Core issue
This issue is due to the presence of Cisco bug ID CSCse36327.
This issue occurs when the IPSec tunnel previously worked and one of these events occurred:
- The crypto map or Internet Security Association and Key Management Protocol (ISAKMP) is removed and reapplied to the interface.
- The PIX Firewall or Cisco Adaptive Security Appliance (ASA) is upgraded to version 7.x from version 6.x.
- The PIX or ASA is rebooted.
- The remote IPSec peer is rebooted.
What is GRE?
Generic Routing Encapsulation (GRE) is a protocol which is used to encapsulate packets in order to route other protocols over IP network. It was originally developed by Cisco.
GRE is a tunneling tool meant to carry any OSI Layer 3 protocol over an IP network, so a private point-to-point connection is created alike VPN
Resolution
As a workaround, use a match address statement for the Generic Routing Encapsulation (GRE) traffic. If the crypto map and/or ISAKMP is removed and reapplied, issue the clear local-host command.
