Erroneous GRE+IPSec tunnels behavior

If connections traversing a GRE+IPSec tunnel suddenly start to slow down dramatically and messages similar to the following turn up in the logs:

%CRYPTO-4-PKT_REPLAY_ERR: decrypt: replay check failed

you can try changing the transform set for that tunnel so that it doesn’t use any hashing algorithm (MD5/SHA). The problem lies in router behavior, not in the configuration – it has been discussed at the Cisco Support Wiki.

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