Traffic drops may be observed when the MTU configured on the GRE interface does not match the MTU configured on the underlying physical interface.

GRE encapsulation adds additional overhead to packets. If the MTU values are not aligned, packets may exceed the supported transmission size, resulting in packet drops and connectivity issues.

• Packet drops observed over the GRE tunnel.
• Intermittent network connectivity issues.
• Small packets may pass successfully while larger packets are dropped.

Configure the same MTU size on both the GRE interface and the corresponding physical interface.

Example:

interfaces {
    gr-0/0/0 {
        unit 0 {
            family inet {
                mtu 1500;
            }
        }
    }
    xe-0/0/0 {
        mtu 1500;
    }
}

After configuring the MTU, commit the configuration and verify the interface settings.

show interfaces gr-0/0/0 extensive
show interfaces xe-0/0/0 extensive

Configuration on SRX Chassis Cluster

For SRX devices configured in a chassis cluster, configure the MTU on the corresponding redundant Ethernet (reth) interface.

Example:

interfaces {
    reth0 {
        mtu 1500;
    }
}

It is not required to configure the MTU separately on the member physical interfaces. The MTU configured on the reth interface is inherited by the associated physical interfaces.