autonegotiation not able to detect the speed

I have a Fa0/7 port in WS-C2950-24 switch connected to a router. I have no access to the router. What I find odd is that in case I enable autonegotiation for speed("speed auto" under Fa0/7 configuration), the "line protocol" of the interface will go down:

WS-C2950-24#sh int Fa0/7 status

Port    Name               Status       Vlan       Duplex  Speed Type
Fa0/7   -> router   connected    596        a-full     10 10/100BaseTX
WS-C2950-24#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
WS-C2950-24(config)#int Fa0/7
WS-C2950-24(config-if)#speed 100 
WS-C2950-24(config-if)#end
WS-C2950-24#sh int Fa0/7 status

Port    Name               Status       Vlan       Duplex  Speed Type
Fa0/7   -> router   notconnect   596          auto    100 10/100BaseTX
WS-C2950-24#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
WS-C2950-24(config)#int Fa0/7
WS-C2950-24(config-if)#speed auto
WS-C2950-24(config-if)#end
WS-C2950-24#sh int Fa0/7 status

Port    Name               Status       Vlan       Duplex  Speed Type
Fa0/7   -> router   notconnect   596          auto   auto 10/100BaseTX
WS-C2950-24#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
WS-C2950-24(config)#int Fa0/7 
WS-C2950-24(config-if)#speed 10
WS-C2950-24(config-if)#end
WS-C2950-24#sh int Fa0/7 status

Port    Name               Status       Vlan       Duplex  Speed Type
Fa0/7   -> router   connected    596        a-full     10 10/100BaseTX
WS-C2950-24#

In case I configure "speed auto" to Fa0/7 it should come up in "a-10"(looks like the router interface is forced into 10BASE-T mode) mode, shouldn't it?

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Comments

vinbuck

Sounds like a physical layer issue...I would try a new cable and verify the config of the far end device. Either Auto/Auto on both sides or hard code speed and duplex. 100 Mbps and up i would autonegotiate. 10 Meg stuff I would hard code.

SharkDiver

I just ran into this on a MetroEthernet router I was installing. The Ethernet switch was pre-programmed and I did not have access to it. The router I was installing would only work when I set the speed to 100 and the duplex to full. When I set the speed to auto, the interface would go down. When I set the duplex to auto, the interface would come up, but would negotiate at half duplex, causing tons of collisions and a very slow connection.

vinbuck

Just FYI....duplex isn't really negotiated unless both sides are auto/auto. When one side is hard coded, duplex cannot be negotiated and the interface falls back to a predetermined duplex setting for the speed of the interface. The standard fallback speed/duplex pairings are:

10 Mbps/Half Duplex
100 Mbps/Half Duplex
1000 Mbps/Full Duplex

SharkDiver

That's correct.

The problem comes in when you have no idea that the device on the other side is set for something other than auto.

vinbuck

You can make a reasonable guess based on the behavior of your interface. Speed can USUALLY be negotiated regardless of whether one side is hard coded because it uses electrical signaling on the copper pairs. So based on your observations, you can deduce that the switch was set for 100 Mbps/Full Duplex.

You can guess whether speed is hard coded by hard coding 10, 100 or 1000 on your side and see if the other side matches. If t doesn't, then it is hard coded for whatever speed will bring the interface active.

SharkDiver

Yes Sir.

That's how I found it.

Good info.