Can't get a new 5400 to use SFP module uplink

[RichardH]

I'm replacing an old Dell switch with a 5420M-24 with a fiber uplink. The old switch worked fine with it's SFP fiber connection but when I moved it to the 5420 I can't get the switch to talk to the network at all. I enabled Ethernet mode on the 5420 with the `disable stacking-support` command but that didn't help.

The U1 port seems to be active and processing traffic (the green light blinks) but the switch still doesn't acquire an IP address from our DHCP server and I can't even ping our firewall.  I also tried installing a new SFP+ module in one of the uplink ports but I get the same result.  As far as I can tell the ports are active and the packet counts are going up, but it still can't see anything on the other side of the fiber connection.

I'm new to the 5xxx series and have more experience with the older X440 stuff.  I feel like I'm missing something simple (and stupid) but I just don't know what it might be.

[Not_Very_Good]

FWIW, I am doing an upgrade from the 440 model to 5520 series and have had a similar issue. I'm guessing, cause I hate to assume, your VLANs match on both ends, etc..  If this is true, I'd suggest swapping optics on both sides. This was my issue even with a link light. This also eliminates the optic as a problem. 

Oh, and don't forget to confirm speed + duplex settings. My predecessor had changed these and I found out through troubleshooting. The joys of no documentation. lol

[RichardH]

That's a good thought about swapping optics on both sides.  The only thing that gets me is that I am using the same SFP module that I was using in the old switch, so the optics should be OK.

I'm not sure about the VLANs matching on both sides.  Our network is pretty simple and I've never configured VLANs at the network level.  The X440 is pretty bone stock and the 5540 is new out of the box.  I set up both of them so I'm pretty sure there's nothing wonky there, but you never know.

[Not_Very_Good]

My concern with the optic is compatibility. If I read this right, you going from a Dell switch to an Extreme switch. I've seen where an optics firmware will not work with a brand of switch. Matter of fact I once spoke with Fujitsu and they straight up said they program their firmware to NOT work with other brands. Otherwise, I'm with you on this that it should be working if there's a link light.

[RichardH · Accepted solution]

It turns out that the problem was stupid simple.  I worked with Extreme technical support and we discovered that each end of the fiber connection was configured differently.  :facepalm:  One side was configured for 1000 full duplex and the other for 10000 full duplex (or something like that).  It wasn't the SFP modules or the fiber link or anything "hard".  I was so focused on thinking that it was something difficult I overlooked the simple stuff.  I guess I thought that both sides of the link were configured to auto-config (like most UTP ports are) and didn't even consider that they might not be.

It was a little weird in that while we were testing it the TX stats would go up but the RX stats were zero; packets would leave the switch but nothing was coming back.  Maybe that can help someone in the future if they have the same problem.