
Description
Starting in Junos OS Release 18.2R1, you can configure an MTU value for a subscriber logical interface in a dynamic profile.