Shared vs Dedicated mode of ports in Nexus 7k

Hi..have you ever seen this error message while trying to configure a port?
“ERROR: Ethernet1/4: Config not allowed, as first port in the port−grp is dedicated”


To understand this, we need to understand what is port-group? Below is the image of N7K-M132XP-12 ine card. This line card has 32 ports and all are 10 Gig port. So what does that mean? Does it mean thate ach one of them is a 10 Gig port and we can have all of these 32 ports connected at the same time and we should be able to get 320 Gbps speed? Not exactly…!!
Yes, they are 10 Gig ports,HOWEVER, that 10 Gig is shared among 4 ports in a group. That group is basically all the ports on same hardware ASIC.

Now take a closer look:
As per the below pic, you can see that even or odd continuous ports are on one side and each group of four ports are on same hardware ASIC. This is a port-group and first port of the group is marked YELLOW as you can see in below diagram.

So, being said that N7K-M132XP-12 has 32 10G ports, it means that each port-group (group of 4 ports for this line card) share 10G speed among themselves. YES!! that is correct. All ports dont get 10G dedicated bandwidth. So, the total capacity of the card is 80G, not 320 (as we were expecting) as there can be 8 port-grp of 4 ports each. This is designed on the concept that “Chances are less that all devices are sending data at the same time”. So, 1,3,5,7 will be in same port-grp and similary 2,4,6,8 and so on…!!
So, 4 ports in a group will share the total available bandwidth of 10G.
What if we have requirement for some critical application that we need dedicated bandwidth of 10 G? In that case, first port of a port-group can be put into “DEDICATED” mode and that port will always be the first one of the group..ie. marked in yellow as shown in above pic. So, 1,2,9,10,17,18,25,26 can be put into dedicated mode and if you have put a port in a port-grp into dedicated mode, all other 3 ports in that group will get disabled. You can not configure them. If you have put Eth1/2 into dedicated mode, and if you try to configure Eth1/4 then you will get : “ERROR: Ethernet1/4: Config not allowed, as first port in the port−grp is dedicated”

Shared mode is the default mode. Command to configure port into dedicated mode is:
We first need to shutdown the port
N7K# config t
N7K(config)#interface Eth1/2
N7K(config-if)#rate-mode dedicated

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