Notes

Connecting Interfaces

There are different types of interface, the most common of which are RJ45 and transceivers. RJ45 is the regular copper cable connector that you probably see the most. Transceivers are added to the switch to set a particular port type. Transceivers are commonly called SFP’s (although technically, SFP is a type of transceiver).

WARNING: People often use the terms CAT6, RJ45, and Ethernet cable interchangeably. CAT6 is the type of copper, RJ45 is the connector on the end, and ‘Ethernet cable’ is completely incorrect in this context.

Despite physical differences, most cabling types use the same higher level protocols, like ethernet, IP, TCP, and so on.

Some ports are capable of different speeds. The speeds need to be the same at both ends of the link, so sometimes this means negotiating a suitable speed between the two devices. This is automatic by default, using a process called autonegotiation.

NOTE: transceiver modules (SFP/QSFP) are locked to a particular speed in hardware, and can’t be changed.

Where supported, interface speed can be set manually. Generally, leave it up to autonegotiation.

Interfaces can work in half-duplex of full-duplex. Half duplex means it can receive OR transmit at one time. Full duplex means it can receive AND transmit at the same time. Full duplex is generally better.

You will probably see half-duplex connections when you connect to a service provider. They often provide a single core of fibre in half-duplex mode. Some will provide dual-core, one for sending, one for receiving (therefore full-duplex).

Duplex settings also need to match on both sides of a link to operate correctly.

MTU and Frame Size

Data is broken into smaller chunks before being sent. At layer-3 these are packets, and at layer-2 these are frames. This has a few advantages; Traffic can be split over multiple paths, and if some traffic is lost, only the lost parts need to be resent.

The largest size of the layer-3 packet is called the Maximum Transmission Unit (MTU). This is 1500 bytes by default in Ethernet. This can be increased to over 9000 bytes (called a jumbo frame), depending on the switch model.

If the MTU is 1500, then the maximum frame size can be just a bit bigger (up to 1522 bytes) to account for the Ethernet headers.

So, the MTU represents the maximum ‘payload‘ size that goes in an Ethernet frame.

Sometimes we will want to use a technology like VPN that adds extra headers to each packet. for this reason, we may sometimes need to shrink the MTU, so the ‘payload’ can still fit into the frame.

If the payload is too big, the packet needs to be fragmented into smaller packets. This will mean that traffic can still reach the destination, but it will come at the cost of performance.

Interface Status

From the show interface status command, we can see:

The interface name
The interface description
Whether the interface is connected
The access VLAN, or trunk port status
Duplex status
The interface’s operational speed
The interface type

From show interfaces summary we see:

Interface queues
Transmit (TX) and Receive (RX) details

In show interfaces <int> we can see:

The interface’s up or down status (physical interface and line protocol)
The interface type
The MAC address / BIA address
The description
The MTU
The Bandwidth
Reliability, txload, and rxload
Duplex
Speed
Media Type
5 minute average input/output rates
Counters (collisions, runts, giants, CRC, etc)

We can clear the counters manually if needed. This can be done to see if errors are increasing, or if they’re historical.

Command Summary

Command	Mode	Description
description <text>	Interface Config	Add a description to an interface
speed <value>	Interface Config	Configure an interface’s speed
interface range <interfaces>	Global Config	Configure several interfaces
duplex [half\|full\|auto]	Interface Config	Set the duplex mode on an interface
mtu <value>	Interface Config	Set an interface’s MTU
show interface description	Privileged Exec	Show all interface descriptions
show interfaces status	Privileged Exec	Show information about all interfaces
show interfaces summary	Privileged Exec	Show information about all interfaces
show interfaces <int>	Privileged Exec	Show information about a specific interface
include / exclude	Privileged Exec	Filter output
clear counters	Privileged Exec	Clear interface counters