Cisco → Re: Curious gotcha between two 3750s

Static dot1q with nonegotiate, vtp transparent on both ends. Static 10 half on the switch it piggybacks from, auto on the other end. It wouldn't connect otherwise. Yes, native vlan is the same on both ends.

DuClassic duplex mismatch setup. Force both ends to 100Mb Full Duplex in the config. Here is my standard Cisco-to-Cisco trunk configuration:
interface GigabitEthernet1/9
description TRUNK to DEVICE_A [ge0/1]
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 56,59
switchport mode trunk
switchport nonegotiate
speed 100
duplex full
spanning-tree portfast trunk

Some switches would not trunk on 10Mb Ports. Or worse, not trunk correctly. Force it on both ends, explicitly define your vlans, remember that some of the commands are default and hidden, and it should rock and roll.

Full duplex on one end and auto on the other, maybe. But it falls back to half duplex, so there shouldnt be any duplex mismatch in this kind of situation.

Also, why hard code a gigabit interface to 100mbit?

The only gig port on these switches is fiber. The switch it piggybacks from has fiber homerun to the core, but not the second one. Else this piggyback situation wouldn't exist.

I would like to get two clean Ethernet runs so I can at least do two-port Etherchannels, and later ask for fiber homerun to core. But when money is tight, this is what we do.

So I went back and tried this on both ends:

no mdix auto
switchp nonego
switchp mod tru
switchp trun allo vla none
switchp trun allo vla add 100
switchp trun allo vla add 101
switchp trun allo vla add 102
switchp trun encap dot1q
spee 100
dup full
 

And still got interface down not-connected.

It only stays up with this on one end:

interface FastEthernet1/0/48
 switchp tru encap dot1q
 switchp tru allow vla 100,101,102
 switchp mod tru
 switchp nonego
 duplex half
 speed 10
 

And this on the other end:

interface FastEthernet1/0/1
 switchp tru encap dot1q
 switchp tru allow vla 100,101,102
 switchp mod tru
 switchp nonego
 

Suffice to say, this is enough for me to consider I'm working with either an extraordinarily long connection barely sufficient for IEEE 802.3 spec or there's EMI interference somewhere preventing this from hitting 100 full.

But I'm open for any ideas.

Might be worth a shot with trying a cable-diag, plus it would give a reasonable estimate of cable length...

test cable-diagnostics tdr interface fa x/y/z
show cable-diagnostics tdr interface fa x/y/z