Hello Ruslan,
One of limitations is given by the maximum count of so-called Interface Descriptor Blocks (IDBs). These IDBs are software data structures that describe the properties of individual interfaces, either physical or virtual, of a router or a switch. There is one IDB allocated for each interface. The maximum counts of IDBs that can be created is strongly dependent on the device platform and IOS version. A device can never have more interfaces than the maximum allocatable number of IDBs. To find out the current number of allocated IDBs and maximum count of IDBs, you can use the show idb command:
Switch# show idb
Maximum number of Software IDBs 5474. In use 86.
HWIDBs SWIDBs
Active 82 82
Inactive 4 4
Total IDBs 86 86
Size each (bytes) 3392 1528
Total bytes 291712 131408
[ ... output omitted ... ]
This particular Catalyst 3560 tells me that the maximum number of IDBs is 5474, and we're already using 86, so there's a plenty of further space.
This is not the only limit, however. Usually, lower-end Catalysts do not support more than 255 or 1005 simultaneously existing VLANs, and without a VLAN, the corresponding interface Vlan will not be usable. So the maximum number of supported VLANs also influences how many Vlan interfaces you can use. You can check this maximum number of supported VLANs in the show vtp output:
Switch# show vtp status
[ ... cut ... ]
Maximum VLANs supported locally : 1005
Number of existing VLANs : 38
VTP Operating Mode : Server
[ ... cut ... ]
It should also be noted that each routed port on a multilayer switch (i.e. a no switchport port) consumes one specific VLAN just for itself (these are so-called internal usage VLANs), so using routed ports instead of Vlan interfaces does not result in any conservation of resources.
There also may be another indirect limits imposed, such as maximum supported number of simultaneously running STP instances (usually, only up to 128 STP instances are supported so either you need to use less than 128 VLANs, or use MSTP to decouple spanning-tree instances from VLANs) or others, but these are the ones I am acutely aware of.
The maximum number of IDBs and VLANs is platform and IOS specific, so it is generally difficult to produce a universally applicable table summing these numbers.
Perhaps this helps somewhat.
Best regards,
Peter
