Hi Zeeshan,
Modular switches are generally high capability switches or we can say high end switches which can be customized as per network requirement for an example 6500,4500 switches. These switches are modular and can add different line cards as per your requirement. Like if you want 48 port ethernet card , this can be installed in 6500 and later you want to remove it and just add 16 port gug module you can do the same. You can also install firewall module, wireless module, ids module and many more on modular switch like 6500. ALso they have higher capability and somewhat have more features.
Now think of fixed switches like 3550, 2960, 3750 etc can you think of changing the ports or type of physical interface on these switches? No. Once you baught 3750 with 48 port 10/100/1000 potr you cannot change the physical interfaces on this as they are fixed on to the box. ALso you cannot upgrade DRAM and FLASH on the same, once installed from factory done.
But modulat switches have the capability that if you want more DRAM of flash it can be upgraded as per the network requirement.
HTH
Ankur
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