This article describes an issue in which vlan tagged MPLS traffic is dropped as it passes through Juniper’s EVO-based QFX switches. The problem occurs when such a switch is acting as an L2 switch with family ethernet-switching configured on its trunk interfaces. In this configuration, MPLS packets with VLAN tag are dropped whenever they are received on a Layer 2 port, and only the L2 switching of MPLS packets carrying a VLAN tag is affected.

As a result, MPLS transit traffic passing through the switch fails, which disrupts the Label Distribution Protocol (LDP) sessions with adjacent devices and causes a loss of MPLS connectivity between them. Non-MPLS traffic through the same interfaces is not affected.

The issue is limited to the following conditions:

1. Platform is an EVO based QFX5230, QFX5240 or QFX5250 series switch. QFX5130 switch is not impacted
2. Switch is acting as an L2 switch with family ethernet-switching configured on trunk interfaces
3. MPLS packets are received with a VLAN tag on a Layer 2 port

Following symptoms are observed:

1. MPLS traffic associated with a VLAN tag is dropped
2. MPLS transit traffic through the switch fails, resulting in loss of MPLS connectivity between adjacent devices
3. LDP sessions between adjacent devices keep flapping
4. Non-MPLS traffic through the same interfaces is forwarded normally
5. There are no log messages indicative of the issue

There is no workaround to this problem. This problem is due to a software bug and has been fixed in the following releases:

1. 25.2X100-D21-EVO
2. 25.2X100-EVO
3. 26.3R1-EVO
4. 26.4R1-EVO
5. 26.4R1