Question Connect two switches with SFP and fiber cable

G.S.

Junior Member
Aug 6, 2022
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Hello,
I would like to connect two parts of my house with two switches using a fiber cable. They are 60 meters away and I could not pass a copper cable into the wall since the tubes are too small.
At one side I have an old 3com 2924-SFP switch, and at the other side I have a brand new QNAP QSW-M2108-2S switch. The former one accept 1 Gbit/s SFP, the latter accept both 1 and 10 Gbit/s SFP(+) connector. A cable guy passed an 80 meters cable, then cutted it down and joined the parts in order to have a 60m cable. He did not tested the cable one joined.
Now, when I connect the switches, the 3com does not get an IP address from the DHCP server connected to the QNAP. I think the connection is not working at all, but the diagnostic logs are very very limited.
I have tried two different SFP on the 3com: the original 3CSFP91 from 3com (1000Base-SX, LC MMF, 850 nm, 275m) and a generic SFP1G-SX-85 (1000BASE-SX, 850nm, DOM, LC MMF, 550m) from fs.com. On the QNAP I only use the generic one.

When I plug the SFP on the 3com, I see this message: "
%Box-I-SFP-PRESENT-CHNG: unit_id 1 SFP 0 status is present".
When I plug the cable on the SFP on the 3com, I see: "
%2SWPHY-I-CHNGCOMBOMEDIA: Media changed from copper media to fiber media (Unknown) on port 21".

I tried both SFP connectors on all 4 SFP ports, having always these messages. Of course, when I plug the SFP in, I see the 3com led switched on, but the port is shown as emplty form the 3com web interface.

On the other side, on the QNAP, there is no available log, but the QSS web interface shows that the port is green when the SFP is connected. It also says: Link is up, 1 Gbps FDX.

How may I better understand what is wrong? Why the connections does not work?

Thank you,
Giuseppe
 

ch33zw1z

Lifer
Nov 4, 2004
35,683
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Switches will have a single IP address. Is the 3com set to DHCP? Have you tried a static IP?

does the QNAP have an IP?

for troubleshooting purposes, move the 3com to the QNAP location and connect them with a prefab fiber. If there’s something wrong with the splice, a prefab should show a change in symptom
 

G.S.

Junior Member
Aug 6, 2022
8
1
36
for troubleshooting purposes, move the 3com to the QNAP location and connect them with a prefab fiber. If there’s something wrong with the splice, a prefab should show a change in symptom
The 3com had the DHCP client active, but I also tried using a fixed address. Tomorrow I'll buy a short fiber cable and I'll try your suggestion.

Thank you,
Giuseppe
 

ch33zw1z

Lifer
Nov 4, 2004
35,683
15,157
146
The 3com had the DHCP client active, but I also tried using a fixed address. Tomorrow I'll buy a short fiber cable and I'll try your suggestion.

Thank you,
Giuseppe
Ok, you can also list what IP range the network is on, and what the QNAP and 3com have for IP’s, if any vlans are involved, and if there are trunk ports
 

G.S.

Junior Member
Aug 6, 2022
8
1
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Ok, you can also list what IP range the network is on, and what the QNAP and 3com have for IP’s, if any vlans are involved, and if there are trunk ports
The two IPs are on the same subnet (QNAP 192.168.222.25/24 via DHCP, 3com 192.168.222.99/24 static). There are no VLAN involved nor port trunks, in fact both switches were just reset to factory setting while testing.

Thank you,
Giuseppe
 

ch33zw1z

Lifer
Nov 4, 2004
35,683
15,157
146
The two IPs are on the same subnet (QNAP 192.168.222.25/24 via DHCP, 3com 192.168.222.99/24 static). There are no VLAN involved nor port trunks, in fact both switches were just reset to factory setting while testing.

Thank you,
Giuseppe
cool, you can test the ips using ping from switch to switch

you can also plug in a client, static ip setup, and test using ping as well

what’s serving DHCP in the network?
 
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G.S.

Junior Member
Aug 6, 2022
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cool, you can test the ips using ping from switch to switch
you can also plug in a client, static ip setup, and test using ping as well
what’s serving DHCP in the network?
Ping works on each side: a client plugged into the QNAP ping the QNAP and cannot ping the 3com. A client connected to the 3com ping the 3com and cannot pint the QNAP.
The DHCP server is a debian machine plugged into the QNAP.

I'll update this thread once tested with a short cable, as you suggested.
 
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G.S.

Junior Member
Aug 6, 2022
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So, I bought a new OM3 MMF LC-LC cable and plugged it into the two switches. There is no change in behaviour compared to the longer cable. Again, I tried both SFP on the 3com.
Moreover, I also connected the two switches using a copper cable with RJ45 connectors and everything worked like a charm.
I am quite confident that the problem is on the SFP compatibility. Since I used the original SFP from 3com on the 3com, I think the problem might be on the QNAP, where I probably used and incompatible SFP.
Is SFP compatibility really a problem?

