Packets are being silently dropped by MX or EX9200s devices due to the MTU size. This article explains the issue and describes a workaround.

Network diagram

Server1--EX Switch--ge-0/0/0 MX960 ge-0/0/1---EX Switch---Server2

Server 1 and 2 are in the same VLAN (for example: VLAN 10).

On MX960:

show configuration interface ge-0/0/0

unit 0 {
     family bridge {
         interface-mode trunk; < trunk mode
         vlan-id-list [ 10 20 ];
     }
}

show configuration interface ge-0/0/1

Logical interface ge-0/0/0.0 (Index 6513) (SNMP ifIndex 8908)
Flags: SNMP-Traps 0x24024000 Encapsulation: Ethernet-Bridge
Input packets : 0
Output packets: 0
Protocol bridge, MTU: 1514 <==== MTU is 1514
Flags: Is-Primary, Trunk-Mode

unit 0 {
    family bridge {
        interface-mode trunk; < trunk mode
        vlan-id-list [ 10 20 ];
    }
}

When server 1 pings server 2 with a packet size of 1469 or greater, the total packet size will be 1469 + 20(IP) + 8(ICMP) + 14(layer2) + 4(802.1q) = 1515 bytes. When the MX960 router receives the packet from the EX server, it will silently drop the packet, as the size is greater than the interface MTU on ge-0/0/1 and is a pure layer 2 network with no IP fragmentation as well.

When the interface is configured in trunk mode, Junos OS will not automatically increase the size of the interface MTU to accommodate the extra 4 bytes of 802.1Q.

The following workarounds can be used to resolve the issue:

• Configure the interface with vlan-tagging or flexible-vlan-tagging, which will increase the interface MTU by 8 bytes:

Logical interface ge-0/0/0.0 (Index 6490) (SNMP ifIndex 8908)
Flags: SNMP-Traps 0x20024000 Encapsulation: Ethernet-Bridge
Input packets : 0
Output packets: 0
Protocol bridge, MTU: 1522  < ====
Flags: Is-Primary, Trunk-Mode

Logical interface ge-0/0/0.32767 (Index 6491) (SNMP ifIndex 8909)
Flags: SNMP-Traps 0x4004000 VLAN-Tag [ 0x0000.0 ] Encapsulation: ENET2 < ==== vlan-tagging
Input packets : 0
Output packets: 0
Protocol multiservice, MTU: Unlimited
Flags: None

• Manually increase the interface MTU. This is an interface command. Refer to mtu Syntax for more information.

2021-03-02: Minor non-technical changes made; article valid and accurate; EX9200s category added