Teaching how to use the COSI switches
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This documents will go into the basics of networking. This is in no way a comprehensive guide to how the network works. For that, I suggest taking CS455, Computer Networks. I highly suggest taking it with Jeanna Matthews, she teaches it very hands-on with tools like Wireshark.

Table of Contents

  • Network Protocols
    • STP
    • CDP
    • VLANS (802.1q)
    • LAG and LACP


STP is the Spanning Tree Protocol. STP handles redundant paths with the tree, and also knows about the topography of the network at any point in time. It also is used to disable ports that are connected to themselves, even across switches.

This prevents switching loops from occurring. Very useful when your users make mistakes, so that the entire network doesn't go down.

STP is typically enabled on a port-by-port basis.


CDP is the Cisco Discovery Protocol. Packets like this are generally not originated on our network, and rather are originated on the Clarkson OIT's network. It helps identify the Cisco switch that the current device is connected to, and allows quicker debugging when in a pinch. It has a lot of information about the connected port, such as the VLAN tag that is delivered from the switch, the IP address for the switch management, the name of the switch, software versions, and more.

CDP is not something that is really useful, but sometimes you will see it. It can be useful for debugging particularly odd events.

CDP is typically enabled on a port-by-port basis.

VLANS (802.1q)

VLANs (also known as Virtual Local Area Networks) are used to separate different Level 2 and higher broadcast domains.

VLAN tags that use the 802.1q protocol have a special field in the packet header, just after the source and destination MAC addresses.

There are two possible port modes on our switches (that are relevant). The first, called Access Mode, breaks out a particular VLAN ID's packets on the switch to the port. This allows untagged packets coming from the network to enter a VLAN (transparently to the connected device) on the switch. The second, called Trunk Mode, breaks out one or more VLANs to a particular port, using the 802.1q protocol. Packets that are received on this interface that do not have a VLAN ID are tagged to the "Native" VLAN (which is typically 1 by default, but can be any number as defined per interface on the switch itself).

In COSI, the default VLAN is always 1 (as of writing), and as such, we don't use it for tagged links.

Structure of a VLAN tag:

VLAN Packet Format

VLAN's are typically enabled on a port-by-port basis.


These protcols are used to do link aggregation (hence, "Link Aggregation Protocol" and "Link Aggregation Control Protocol"). Link aggregation takes place at the level 2 layer, and is lower than VLANs and other types of broadcast networks. This means that you can have VLAN tagged networks and other protocols on top of LAG's without any tunneling or other special sauce.

LAG's are used mostly for redundant physical paths for a particular network, as well as increasing total bandwidth possible, by stitching together physical links.

LACP extends LAG's by making the process automatic - you define active and passive participants. The active participant will try to talk with a connected passive participant, and negotiate a LAG, as well as maintaining the state of the lag.

LAGs and LACP are typically enabled on a port-by-port basis.