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Community Member

Ethernet Design

Currently, I have been asked the best way to extend a Fast Ethernet or Gigabit LAN over a distance of 10-15 miles. Within this network there are about 12 access locations that will deploy streaming video over IP as well as 15 PCs (non-critical data). These 12 locations are a little over a mile apart and will be connected via single mode fiber.

Understanding that there are distance limitations with Ethernet technologies and I want to minimize ARP and general broadcasts storms - are there any articles on best practice design? the true limitations of Ethernet technologies using repeaters, etc...?? and what type of device would be most appropriate at these 12 locations (3550, 6500, 4500, etc...).

1 ACCEPTED SOLUTION

Accepted Solutions

Re: Ethernet Design

Yes, they do. Even with layer2 switches you will be safe. This is because layer2 termination (equivalent to bridging) also takes care of terminating the collision domains. The collision domain is the second distance-limitator besides signal loss. Its size is restricted due to the propagation delay for the signals. Stretching it beyond the specs will result in very high collision rates.

With Layer2 you will have one broadcast domain, which could affect performance when you have large numbers of nodes. (> 1000)

With layer3 you will have smaller broadcast domains which is better.

With todays pricing & performance ratio between L2 vs L3 I would opt for L3 if I could afford it. It is the superior design for this environment.

7 REPLIES
Community Member

Re: Ethernet Design

You might wish to consider the 3550.

Using the LX GBIC you are well withing the needed distance (use 62.5 um multimode fiber) and you will have band width to spare. The 3550 is a layer3 switch, therefore you might have each site in his own broadcast domain and route between them.

Fabio

Re: Ethernet Design

Depending on your budget and how resilient you want it you could use any of the products you mentioned. I'd try and make every site its own IP subnet and interlink everything using Layer-3. Probably use something like 2950's at the sites (Layer-2) and either 4000's or 3550's as the core devices

Gigabit Ethernet Fibre distances:

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat5000/hardware/modules/02prep.htm#xtocid261767

Community Member

Re: Ethernet Design

This is good. I'm just trying to conform to IEEE so I have two further questions:

Are these the maximum distances between any two workstations ACROSS THE ENTIRE NETWORK)?

or

Are these the maximum lengths of the cable segments BETWEEN ANY TWO LOCATIONS?

Re: Ethernet Design

You do not need to bother about the max-distance between two workstations.

This could only become an issue when you were using (fiber) repeaters. (the max 4 repeater rule) The proposed design uses routers and your frames will be regenerated at each hop. Therefore you only need to worry about the distance limitations between two nodes.

Layer3 termination (on the routers) will reset all ethernet layer2 distance limitations.

Community Member

Re: Ethernet Design

Your answer is exactly what I was seeking. Does your response change if I am only using layer 2 switches such as the Cisco 2950 at each island? I am assuming that layer 2 switches don't regenerate the frame, but I could be wrong.

I will really have a comprehensive understanding based on your reply to this question.

Re: Ethernet Design

Yes, they do. Even with layer2 switches you will be safe. This is because layer2 termination (equivalent to bridging) also takes care of terminating the collision domains. The collision domain is the second distance-limitator besides signal loss. Its size is restricted due to the propagation delay for the signals. Stretching it beyond the specs will result in very high collision rates.

With Layer2 you will have one broadcast domain, which could affect performance when you have large numbers of nodes. (> 1000)

With layer3 you will have smaller broadcast domains which is better.

With todays pricing & performance ratio between L2 vs L3 I would opt for L3 if I could afford it. It is the superior design for this environment.

Community Member

Re: Ethernet Design

Wouldn't it be better to use FE vs. GE? The additional time (gained by using a slower datarate) should compensate for the time diameter (size) of the network. Just a thought. Also, by positioning your streaming sources into the center of the diameter (vs in line), can cut your time (size) in half. The only real problem then is the cost of the SM media converters (almost the same as LX GBIC) and not usually as reliable. Depends on what you have and where you might be going in the future.

Good Luck

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