Is there any issue if one end interface speed is set to 100mbps?

[Herman2018]

Hi, is there any issue if one end interface speed of direct link is set to 100mbps and the other end is set to auto-negociated or set to 1Gbps? can anyone please advise? thanks in advance!

[M02@rt37]

Hello @Herman2018,

When connecting two devices with different interface speed configurations, it can potentially lead to connectivity issues. In most cases, the link will not come up at all if the two ends have incompatible speed settings.

With most of networking equipment, auto-negotiation is typically supported and should work correctly in most scenarios. However, to ensure compatibility and prevent any potential issues, it is a good practice to set both ends of a direct link to the same specific speed and duplex settings if you have control over both ends.

[Joseph W. Doherty]

"... it is a good practice to set both ends of a direct link to the same specific speed and duplex settings ..."

I believe, for decades now, network vendors have recommended using auto except when you know it doesn't work correctly.

BTW, the "good practice" of hard coding speed/duplex, especially for infrastructure links, was because auto wasn't reliable when first introduced, and such equipment was in the field for a decade or more.  Unless you're still using last century equipment, auto unlikely to be a problem.

Oh, when you disable auto, believe you also disable some low level error detection and often auto MDI/MDI-X too.

[Kasun Bandara]

@Herman2018 Hi yes. there will be issues. your link may not come-up if other side cannot properly match the speed. if auto negotiation works well, its OK. but recommendation is to do change on both sides if you changing manually. do not configure 1G at one end and 100Mbps at another end. 

[Joseph W. Doherty]

For Ethernet, multi-speed interfaces. . .

I recall, if both sides have different speeds hard coded, link will not come up.

Further recall, is one side has hard coded speed or is a single speed interface, and the other side does not, but it supports a similar speed, it will match it and come up.

Unlike duplex, multi-speed hardware can "see" signal speed.

Note, above has "classic" 10/100 or 10/100/1000 in mind.  I've read, within these forums, mGig might work differently.