LAN speed mismatch

[jyuffa]

What will be an ethernet interface status (line and protocol) if I have 10 Mbps and 100 Mbps interfaces on the same LAN? How to hard-code interface to be 100 Mbps and full duplex?

Thanks.

JY

[robho]

If the interfaces are directly connected to each other, than Network Link Pulse (NLP) should detect the signal and properly set the port to the correct speed. There should not be a speed mismatch between the 2 devices. The interface on the router/switch should report X/down. The command under the interface would be "speed 100" Make sure to hard-code the connecting device as well.

But, if you are referring to a mixed LAN environment where some go to 10Mbps and others to 100Mbps, than I do not see a problem. That is, if they match between 2 directly connected neighbors.

[jyuffa]

Hi rodho,

The my question actually came out from an exam. It was actually 2 routers with fa interfaces directly connected. I was troubleshooting them using a simulator. I found in one router under show int fa0/0 command , that int is up, protocol is down, and then discovered that speed is 10. I was running low on time and didn't complete my trubleshooting. So, couls speed mismatch make a protocol on 1 ether card down? What other indications (errors, collisions and etc) are may point to such mismatch?

Thanks.

[aslam]

Hi,

There are two different results due to mismatch in speed and duplex respectively.

1. In case of a speed mismatch, the Link will not come up.

2. A Duplex mismatch will result in Performance Issues. You can see Runts and other packet errors on the interface. You will see collisions if both the sides are configured for Half-duplex and its normal to see some collisions then.

In order to troubleshoot, if both the devices are Cisco devices, put them in Speed Auto and Duplex Auto and see what is the result of the negotiation.

If they are different vendors, its good to start at 10 mbps Half-duplex and proceed towards 100 mbps full duplex. Keep observing the status and interface statistics and find the optimum combination.

From a Test standpoint, yes a speed mismatch can bring the link status down.

Check this link for more information

http://www.cisco.com/en/US/partner/products/hw/switches/ps700/products_tech_note09186a00800a7af0.shtml#auto_neg_valid

HTH

Aslam.

[jyuffa]

Hi Aslam,

Detailes you provided are great!! But if speed mismatch can bring link down, will it? Or sometimes linkk can be up, protocol is down (the way it was on my test)?

Tahnks in advance.

Joseph

[robho]

Yes, this is definitely a L1 issue. A speed mismatch should not keep the interface up ... and the up/down condition you see is the correct output. You may want to check the remote end to see if it is manually configured. Possible that one is is hard-coded to 10 and the other at 100. Try removing any "speed xx" commands and let the interface autonegotiate. If still not come up, change the cable and make sure that it is cross-over.

[jyuffa]

A little confused: if speed is L1 issue, how int state can be up/protocol is down, i.e the way it was?

[robho · Accepted solution]

I just tried it on my lab and the interface is up/down when there is a speed mismatch.

[jyuffa]

Thanks for your efforts! For a 'desert' here is a little one question: is it possible to associate router's name with loopback ip address through arp and telnet to the router even when an access link in up/protocol down state?

[robho]

If the ingress interface is the interface that is up/down, then there is no possible way to reach the loopback interface.

Also, I do not understand your question regarding the association of the router's name and loopback. In ARP, you associate the MAC-address to an known IP address. You do no associate a router's name.

[jyuffa]

OOO-ps, mixed DNS function with ARP.It is hard to beleive that test creators (for CCNP level certification) made isolated faulty router in simulator question on purpose to fail candidates! But it is a fact now. I'll fight for my score. Promise to keep your name anonimous. Thanks for your help again!!!

Joseph