Fails to learn MAC address - Fiber Interface issues - Weird

Unanswered Question
Dec 29th, 2008

Hi,

We have a problem to bring a new 3750 switch interface up with the ISP.

Current interface configuration on the router 7500 with SC/Single mode 1000 Base LX is

interface GigabitEthernet4/0/0

description ###### ISP #######

ip address 1.1.1.2 255.255.255.252

no ip redirects

no ip unreachables

load-interval 30

no negotiation auto

no cdp enable

end

works perfectly fine.

we are trying to move this link to a Cisco 3750G on SFP single mode 1000 baase LX with the same configuration as below

interface GigabitEthernet1/0/51

no switchport

ip address 1.1.1.2 255.255.255.252

load-interval 30

no ip redirects

no ip unreachables

no cdp enable

speed nonegotiate

we dont get any errors on the link but it fails to learn the mac address from the isp.

checked the following.

1. tried changing the SFP and the fiber.

2. checked internally connecting back to back with another cisco device - works fine .

3. checked with the isp for any static arp on their side and it is a no.

I am wondering why it fails to learn the mac-address when it can self ping its own ip address and also the layer statys up with no errors on both the sides.

Thanks

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Giuseppe Larosa Tue, 12/30/2008 - 00:05

Hello Subramania,

try to add

int g1/0/51

switchport nonegotiate

sometimes this helps

Hope to help

Giuseppe

subra4u Tue, 12/30/2008 - 05:11

Hi,

Thanks for your quick response. Finally it suddenly worked without my or service providers interventions. its a bit weird though.

Thanks

subra

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