Home LAN/MAN Gigabit Ethernet Issue on Cisco GigE port with FastEthernet port on 3Com switches

As you know, Cisco Systems is one of top vendor in Router and Switch market in the world since early 90's. Issue on Cisco GigE port with FastEthernet port on 3com switches - logoIf someone asks who  #1 market leader is, then 99 out of 100 would say "Cisco Systems". Well, in the router market, Cisco was obvious, but who was 2nd leader on switch market? There were few candidates, Cabletron(current Enterasys), Bay Networks, Madge(token ring), HP,  3Com and so on. Most of makers were out of picture, but 3Com is still one of popular switch vendor in these days. Actually, 3com one of few vendor who has IT total solution.

Cisco and 3Com are IT leaders and they developed a lot of technology and some of new technique is under way to be IT standard. Cisco and 3Com have their own proprietary techniques which are more than improving IT industry. That's good for industry, but not for consumers. why? Sometime it brought compatibility issue like today's case. 

Slow performance issue was raised due to duplex optioning mis-match between Cisco device and 3Com switches. Well, ended up we found a solution, but it was definitely troublesome.  Here is what the issue I was facing and how that was resolved.




Network layout;

ISP - Router - 3com switch - firewall - PC

ISP = ?

Router = Cisco 3845, WAN(PA-T3), LAN(2port GigE), ISO 12.4

Switch = 3Com 4500 series(Fast Ethernet ports)

Initial issue;

Slow upload speed compared to download speed. Download speed was about 35Mbps, but Upload speed was 3Mbps only. Why? Bad circuit? Defected devices? ISP issue?...... many things in my mind. After checking both side of device, I realized it was neither link issue nor device issue. It was duplex mismatch.

Why half duplex mis-matching is caused speed problem?

Here is quick explanation about Full duplex and Half duplex. Ethernet uses CSMA/CD (Carrier Sense Multiple Access With Collision Detection) as a access control protocol. Basically, each node (PC) is monitoring a shard link (Ethernet) and if no one is using it, a node sends data. If two nodes send data at the same time, both nodes declare "Collision". This mechanism is CD (Collision Detection) function. Why we need it? B/c Ethernet technology is using shared media access control machanism. However if you are using Ethernet link for only two nodes, you dont' need CD (Collision Detection) function. Just send data out whenever you want, b/c no collision is expected. This is Full duplex. Like serial links. So if you are using Ethernet technology on point-to-point link, you should make sure it is working full duplex if you are expecting fast transport speed.

Troubleshoot Option issue;

We set (hard coded) both devices with full duplex speed 100. Both devices took the setting and seems working, no collisions and indicated "Full duplex / speed 100" both end, but traffic from Cisco to 3com (download) was fine, but other direction is not. Once we change setting on Cisco side to duplex Auto/ speed Auto, then interface status changed to "Half duplex" at speed 100. It leads me to think hard coded setting on 3Com with Full duplex / speed 100 didn't work properly.
Most of tech prefers to use hard coded option, b/c they don't want to see the option keep on changing by negotiation. Wll, hard coded option is not always good in my experience.
If you are configuring devices that are produced by different vendors, then you gotta think option is the best TOOL to make it works. Meaning you need to play with option to make what you want. In this case, true full duplex on both ways.

We tried most of options, but all failed. Only option was work below combination;

Cisco GigE port  - Duplex : Full(hard coded) / Speed : auto
3Com 4500 port - Duplex : auto / Speed : auto

 ** Some cases such as Cisco's GigE and Notebook, Cisco GigE switch and other switch, you might want to try below option.

Cisco GigE port  - Duplex : auto  / Speed : 100 (Hard coded)
3Com 4500 port - Duplex : auto / Speed : 100 (Hard coded)

When I am troubleshooting an issue, especially an issue between different vendor products, always looking for best combination of option to make it work. That is my choice. Sometime, theory is just not more than theory.

Good Luck!


Last Updated (Monday, 01 June 2009 12:04)

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