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Fails to learn mac address on Fiber interface with ISP


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

no ip redirects

no ip unreachables

load-interval 30

no negotiation auto

no cdp enable


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

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 stays up with no errors on both the sides.



Re: Fails to learn mac address on Fiber interface with ISP

Doesn't feel like a fiber/optical issue but a configuration mismatch on one of the end devices.