[MX/EX9200s] Packets being silently dropped due to MTU size

Summary:

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

Symptoms:

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.

Cause:

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.

Solution:

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.

Modification History:

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

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