This document explains the Interface Descriptor Block (IDB) limit, and provides the limits for the different Cisco IOS® software-supported platforms and Cisco IOS software releases.
There are no specific requirements for this document.
The information in this document is based on the software and hardware releases that the IDB Limits Per Platform section lists.
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.
Refer to Cisco Technical Tips Conventions for more information on document conventions.
An Interface Descriptor Block (IDB) is a special control structure internal to the Cisco IOS software that contains information such as the IP address, interface state, and packet statistics. Cisco IOS software maintains one IDB for each interface present on a platform and one IDB for each subinterface.
There are two main types of IDBs:
Hardware IDBs (HWIDBs)
Software IDBs (SWIDBs)
A HWIDB represents a physical interface, which includes physical ports and channelized interface definitions. A SWIDB represents a logical sub-interface (Permanent Virtual Circuit (PVC) or virtual LAN (VLAN)), or a Layer 2 encapsulation (Point-to-Point Protocol (PPP), High-Level Data Link Control (HDLC), and so forth).
Each physical interface on the router consumes a minimum of two IDBs:
One HWIDB for the physical port
One SWIDB for the Layer 2 encapsulation
A channelized port consumes N+1 HWIDBs, where N is the number of channels within the physical port, plus a minimum of N SWIDBs (Level 2 encapsulation per channel). Any sub-interfaces that you define each add another SWIDB.
Each tunnel interface definition, such as Universal Transport Interface (UTI), Generic Routing Encapsulation (GRE), Multiprotocol Label Switching Traffic Engineering (MPLS TE), or Any Transport over MPLS (AToM) consumes an HWIDB plus one SWIDB per tunnel, plus an additional SWIDB for each additional sub-interface, for example, a Frame Relay PVC, that is tunneled. The tunnel IDBs are in addition to the original interface(s) that are tunneled.
Layer 2 Tunnel Protocol Version 3 (L2TPv3), which replaces UTI in Cisco IOS Software Release 12.0(23)S, does not consume IDBs, because L2TPv3 is a session-based pseudo-wire implementation rather than a defined tunnel interface such as UTI.
The maximum number of interfaces (physical, subinterface, or virtual) a router can handle depends on the maximum number of SWIDBs that the router can use. This limit used to be set to 300 for all platforms, but with the emergence of features such as frame-relay subinterfaces, multilink Point-to-Point Protocol (PPP), and virtual private dial-up network (VPDN) that uses virtual interfaces, this value has proven to be insufficient on some platforms.
Cisco has performed extensive work to scale Cisco IOS software to these new requirements. From Cisco IOS Software Release 11.3T and later, the IDB limit depends on the platform and the Cisco IOS software release. The IDB limit now indicates the maximum number of interfaces a router can handle, if you assume that other resources, such as memory, CPU, and so forth, are available.
In order to see the maximum number of IDBs, and the number of IDBs currently in use, along with their memory consumption, use the show idb IOS command. This command is available in Cisco IOS Software Releases 12.1(9), 12.1(9)E, 12.1(9)EC, 12.0(18)S/ST, 12.2(x), 12.2(x)T, and 12.2(2)B.
If you monitor the number of IDBs currently in use, you can re-configure or add capacity as the IDB limit is approached for dial and aggregation purposes.
