How to Configure a Gigabit Ethernet Interface on Junos Devices

There is no difference between configuring a gigabit Ethernet interface and any other interface in Junos OS. In fact, you can usually leave all other properties at their default values. Properties such as whether the link should use flow control can be set, but usually the default (no flow control) is fine.

However, one additional physical interface property (meaning it is set at the ge-1/2/0 level) is important: the maximum transmission unit (MTU) size. This property is the maximum size of the information field that can be packed inside an Ethernet frame. The MTU is important because there is no sub-unit to a frame.

All frames are processed immediately upon receipt. If an IPv4 packet stuffed into the frame exceeds the MTU size, the overflow bits are not sent, and the received frame will have an error. Even if the IP packet fits, if the additional fields (MPLS tags, VLAN tags, and so on) cause the frame to exceed the MTU size, there will be an error at the receiver.

For Gigabit Ethernet interfaces, the default MTU size of the IP packet is 1500 bytes (which is usually not a problem). The MTU size for the link (Layer 2) is 1514 bytes, meaning that the link allows no more than 14 bytes of overhead in the frame header and trailer.

For Gigabit Ethernet, if you intend to send more, you can increase the MTU size up to a maximum of 9018 bytes (some interface boards support only 9014 bytes).

Most people ignore the MTU size until it causes an issue and traffic on an otherwise functioning link suddenly disappears. Many network operators' first challenge is to track down and figure out an MTU issue!

You can change the Gigabit Ethernet MTU size on ge-1/2/0 to 9000 bytes (larger MTUs are more efficient) as follows:

user@junos-device# set interfaces ge-1/2/0 mtu 9000

Remember, physical interface properties must match at both ends of the link.

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