3750G connect to 6506 via fiber problem

Unanswered Question
Dec 19th, 2007
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I have a trunk connecting my 3750G switch with a Fiber ZX SFP module:


Model: WS-C3750G-24PS

Type: 1000BaseZX SFP


connecting to a ZX SFP on a 6506 switch.


On 3750G

interface GigabitEthernet1/0/25

switchport trunk encapsulation dot1q

switchport trunk allowed vlan 120

switchport mode trunk

speed nonegotiate



If I set the speed to "no speed nonegotiate", the link does not come up at all. If I set "speed nonegotiate", the link flaps as seen below.


5w2d: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/25, changed state to down

xxx#

5w2d: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/25, changed state to up

xxx#

5w2d: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/25, changed state to down

xxx#

5w2d: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/25, changed state to up

xxx#

5w2d: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/25, changed state to down

xxx#

5w2d: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/25, changed state to up

xxx#

5w2d: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/25, changed state to down

xxx#

5w2d: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/25, changed state to up


The config on both sides is the same. I have tried all different combinations of speed on both sides.


speed nonegotiate with negotiate

speed nonegotiate with nonegotiate


no speed nonegotiate with negotiate

no speed nonegotiate with nonegotiate


I cant get this link to come up.


Can this have anything to do with spanning-tree?


Any help ?

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wimccall Wed, 12/19/2007 - 19:27
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Spanning tree won't cause this to happen. The line protocol refers to the actual ethernet link establishment (so even if STP put this port in the block/discard state, the line would remain up)


In this case, I'd try another SFP or check out my fiber. This looks like a layer 1 issue at first review.

cisconoobie Wed, 12/19/2007 - 20:13
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That was my guess also. I will try and reseat the fiber and sfp tomorrow.

everestm Wed, 12/19/2007 - 20:26
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How long is the fiber run between the two switches? I connected a 6509 and 4510 with the ZX SFPs, and after a 12km run was only at -1db of loss. I remember reading that Cisco recommends if you have anything less then -8 to add a 10db attenuator to the link.

wimccall Sat, 12/22/2007 - 13:54
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Good point, Everest. Cisco says that if there is less than 8db of total attenuation that you should add a 10db attenuator (because the SFP will shoot between 5 and 0 db). In this case, I'd, optimally, measure the power on the fiber and see if I need to add the attenuator. If you can't measure the power, add the attenuator and see if that doesn't solve your problem.


If you add the attenuator and the problem is still there, the SFPs may be dead from receiving too much power.


(Source: http://www.cisco.com/en/US/products/hw/modules/ps5455/products_data_sheet0900aecd8033f885.html)

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