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[EX] MTU consideration for inter VLAN routing with jumbo frames on EX-series switches

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Article ID: KB22255 KB Last Updated: 05 Mar 2017Version: 4.0
Summary:
This article provides information about the MTU consideration for inter VLAN routing with jumbo frames on EX-series switches.
Symptoms:

A new feature for the path MTU has been introduced in Junos 10.2 (and later releases) for the EX8200 and Junos 11.2 (and later releases) for the EX4200 platforms. With this feature, the following behavior change is exhibited, as per the Jumbo frame support for data packets on RVI:

'For RVIs (Routed VLAN Interface) > Jumbo frames of up to 9216 bytes are supported on the routed VLAN interface (RVI), which is named vlan. The RVI functions as a logical router. To route jumbo data packets on the RVI, you must configure the jumbo MTU size on the physical member L2 interfaces of the RVI, and not on the RVI itself.'

With this feature change, the packets will get fragmented, if the jumbo frame MTU size is not configured on the VLAN interface.

Cause:
 Changes in JUNOS code.
Solution:
The following information is applicable to Junos 10.2 or later for EX8200 and Junos 11.2 or later for EX4200:

  • Junos 10.2 for the EX8200 platforms

  • Junos 11.2 for all other EX platforms

To route jumbo data packets on the RVI, you must configure the jumbo MTU size on the member physical interfaces and also on the RVI itself (VLAN interface). If the Jumbo MTU size is not configured for RVI, all Jumbo data packets will get fragmented.

To modify the default media MTU size for a RVI interface, include the MTU statement at the [edit interfaces interface-name] hierarchy level:
[edit interfaces vlan]
set mtu bytes;
Caution: Setting or deleting the Jumbo MTU size on the RVI (the routed vlan interface), when the switch is transmitting packets, might result in dropped packets.

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