Question about autonegotiation on Catalyst switches
daan5000
Member Posts: 34 ■■■□□□□□□□
in CCNA & CCENT
Hello
I'm studying for the ICND1 exam, and have a question concerning autonegotiation.
So, according to Wendell's book, the following rules apply if the other end of the link does not send autonegotiation messages (autonegotiation disabled on one end of the link):
Speed: sense the speed (without using autonegotiation), if that fails, use IEEE rules which state the slowest supported speed for that interface should be used.
Duplex: use the IEEE defaults, if speed is 10/100, use half duplex, otherwise, use full duplex.
So I set up a very simple lab with two switches (2 x Catalyst 3560G running IOS 15.0). I connected port gi0/24 on one switch to port gi0/24 on the other switch. Then I configured one of the ports with 10/full duplex. I left autonegotiation enabled on the other port.
The result I would expect is that the port having autonegotiation enabled would sense the speed setting of 10Mbps and use half duplex because of the second IEEE rule.
However, when running show interface status on both switches, I see the following output:
Port Name Status Vlan Duplex Speed Type
Gi0/24 connected 1 full 10 10/100/1000BaseTX
Port Name Status Vlan Duplex Speed Type
Gi0/24 connected 1 a-full a-10 10/100/1000BaseTX
When I change the duplex setting to half duplex on one port, the other port automatically adjusts to half-duplex.
Can anyone explain why it also configures duplex correctly? According to Wendell, the duplex mode cannot be "sensed".
Thank you
I'm studying for the ICND1 exam, and have a question concerning autonegotiation.
So, according to Wendell's book, the following rules apply if the other end of the link does not send autonegotiation messages (autonegotiation disabled on one end of the link):
Speed: sense the speed (without using autonegotiation), if that fails, use IEEE rules which state the slowest supported speed for that interface should be used.
Duplex: use the IEEE defaults, if speed is 10/100, use half duplex, otherwise, use full duplex.
So I set up a very simple lab with two switches (2 x Catalyst 3560G running IOS 15.0). I connected port gi0/24 on one switch to port gi0/24 on the other switch. Then I configured one of the ports with 10/full duplex. I left autonegotiation enabled on the other port.
The result I would expect is that the port having autonegotiation enabled would sense the speed setting of 10Mbps and use half duplex because of the second IEEE rule.
However, when running show interface status on both switches, I see the following output:
Port Name Status Vlan Duplex Speed Type
Gi0/24 connected 1 full 10 10/100/1000BaseTX
Port Name Status Vlan Duplex Speed Type
Gi0/24 connected 1 a-full a-10 10/100/1000BaseTX
When I change the duplex setting to half duplex on one port, the other port automatically adjusts to half-duplex.
Can anyone explain why it also configures duplex correctly? According to Wendell, the duplex mode cannot be "sensed".
Thank you
Comments
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OctalDump
Member Posts: 1,722
My guess at an answer is that you'd need to explicitly disable auto-negotiation also on the port you manually configured 10/full duplex on. The device is Gigabit, which means that it must support auto-negotiate, which is a different circumstance from a legacy 10baseT device which doesn't support auto-negotiate, so does send any auto-negotiate information ever. It is possible for duplex to be negotiated per the standard, specifically in the technology ability field in the base link code word, transmitted as part of the fast link pulse burst.
The IEEE standard describing how it is meant to work is available here in PDF. It is section 28, starting page 283.2017 Goals - Something Cisco, Something Linux, Agile PM -
daan5000
Member Posts: 34 ■■■□□□□□□□
My guess at an answer is that you'd need to explicitly disable auto-negotiation also on the port you manually configured 10/full duplex on. The device is Gigabit, which means that it must support auto-negotiate, which is a different circumstance from a legacy 10baseT device which doesn't support auto-negotiate, so does send any auto-negotiate information ever. It is possible for duplex to be negotiated per the standard, specifically in the technology ability field in the base link code word, transmitted as part of the fast link pulse burst.
The IEEE standard describing how it is meant to work is available here in PDF. It is section 28, starting page 283.
Thank you for taking the time to look into this issue. Okay, you're saying I need to manually disable autonegotiation on the port that has the statically configured settings. Now, according to the documentation I read, autonegotiation is disabled as soon as you configure both a speed and a duplex setting on an interface, which I did. I looked around a bit and I can't find a command that explicitly tells the switch to disable autonegotiation. I guess what you're saying is that, even though I configure both speed and duplex setting on a port, that port will still tell the other port what settings it should use (in this case 10/full).