Switch Troubleshooting

A basic switch is considered a layer-two device. When we use the word layer, we are referring to the 7-layer OSI model. A switch does not just pass electrical signals along, like a hub does; instead, it assembles the signals into a frame (layer two), and then decides what to do with the frame. A switch determines what to do with a frame by borrowing an algorithm from another common networking device: a transparent bridge. Logically, a switch acts just like a transparent bridge would, but it can handle frames much faster than a transparent bridge could (because of special hardware and architecture). Once a switch decides where the frame should be sent, it passes the frame out the appropriate port (or ports). You can think of a switch as a device creating instantaneous connections between various ports, on a frame by frame basis.


Since the switch decides on a frame by frame basis which ports exchange data, it is a natural extension to put logic inside the switch to allow it to choose ports for special groupings. This grouping of ports is called a Virtual Local Area Network (VLAN). The switch makes sure that traffic from one group of ports never gets sent to other groups of ports (which would be routing). These port groups (VLANs) can each be considered an individual LAN segment.

VLANs are also described as broadcast domains. This is because of the transparent bridging algorithm, which says that broadcast packets (packets destined for the all devices address) be sent out all ports that are in the same group (that is, in the same VLAN). All ports that are in the same VLAN are also in the same broadcast domain.

Transparent Bridging Algorithm

The transparent bridging algorithm and spanning tree are covered in more detail elsewhere (Chapter 20: Troubleshooting Transparent Bridging Environments). When a switch receives a frame, it must decide what to do with that frame. It could ignore the frame; it could pass the frame out one other port, or it could pass the frame out many other ports.

In order to know what to do with the frame, the switch learns the location of all devices on the segment. This location information is placed in a Content Addressable Memory table (CAM - named for the type of memory used to store these tables). The CAM table shows, for each device, the MAC address of the device, out which port that MAC address can be found, and with which VLAN this port is associated. The switch continually does this learning process as frames are received into the switch. The CAM table of the switch is continually updated.

This information in the CAM table is used to decide how a received frame is handled. In order to decide where to send a frame, the switch looks at the destination MAC address in a received frame and looks up that destination MAC address in the CAM table. The CAM table shows which port the frame must be sent out in order for that frame to reach the specified destination MAC address. Here are the basic rules that a switch uses to carry out the frame forwarding responsibility:

  • If the destination MAC address is found in the CAM table, the switch sends the frame out the port that is associated with that destination MAC address in the CAM table. This is called forwarding.

  • If the associated port to send the frame out is the same port that the frame originally came in on, there is no need to send the frame back out that same port, and the frame is ignored. This is called filtering.

  • If the destination MAC address is not in the CAM table (the address is unknown), the switch sends the frame out all other ports that are in the same VLAN as the received frame. This is called flooding. It does not flood the frame out the same port on which the frame was received.

  • If the destination MAC address of the received frame is the broadcast address (FFFF.FFFF.FFFF), the frame is sent out all ports that are in the same VLAN as the received frame. This is also called flooding. The frame is not sent out the same port on which the frame was received.

Spanning Tree Protocol

As we have seen, the transparent bridging algorithm floods unknown and broadcast frames out of all the ports that are in the same VLAN as the received frame. This causes a potential problem. If the network devices that run this algorithm are connected together in a physical loop, flooded frames (like broadcasts) are passed from switch to switch, around and around the loop, forever. Dependent upon the physical connections involved, the frames can actually multiply exponentially due to the flooding algorithm, which can cause serious network problems.

There is a benefit to a physical loop in your network: it can provide redundancy. If one link fails, there is still another way for the traffic to reach its destination. In order to allow the benefits derived from redundancy, without breaking the network because of flooding, a protocol called spanning tree was created. Spanning tree was standardized in the IEEE 802.1d specification.

The purpose of the spanning tree protocol (STP) is to identify and temporarily block the loops in a network segment or VLAN. The switches run the STP, which involves electing a root bridge or switch. The other switches measure their distance from the root switch. If there is more than one way to get to the root switch, there is a loop. The switches follow the algorithm to determine which ports must be blocked in order to break the loop. STP is dynamic; if a link in the segment fails, ports that were originally blocking can possibly be changed to forwarding mode.


Trunking is a mechanism that is most often used to allow multiple VLANs to function independently across multiple switches. Routers and servers can use trunking, as well, which allows them to live simultaneously on multiple VLANs. If your network only has one VLAN in it, you might never need trunking; but if your network has more than one VLAN, you probably want to take advantage of the benefits of trunking.

A port on a switch normally belongs to only one VLAN; any traffic received or sent on this port is assumed to belong to the configured VLAN. A trunk port, on the other hand, is a port that can be configured to send and receive traffic for many VLANs. It accomplishes this when it attaches VLAN information to each frame, a process called tagging the frame. Also, trunking must be active on both sides of the link; the other side must expect frames that include VLAN information for proper communication to occur.

There are different methods of trunking dependent upon the media that is used. Trunking methods for Fast Ethernet or Gigabit Ethernet are Inter-Switch Link (ISL) or 802.1q. Trunking over ATM uses LANE. Trunking over FDDI uses 802.10.


EtherChannel is a technique that is used when you have multiple connections to the same device. Rather than each link function independently, EtherChannel groups the ports together to work as one unit. It distributes traffic across all the links and provides redundancy if one or more links fail. EtherChannel settings must be the same on both sides of the links involved in the channel. Normally, spanning tree would block all of these parallel connections between devices because they are loops, but EtherChannel runs underneath spanning tree, so that spanning tree thinks all the ports within a given EtherChannel are only a single port.


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