Speed and Duplex on a fibre port

[Umesh Shetty]

Hi All,

I have aquery with regards to speed and duplex setting on a fibre port. On my ASR1002 router when I connect a fibre sfp on a port it does not give me the option for setting the speed and duplex, however on the same port if i connect a copper sfp it allows me to configure speed and duples, can any one explain why? Below are the two modules connected.

NAME: "subslot 0/0 transceiver 0", DESCR: "GE SX"

PID: N/A                 , VID: A1C , SN: OPA10263660

NAME: "subslot 0/0 transceiver 2", DESCR: "GE T"

PID: N/A                 , VID: B2  , SN: AGM164222VQ

Thanks in advance

Regards

Umesh Shetty

[Reuben Farrelly · Accepted solution]

Because the speed is set by the type of module it is (1000M) and the duplex is by the very nature of fibre, full duplex.  There's a separate physical strand for TX and RX so there's no situation where you'd ever want to or need to run half duplex.

[Parvesh Paliwal]

What I can say is :

the GE fiber specification only makes provisions for the           negotiation of duplex, flow control, and remote fault detection. GE fiber ports           do not negotiate port speed.

--

Parvesh

[Leo Laohoo · Accepted solution]

I have aquery with regards to speed and duplex setting on a fibre port. On my ASR1002 router when I connect a fibre sfp on a port it does not give me the option for setting the speed and duplex,

Fibre optic only has one speed.  GLC-T, however, is multi-speed (depending on hardware).  Stick the same GLC-T on a 6500 and it becomes a 1Gbps-only.

[Umesh Shetty]

Hi Leo,

How do I identify what device will support spped negotiation and what device will not. Under the device specifications what should I actually look for to identify if the interface will support speed or duplex setting.

Thanks in Advance

Regards

Umesh Shetty

[Leo Laohoo]

How do I identify what device will support spped negotiation and what device will not.

Just presume all SFP will be single-speed.

[Joseph W. Doherty · Accepted solution]

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Posting

Although the port you're using uses Ethernet, copper and fiber Ethernet are different standards.  As noted by another poster, fiber always has dedicated tx/rx so there's no reason for it not be full duplex whereas original Ethernet copper didn't support full duplex, full duplex later became an option with BaseT.

As to speed, in theory, fiber could support multiple speeds, but there wasn't much of a demand for it.  Also it would be much more complex to do so because of the different fiber specifications (e.g. SX vs. LX).

Original copper Ethernet and FastEthernet ports were single speed, but dual speed (and later triple speed) became an option to support different hosts speed capabilities.

[ajipradh]

Hi Umesh, in case of fiber sfp, you need to put the command " no negotiation auto" first, then you can see speed and duplex manually.

"Negotiation auto" being the default in fiber sfps.

[Joseph W. Doherty]

@ajipradh wow!

I didn't know Fiber SFPs can be manually configured to run at different speeds.

Does this support apply to all SFPs on all Cisco ports that support SFPs?  Are this any limitations based on kind of SFP, or brand of SFP?

Does this also work with SFP+ and/or other non-SFP (gig?) transceiver modules?

When you reduce speed of a SFP, say from gig to 100 Mbps, do you get increased distance and/or can you use lower quality fiber for the same distance?

Any recommendations about when running half duplex should be used?

[ajipradh]

Hi, That “negotiation auto” mostly applies to fiber SFPs. I think it also applies to SFP+.
I would not recommend to use Half duplex as it will decrease the performance. The only case where we should use half duplex is when the device at the other end does not support full duplex.