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10G and Multigigabit

andreagiorgi
Level 1
Level 1

Hello,

is a 10G rj45 switch port also a multigigabit or only a 10G?

In other words, what happens if I connect a 2.5G port (for example an hotspot) in a 10G port? The resulting speed is 2.5G or the link will not work?

thanks in advance to everyone

andrea

1 Accepted Solution

Accepted Solutions

Joseph W. Doherty
Hall of Fame
Hall of Fame

"is a 10G rj45 switch port also a multigigabit or only a 10G?"

If the port is "just" 10g, it's just that, i.e. 10g only.

Multi-gig ports are also that, i.e. multispeed; somewhat like a 10/100/1000 copper ports, but 1/2.5/5/10 gig (and possibly [?] 100 Mbps).

BTW, perhaps the big advantage of multi-gig ports, you can run 2.5 or 5 Gbps on CAT 5e or CAT 6.  For 10g, you should be using CAT 6A.

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10 Replies 10

Reza Sharifi
Hall of Fame
Hall of Fame

Hi,

Here is an example of a switch supporting multi-gig including 10gig and 2.5 Gig. See datasheet for more details"

C9300-24UX

https://www.cisco.com/c/en/us/products/collateral/switches/catalyst-9300-series-switches/nb-06-cat9300-ser-data-sheet-cte-en.html

HTH

Joseph W. Doherty
Hall of Fame
Hall of Fame

"is a 10G rj45 switch port also a multigigabit or only a 10G?"

If the port is "just" 10g, it's just that, i.e. 10g only.

Multi-gig ports are also that, i.e. multispeed; somewhat like a 10/100/1000 copper ports, but 1/2.5/5/10 gig (and possibly [?] 100 Mbps).

BTW, perhaps the big advantage of multi-gig ports, you can run 2.5 or 5 Gbps on CAT 5e or CAT 6.  For 10g, you should be using CAT 6A.

Leo Laohoo
Hall of Fame
Hall of Fame

Two vital questions about mGIG that needs to be considered:

  1. What happens if a 10 Mbps wired client is connected into an mGIG port? 
  2. Why is it if a 100/1000 Mbps wired client is connected to an mGIG port and stops talking but if I connect it to a non-mGIG port it works? 

When a 10 Mbps wired client connects to an mGIG switch, the link(s) will not go up.  

When a 100/1000 Mbps wired client connects to an mGIG switch, the link will go up, however, if the 100/1000 Mbps wired client requires 10 Mbps for channelling and signalling, the wired client will not talk nor get any DHCP.  

Our usual deployment of 9300 is a non-mGIG and an mGIG switch in a stack.  

This is an interesting remark: "When a 100/1000 Mbps wired client connects to an mGIG switch, the link will go up, however, if the 100/1000 Mbps wired client requires 10 Mbps for channelling and signalling, the wired client will not talk nor get any DHCP."

What do you mean with 10 Mbps for "channeling and signalling" ? We have some clients that seem to have big problems on multigig interfaces. yes, they are old, but work flawlessly on a 10/100/1000 copper SFP port at 100 full duplex speed. Put them on a multigig interface, and nothing but troubles (very unstable even with static ips, ARP often doesn't work).
I believe that older devices that start the auto-negotiation at 10 -> 100 -> 1000 might have problems on multigig interfaces, because the 10 speed is missing on multigig (and also half-duplex capability). 

Possibly, the key in dealing with multi-gig ports, are they are just that multi-gig, i.e. 10/100 NICs might not be compatible, you might need gig capable NICs, even if 10/100/1000.  Or, even with 10/100/1000 NICs, you might need NICs that are "certified" multi-gig compatible.  (BTW, I've lived through the Ethernet to FastEthernet and then to Gigabit Ethernet transitions - it wasn't always "smooth sailing".)

One of my "sayings" is, there's no real surprise in there being "bugs" in software or hardware, when you really dive into the underlying complexities, the real surprise it that some software or hardware works at all.

This is especially true when software or hardware has to meet some (arbitrary) ship or delivery date.  ;  ) 


@gnijs wrote:
What do you mean with 10 Mbps for "channeling and signalling" ?

We had a site which had several of this meeting room clients and they can negotiate to 1 Gbps.  So we said, "cool, since they work with 1 Gbps, let's plug them into mGIG ports" and we walked away. 

We got numerous calls (some from very angry executives) and reported that every one of these clients are not working -- The clients were not pulling DHCP.  Several hours of troubleshooting later (including Fluke TDR*), we decided "for-sh*ts-n-giggles, let's plug those clients to a non-mGIG switch".  

Voilà!  Works immediately.  

* We have stopped using TDR on IOS-XE because it is broken since 16.X.X (CSCwd97177, CSCvw97924).  Any TDR result taken from a switch on IOS-XE versions 16.X.X & 17.X.X is no longer to be trusted nor reliable.  

andreagiorgi
Level 1
Level 1

Hi all,

I’ve eventually bought a cisco switch and found that the answer to my question is hardware specific, i.e. some RJ45 ports are only 10Gb while others are multigigabit ones.

I must say it’s not straightforward to understand it case by case via the data sheet and/or documentations which is sometime tricky! I remember time ago there exist hardware specific documentation for every product.

 

Thank you all,

andrea

"I’ve eventually bought a cisco switch and found that the answer to my question is hardware specific, i.e. some RJ45 ports are only 10Gb while others are multigigabit ones."

Confirming what I posted earlier, correct?  (BTW, just noticed your follow-up with the entry gnijs's posting.  You may have marked my posting as "solved", at that time.[?])

At time I marked as solved, I thought it was a satisfying answer! Today it's still a correct answer, indeed my personal experience is that is hardware specific: when they label a port as multigig is always multigig, but sometimes they label the port as 10G and then you find out it multi gig and not only 10G

Ah, mgig capable ports only labeled as 10g, interesting!

Well, one might argue this is okay provided port works correctly at 10g.

That such a port appears to also provide mgig is possibly an undocumented "feature" (which should not be relied upon).

If additional documentation does note port is mgig, certainly no additional device port labeling is annoying.

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