Network Ninja

Transparent Bridges were first developed in the 1980s by Digital Equipment Corporation which handed their work to the IEEE which incorporated the work into the IEEE 802.1 standard. When one looks at the operation of transparent bridges the basic operation reminds one of a switch because the basic operation is identical on a per-VLAN basis.

By definition end-devices don’t know that they aren’t directly connected to a server or other resources. This would mean that end-devices require no additional configuration to make them work with transparent bridges. The problem with transparent bridges is that they lack the capability to dynamically discover network topology and ensure only one path through the network.

The characteristics of a transparent bridge is that:

1. Transparent Bridges don’t modify frames that it forwards
2. Transparent Bridges learns addresses by listening and analyzing on ports for source MAC addresses. It uses these MAC addresses like a return-to-sender system where if a source MAC address for Host A is found on Port 1 when Host A sends something the Transparent Bridge assumes that by sending frames back out Port 1 for Host A it will reach Host A. The bridge then builds a table of MAC source addresses and Ports which is updated constantly.
3. Transparent Bridges forwards all broadcasts out all ports except the port the port that the broadcast came in on.
4. When a destination is unknown it forwards out all ports except the port that the unknown frame came in on (unicast flooding).

This all having been said STP provides the missing link in that it gives Ethernet and other protocol transparent bridges what it lacks. STP provides a way by which Ethernet transparent bridges can dynamically discover network topologies and ensure only one path is active through a network; thus eliminating loops.

Without STP there is no fool proof way to make a transparent bridge environment redundant. STP also protects against idiots accidental cabling things incorrectly because it prevents bridging loops.

I have been called in on a problem like this where someone decided a network cable looked untidy and plugged both ends of a RJ-45 cable into a access switch. Expert opinion said the switches in the rack server room had blown, the server was crashing, there was a virus on the network. Walking around the office and unplugging the looped network cable sorted out all problems.

Identifying Traffic Loops

Traffic loops are observed when a frame is forwarded continually and redundantly; this happens when there is no dynamic mechanism to manage multiple paths to the same destination. When a loop occurs firewalls think they are in WW3 and shut down access, switches start forwarding out all ports except the port it came in on (the LEDs look like X-Mas lights on steroids with activity), the load and activity on the network means no one is going to be doing anything until it is sorted out. Examples of this can be found in the above document or another Cisco document.

I made some pictures (I was bored) we have a sample network

That wants to send a frame from Work-Station-01 on Segment 1 to to Work-Station-02 on Segment 2 and so it begins. Work-Station-01 sends and both the bridges on Segment A see the frame making annotations in their tables that Work-Station-01 is in Segment A.

Our Packet Reaches its destination with Work-Station-02 but other things also happen like both bridges see a frame from Work-Station-01 from Port 2/2 and Port 1/2 and they make the adjustments in their tables that Work-Station-01 is in Segment B.

Work-Station-02 having received the frame wants to reply / acknowledge receipt to get the rest and sends a frame. Both bridges receive the frame and give Work-Station-02 the Access Denied sign and drop the frame because Work-Station-01 according to them is in Segment 2.

Now this all gets more interesting when anyone broadcasts something on the network. In this Work-Station-01 is going to broadcast something on the network and the fun begins. The Frame sent from Work-Station-01 is received by both bridges and forwarded out all ports except the received port like they should. The frame is received on Segment 2 by Work-Station-02 but also by both bridges (port 1/2 and port 2/2) and the bridges follow the rules and forward the frame again out all except the one it came in on. Repeat this about 100 times or until someone switches off one of the bridges or one of them just cant take the load anymore.

Notes and Notices:

This is a part of my personal BCMSN notes and research to assist myself in learning and understanding the concepts and theory for the BCMSN exam. I learn by making notes reading and writing things down and wish to file them where I can’t lose them. These notes are not to be seen, judged or mistaken for replacements to Cisco recognized and authorized training which I personally support and attend and suggest you undertake if you are going for the BCMSN Certification.