Connecting Ethernet interface to Fastethernet interface

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Aug 13th, 2007
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Can Ethernet interface on a 1700 series cisco router be connectrd to the fast ethernet interface of a cisco (Catalyst 2950)switch. I have connected both with half duplex settings on both sides, and I am observing a lot of collisions.

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Overall Rating: 3.5 (2 ratings)
b.petronio Mon, 08/13/2007 - 04:38
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The answer is YES. But could only perform 10Mbps.

Try to match both side, Router and Switch, with 10Mb/s and a full-duplex mode.

Half-Duplex is making the collisions.

Best Regards,

Bruno Petr?nio

Richard Burts Mon, 08/13/2007 - 04:52
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Perhaps we need some clarification about your environment. Your post describes the interface as Ethernet. My experience of 1700 routers is that the built in Ethernet is usually a 10/100 Ethernet. The 1700 can also have an Ethernet card which is a 10 Mb Ethernet. Can you clarify whether you are using the built in 10/100 or the 10 Mb card?

If it is the 10 Mb card my experience is that most of them will only do half duplex. As Bruno says it is important to match duplex on both ends of the connection. If the router is doing half duplex it is important that the switch is also half duplex. If the router is half duplex and the switch is full duplex this will increase the number of collisions (and late collisions) experienced by the router.



b.petronio Mon, 08/13/2007 - 07:39
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Hi Rick,

I have confirmed, and by default the Built In Ethernet port is 10/100. Tks.

About the duplex configuration on 10 Mbps cards, i have confirmed in a 1 WIC-1ENET, it could be different in others. I really dont know.

Take a look on this output:

Router1760(config)#interface e0/0

Router1760(config-if)#duplex ?

full Force full duplex operation

half Force half-duplex operation

Nevertheless, the main issue for Sharad is the matching configuration in both sides, and if you have the duplex-full option, u should use it.

Hope it helps,

Best Regards,

Bruno Petr?nio

vatsey.sharad Mon, 08/13/2007 - 10:25
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Hi, Thanks a lot for the prompt reply,But when i give full duplex on the router's interface it gives me duplex mismatch error logs. which means that the unmamanged switch is working on half duplex. Adding to my 1700 does not support auto negotiation mode. Please advise

b.petronio Mon, 08/13/2007 - 10:35
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U have two options here.

1st- Force HalfDuplex in Router's side, having always collisions.

2nd- Force Switch side to Full-Duplex mode, if u could manage it. To be configured Half-Duplex on switch, someone configured it.

The auto-negotiation works better if both sides have it configured. It seems the switch have it forced.

Some times, when the patching path is bad, i have experienced other kind of errors, like crc, and others, in 100 Mbps configuration, and forcing the connection to 10Mbps i always get a better connection. (with collisions but no other)

I could be the previous case on the switch.

glen.grant Mon, 08/13/2007 - 14:23
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The switch port on the 2950 is set as auto so when you try to set the router to full duplex you will get a duplex mismatch because the 2950 in auto mode will default to half duplex because it cannot negotiate the duplex because the router does not do auto.


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