Tuesday, August 30, 2011

Maximum Number of Interfaces and Subinterfaces for Cisco IOS Platforms: IDB Limits


Background Information

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)

Maximum Number of Interfaces

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?"

Maximum Number of VLANs

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.


IDB Limits Per Platform

Table 1 lists the IDB limit for the different Cisco IOS software-supported platforms and Cisco IOS Software Releases 11.3T and later:
Table 1 – IDB Limits
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
as5200 300 300 300 n/a 300 300 300 300 300 n/a n/a
as5300 700 700 700 n/a 800 800 800 800 800 800 800
as5400 n/a n/a n/a n/a n/a n/a 2000 3000 3000 3000 3000
as5800 n/a 2048 2048 n/a 2048 2048 2048 2048 2048 2048 2048
800 n/a n/a n/a n/a 300 300 300 300 300 300 300
ubr900 n/a n/a n/a n/a 300 300 300 300 300 300 300
1000 300 300 300 n/a 300 300 300 300 300 n/a n/a
1700/c1600 300 300 n/a n/a 300 300 300 300 300 300 300
2500 300 300 300 n/a 300 300 300 300 300 300 300
2600/2600XM 300 300 300 n/a 300 300 300 300 800 800 800
3600 800 800 800 n/a 800 800 800 800 800 800 800
3660 n/a n/a n/a n/a 1400 1400 1400 1400 1400 1400 1400
3725 n/a n/a n/a n/a n/a n/a n/a n/a 800 800 800
3745 n/a n/a n/a n/a n/a n/a n/a n/a 1400 1400 1400
3800 300 300 300 n/a 300 300 300 300 300 n/a n/a
mc3810 n/a n/a 300 n/a 300 300 300 300 300 300 300
4000 300 300 300 n/a 300 300 300 300 300 n/a 300
4500/4700 300 300 300 n/a 300 300 300 300 300 300 300
7100 300 300 3000 3000 3000 3000 10000 10000 10000 20000 20000
7200 300 300 3000 3000 3000 3000 10000 10000 10000 20000 20000
MSFC n/a n/a n/a n/a 3000 3000 3000 3000 3000 n/a n/a
ls1010 300 300 300 n/a 300 300 300 300 300 n/a n/a
6400 (nrp) n/a n/a n/a n/a 3000 4500 4500 4500 4500 4500 4500
7500 (rsp/vip) 300 1000 1000 2048 2048 2048 2048 2048 2048 2048 2048
12000 (grp/lc) n/a n/a n/a 4096 n/a n/a n/a n/a n/a n/a n/a

Note: 
  • 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.
Table 2 – ESR 10000 and ESR 10700 IDB Limits and the Supported Cisco IOS Software Releases
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

Additional IDB Limits for All Platforms

Table 3 indicates the IDB limit for the different Cisco IOS software-supported platforms and Cisco IOS software releases (earlier than 11.3T):
Table 3 – IDB Limit for Cisco IOS Software-Supported Platforms and Releases (11.3T and Earlier)
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
All platforms 300 300 300 300 1024 1024 256

IDB limits for various ISR platforms

Table 4 – IDB Limits
Platform/IOS Cisco IOS Software Release 12.3T
1841 700
2801 800
2811 800
2821 900
2851 1000
3825 1200
3845 1400

Ref: http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_tech_note09186a0080094322.shtml
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