Strange note: when I plug/unplug the cable from the 3com, on the QNAP I see a note about link up/down. So something works, at least.
 

ch33zw1z

Lifer
Nov 4, 2004
35,683
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So, I bought a new OM3 MMF LC-LC cable and plugged it into the two switches. There is no change in behaviour compared to the longer cable. Again, I tried both SFP on the 3com.
Moreover, I also connected the two switches using a copper cable with RJ45 connectors and everything worked like a charm.
I am quite confident that the problem is on the SFP compatibility. Since I used the original SFP from 3com on the 3com, I think the problem is on the QNAP, where I probably used and incompatible SFP.
Is SFP compatibility really a problem?
Yes, SFP compatibility is a real world problem.


here's a compatible cisco SFP on fs.com for $20


or intel

 
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G.S.

Junior Member
Aug 6, 2022
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Yes, SFP compatibility is a real world problem.
That list only contains SFP+ for a 10gbit/s link, but the 3com only works with SFP socket and with a speed of 1gbit/s. May I mix them? May I put an SFP of 1gbit/s on the 3com and one of 10gbit/s on the QNAP? I don't know if this is possible, at first.

Thank you,
Giuseppe
 

ch33zw1z

Lifer
Nov 4, 2004
35,683
15,157
146
That list only contains SFP+ for a 10gbit/s link, but the 3com only works with SFP socket and with a speed of 1gbit/s. May I mix them? May I put an SFP of 1gbit/s on the 3com and one of 10gbit/s on the QNAP? I don't know if this is possible, at first.

Thank you,
Giuseppe
you can try it, but it could be incompatible. Both would support 1gbps, which is what you want. SFp+ doesn’t support lower though

you could always run a copper cable since that worked for you. 1gbps is 1gbps, for almost everyone the difference between copper and fiber is negligible
 

Fallen Kell

Diamond Member
Oct 9, 1999
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you can try it, but it could be incompatible. Both would support 1gbps, which is what you want. SFp+ doesn’t support lower though

you could always run a copper cable since that worked for you. 1gbps is 1gbps, for almost everyone the difference between copper and fiber is negligible
Except that the first two sentences in the post says:
Hello,
I would like to connect two parts of my house with two switches using a fiber cable. They are 60 meters away and I could not pass a copper cable into the wall since the tubes are too small.
In terms of using mixing/matching SFP/SFP+ optical transceivers, you will need to really dig into the documentation of the switches you have. Some SFP+ switches can accept a SFP transceiver (and some can not). The best solution is to use a SFP transceiver on both ends of the cable (matching mode of the transceiver with the type of optical cable). IF (and that is a BIG IF), you match all the characteristics (same wavelength, same optical mode, same transmit/receive signal strengths, same speed/bandwidth (i.e. 1Gbps)) between both transceivers, it is possible to have one be a SFP and the other a SFP+. It is usually NOT recommended to do this, but instead purchase a switch that is compatible with both SFP and SFP+ transceivers and use matching SFP modules on both sides.

For reference, my main switch, a Brocade ICX 6610-24f, supports both SFP+ and SFP transceivers. Most switches will have a transceiver compatibility matrix published somewhere. Some stores that focus on selling optical cables and transceivers may also publish such information. Case in point, here is one such for my switch:

Doing some digging and it does not appear that the QNAP is compatible with any SFP transceivers (at least officially, sometimes unofficially, it might work, but most places would advertise it as it is a beneficial feature that could result in additional sales):

Also note, I am not recommending you purchase my same switch without doing extensive research (it is a fully managed enterprise piece of gear and can be very load).
 
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ch33zw1z

Lifer
Nov 4, 2004
35,683
15,157
146
Except that the first two sentences in the post says:


In terms of using mixing/matching SFP/SFP+ optical transceivers, you will need to really dig into the documentation of the switches you have. Some SFP+ switches can accept a SFP transceiver (and some can not). The best solution is to use a SFP transceiver on both ends of the cable (matching mode of the transceiver with the type of optical cable). IF (and that is a BIG IF), you match all the characteristics (same wavelength, same optical mode, same transmit/receive signal strengths, same speed/bandwidth (i.e. 1Gbps)) between both transceivers, it is possible to have one be a SFP and the other a SFP+. It is usually NOT recommended to do this, but instead purchase a switch that is compatible with both SFP and SFP+ transceivers and use matching SFP modules on both sides.
yes I saw that 😉
 

G.S.

Junior Member
Aug 6, 2022
8
1
36
Hello,
the QNAP site only lists SFP+ compatible transeivers, but a note on the same site affirm SFP compatibility.
After all my tests failed, I tried upgrading the QNAP firmware. I have not been willing to do that since newer versions blocked the access via serial console, where more commands are available compared to the web interface.
Nevertless, with latest firmware, everything works well.

Thank you all for your help.
 

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