Cisco routers and switches use interface descriptor blocks (IDBs) to keep track of the information for each interface. These include hardware IDBs (HWIDB) and software IDBs (SWIDB). The former control hardware interfaces (at least two are needed per interface), and the latter control sub-interfaces.
With today’s heavy use of virtual interfaces, IDBs can be a problem on older equipment, because there’s a limit to how many IDBs they support. Nowadays, the number of available IDBs is much higher, but it’s still possible that one may reach the limit with certain intense applications.
To know the number of IDBs your hardware supports, as well as the current status of the IDBs, issue the following command:
show idb
You can also find additional information, such as how many HWIDBs are available for a device, from Cisco documentation at the following page: