MTU!

Hello

I installed VPN(HUB-spoke) and the VPN connection is OK. Ping is also OK. But when I access the websites of H.O via VPN, the page does not appear.

That's the problem with the MTU size?

My router is a Cisco ISR 2821 with IOS 12.3 (11) T3. This router acts as the firewall and VPN devices.

Does router cisco automatically changes the MTU size for VPN tunnel, when the wan interface is used for the VPN and internet access with the NAT/PAT settings?

-Aline

Vpn IPSec traffic adds about 70 bytes for headers in tunnel mode (20 for the new ip header, 24 for the header ah / esp and around 10-20 more if GRE is used). IPSec VPNs also encapsulate and then fragment, so if you block the fragmented traffic to the HO and then we saw the issue of not getting the trafifc.

With 12.3 IOS T, I believe that there is a command to use a tcp mss/mtu of adjustment, or a substitution of DF (to unplugged the DF bit to allow the fragmentation of the image) on the crypto card and/or the outgoing interface for the router to make the adjustment.

Search Path MTU can not take place if only icmp traffic you allow echo and echo-reply. If you allow inaccessible messages that pmtu can work and you should be able to view the pages. However, that open security holes in order to substitute the mtu/df is the best way to proceed.

Run this test to see if the mtu is causing this issue: on a workstation, set its mtu equal to or less than 1400 max and see if you can view the pages.

If mtu is the case, one or two of these links can help you to understand and resolve the issue.

http://www.Cisco.com/en/us/products/SW/iosswrel/ps1839/products_feature_guide09186a00804247fc.html#wp1052526

http://www.Cisco.com/en/us/customer/products/SW/iosswrel/ps1839/products_feature_guide09186a0080115533.html

Let me know if you need more information.

Tags: Cisco Security

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