The output of the show idb command looks similar to this:
Router#show idb Maximum number of IDBs 4096 42 SW IDBs allocated (2440 bytes each) 40 HW IDBs allocated (5760 bytes each) HWIDB#1 1 SRP0/0 (HW IFINDEX, SRP) HWIDB#2 2 POS1/0 (HW IFINDEX, SONET, Serial) HWIDB#3 7 FastEthernet3/0 (HW IFINDEX, Ether) HWIDB#4 8 FastEthernet3/1 (HW IFINDEX, Ether) HWIDB#5 9 FastEthernet3/2 (HW IFINDEX, Ether) HWIDB#6 10 FastEthernet3/3 (HW IFINDEX, Ether) HWIDB#7 11 FastEthernet3/4 (HW IFINDEX, Ether) HWIDB#8 12 FastEthernet3/5 (HW IFINDEX, Ether) HWIDB#9 13 FastEthernet3/6 (HW IFINDEX, Ether) HWIDB#10 14 FastEthernet3/7 (HW IFINDEX, Ether) HWIDB#11 15 POS4/0 (HW IFINDEX, SONET, Serial) HWIDB#12 16 POS4/1 (HW IFINDEX, SONET, Serial) HWIDB#13 17 POS4/2 (HW IFINDEX, SONET, Serial) HWIDB#14 18 POS4/3 (HW IFINDEX, SONET, Serial) HWIDB#15 19 GigabitEthernet6/0 (HW IFINDEX, Ether) HWIDB#16 21 POS10/0 (HW IFINDEX, SONET, Serial) HWIDB#17 22 POS11/0 (HW IFINDEX, SONET, Serial) HWIDB#18 23 Loopback0 (HW IFINDEX) HWIDB#19 24 Loopback1 (HW IFINDEX) HWIDB#20 25 Tunnel100 (HW IFINDEX) HWIDB#21 26 Tunnel909 (HW IFINDEX) HWIDB#22 27 Ethernet0 (HW IFINDEX, Ether)
Every interface uses an IDB. Therefore, the IDB limit indicates the maximum number of interfaces a router can handle.
The IDB limit is, therefore, the answer to the common question "How many (sub)interfaces can be configured on this platform?"
Each Virtual LAN (VLAN) requires one IDB. Any Cisco IOS software release can support up to 4096 VLANs (0-4095, where the number range is 1 to 4094 and in which 0, 4095 are reserved), if the platform supports at least 4000 IDBs
There is a limitation of 256 bridge groups in the Cisco IOS software release if you use VLAN bridging.
Table 1 lists the IDB limit for the different Cisco IOS software-supported platforms and Cisco IOS Software Releases 11.3T and later:
|Platform/IOS||Cisco IOS Software Release 11.3T||Cisco IOS Software Release 11.3AA||Cisco IOS Software Release 12.0||Cisco IOS Software Release 12.0S||Cisco IOS Software Release 12.0T||Cisco IOS Software Release 12.1||Cisco IOS Software Release 12.1T||Cisco IOS Software Release 12.2||Cisco IOS Software Release 12.2T||Cisco IOS Software Release 12.3||Cisco IOS Software Release 12.3T|
Limits in bold denote value changes.
The numbers in this table are nominal values. Real values might vary. Consult your Cisco Sales Engineer (SE) for details.
|Platform/ IOS||Cisco IOS Software Release 12.0.28.S||Cisco IOS Software Release 12.2||Cisco IOS Software Release 12.3(7)X12|
|ESR 10000||Yes (Can have up to16383)||Yes||Yes (Can have up to 65530)|
|ESR 10700||Yes (12.0SP)||No||No|
Table 3 indicates the IDB limit for the different Cisco IOS software-supported platforms and Cisco IOS software releases (earlier than 11.3T):
|Platform/IOS||Cisco IOS Software Release 11.3||Cisco IOS Software Release 11.2||Cisco IOS Software Release 11.2P||Cisco IOS Software Release 11.1||Cisco IOS Software Release 11.1CC||Cisco IOS Software Release 11.1CA||Cisco IOS Software Release 11.0|
|Platform/IOS||Cisco IOS Software Release 12.3T|
Table 5 lists the IDB limit for the Cisco IOS Software Release15.0 M routers. Earlier Cisco IOS software releases may have the same IDB limits.
|812, 819, and 860||300|
|880 and 890||300|
|1861 and 1861E||300|
|2911 & 2921||1400|
|3825 & 3845||1400|
|3925 & 3945||2400|
|3925E & 3945E||4800|
|ASR1000 ESP 2.5||65535 / 16K *|
|ASR1000 ESP 5||65535 / 32K *|
|ASR1000 ESP 10||65535 /32K *|
|ASR1000 ESP 20||65535 / 64K *|
|ASR1000 ESP 40||65535 / 64K *|
Note: *ASR1000 IOS XE allows a maximum of 65535 IDBs. But, the maximum number of supported logical interfaces is lower and varies by model of ESP in use. For example, in ASR 1000 ESP 2.5, the router uses ESP 2.5
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