Max amount of GRE tunnels

[Ekaterina Romanova]

Hi,

could you tell me how much GRE tunnels can i configure on a router? And do we have any limitations?

thank you

[Richard Burts · Accepted solution]

There is not a limit on the number of GRE tunnels other than the limit on the total number of interfaces supported on that router model. Each interface (both physical and virtual) requires an Interface Descriptor Block and the number of IDBs has a limit. The exact limit varies by platform (and sometimes by version of code). You can configure as many GRE tunnels as you want until you reach the number of IDB for your platform.

HTH

Rick

[sandeeprajapati]

Hello, Can we configure 2 GREs in a CISCO 3925 router in a Primary/Secondary way.

[Leo Laohoo]

It'll depend on the exact model of your router.

Gets even worst if your router doesn't have a VPN card. 

[Sandeep Choudhary · Accepted solution]

Hi,

Adding to Richard and Leo.........

The limitation of the interfaces is bind to the IDB (Interface Descriptor Blocks) limit which is a function of the IOS and the Platform. You can use the show idb command on your platforms to see what the limits will be on your gear.

I am not much aware about 1800/2800/3800, but i think 7200 series router support 8000 GRE tunnel.

Please find link for your ref.

http://cisco.com/en/US/products/sw/wirelssw/ps873/products_data_sheet09186a00801c33a8.html

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?"

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

Reagrds

[sandeeprajapati]

Hello, Can we create 2 GRE tunnels in a Cisco 3925 router in Primary/Secondary way.

[Georg Pauwen]

Hello,

do you mean one tunnel as the backup of the other ? In what context (e.g. DMVPN) ?

[Ekaterina Romanova]

thank you, guys! now it s clear for me

[Richard Burts]

I am glad that our responses helped to clarify this for you. Thank you for using the rating system to mark this question as answered.

HTH

Rick

[Joseph W. Doherty]

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Posting

I just want to note, it's unusual to exceed the number of allowed GRE tunnels, as the limits are rather large.

What is more problematic, is whether your device can handle the additional load imposed by using GRE tunnels, especially if there's IP packet fragmentation.  When dealing with GRE load issues, it's the volume and "kind" of traffic that often matters more than the number of tunnels.

If your device supports it, using the tcp mss-adjust command can be very helpful.

Also note, not all Cisco L3 switches support GRE tunnel (endpoints), and occasionally, they may perform GRE tunnel end-point processing in "software" (which is often much, much slower than their "normal" hardware packet forwarding performance).