GRE Tunnels on Cisco 2921 router

[Tan Han Ming]

Is there a recommended number of GRE tunnels that Cisco 2921 ISR router with default configuration (512MB DDR2 ECC DRAM) can support?         

[Leo Laohoo]

Not really but your bandwidth will play a significant role.

[Joseph W. Doherty]

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Posting

I agree with Leo, bandwidth usage is generally much more important then number of GRE tunnels.  Additionally, percentage of traffic that will need fragmentation.

[Tan Han Ming]

I happened to find this link regarding Interface Descriptor Block (IDB).

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

It states 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

  
  

Does this translate to the maximum of GRE and other Layer 2 encapsulations the router could support?

I'm not sure the maximum number of IDBs the router could support on a particular IOS is based on the default memory installed or maximum memory installed?

[Leo Laohoo]

Does this translate to the maximum of GRE and other Layer 2 encapsulations the router could support?

Tan Han,

Please read my first post and Joseph's post. 

[Joseph W. Doherty]

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

Does this translate to the maximum of GRE and other Layer 2 encapsulations the router could support?

More or less, yes.  But not just L2 encapsulations, maximum number of interfaces.

I'm not sure the maximum number of IDBs the router could support on a particular IOS is based on the default memory installed or maximum memory installed?

Should be maximum amount of memory too.

However, Cisco recommends a 2921 for up to 50 Mbps of WAN bandwidth.  So if you did have about 1,400 GRE tunnels, that's about 36 Kbps each, about half a DS0.  Normally, you'll run out of CPU performance before hitting the limit of how many tunnels you can define.