Cisco VPN Connectivity Issues "DEL_REASON_PEER_NOT_RESPONDING"

I am having an issue getting some VPN clients to connect to the VPN concentrator and actually establish the connection.

I am using the following:
Cisco 3000 series VPN concentrator
Cisco VPN Client Version 4.0 (REL)
Authentication is done via PKI authentication (Entrust/Entelligence 6.1.100.624)

The issue:
When some users attempt to connect they get the following error message: "Secure VPN Connection terminated locally by the Client. Reason: The remote peer is no longer responding."

This does NOT affect all users, only a few. Thus far I have asked users to modify the MTU settings on the client system itself via the MTU tool included in the VPN client software. That has worked, however; I am still having users get the same error message even after changing the MTU. The MTU value I had them set was 1300 (seemingly the default in the client) but again, for some; no go.

Firewalls have also been disabled and each machine is built from one Windows XP SP2 image, still the same issue after disabling desktop firewalls.

The VPN concentrator log shows the user logging in and being authenticated successfully but then kills the connection citing an error with "Oakley Aggressive Mode" (I do not have complete logs due to this being a managed service. However, since the workstation modifications seemed to work, I do believe this is a client issue versus a concentrator issue especially since other users login fine.)

The users are having problems from home and I am wondering if the issue could be coming from home cable/DSL routers having set the MTU too low or either blocking ISAKMP/IKE traffic. Any and ALL ideas/solutions would be appreciated. Any ideas?

Please keep in mind that other users, most users actually, can connect and maintain the connection to the VPN concentrator.



Here is the complete log from a non-working client:
1      18:22:54.187  06/29/07  Sev=Info/4      CM/0x63100002
Begin connection process

2      18:22:54.207  06/29/07  Sev=Info/4      CM/0x63100004
Establish secure connection using Ethernet

3      18:22:54.247  06/29/07  Sev=Info/4      CM/0x63100024
Attempt connection with server "204.250.118.194"

4      18:22:54.257  06/29/07  Sev=Info/6      IKE/0x6300003B
Attempting to establish a connection with 204.250.118.194.

5      18:22:55.599  06/29/07  Sev=Info/4      IKE/0x63000013
SENDING >>> ISAKMP OAK MM (SA, VID(Xauth), VID(dpd), VID(Nat-T), VID(Frag), VID(Unity)) to 204.250.118.194

6      18:22:55.609  06/29/07  Sev=Info/4      IPSEC/0x63700008
IPSec driver successfully started

7      18:22:55.609  06/29/07  Sev=Info/4      IPSEC/0x63700014
Deleted all keys

8      18:22:55.769  06/29/07  Sev=Info/5      IKE/0x6300002F
Received ISAKMP packet: peer = 204.250.118.194

9      18:22:55.769  06/29/07  Sev=Info/4      IKE/0x63000014
RECEIVING <<< ISAKMP OAK MM (SA, VID(Nat-T), VID(Frag)) from 204.250.118.194

10     18:22:55.979  06/29/07  Sev=Info/5      IKE/0x63000001
Peer supports NAT-T

11     18:22:55.989  06/29/07  Sev=Info/5      IKE/0x63000001
Peer supports IKE fragmentation payloads

12     18:22:55.989  06/29/07  Sev=Info/6      IKE/0x63000001
IOS Vendor ID Contruction successful

13     18:22:55.989  06/29/07  Sev=Info/4      IKE/0x63000013
SENDING >>> ISAKMP OAK MM (KE, NON, NAT-D, NAT-D, VID(?), VID(Unity)) to 204.250.118.194

14     18:22:56.170  06/29/07  Sev=Info/5      IKE/0x6300002F
Received ISAKMP packet: peer = 204.250.118.194

15     18:22:56.170  06/29/07  Sev=Info/4      IKE/0x63000014
RECEIVING <<< ISAKMP OAK MM (KE, NON, CERT_REQ, VID(Unity), VID(Xauth), VID(?), VID(?), NAT-D, NAT-D) from 204.250.118.194

16     18:22:56.170  06/29/07  Sev=Info/5      IKE/0x63000001
Peer is a Cisco-Unity compliant peer

17     18:22:56.170  06/29/07  Sev=Info/5      IKE/0x63000001
Peer supports XAUTH

18     18:22:56.170  06/29/07  Sev=Info/5      IKE/0x63000001
Peer supports DWR Code and DWR Text

19     18:22:56.210  06/29/07  Sev=Info/4      IKE/0x63000013
SENDING >>> ISAKMP OAK MM *(ID, CERT, CERT_REQ, SIG, NOTIFY:STATUS_INITIAL_CONTACT) to 204.250.118.194

20     18:22:56.210  06/29/07  Sev=Info/4      IKE/0x63000013
SENDING >>> ISAKMP OAK MM (FRAG) to 204.250.118.194

21     18:22:56.210  06/29/07  Sev=Info/4      IKE/0x63000013
SENDING >>> ISAKMP OAK MM (FRAG) to 204.250.118.194

22     18:22:56.210  06/29/07  Sev=Info/4      IKE/0x63000013
SENDING >>> ISAKMP OAK MM (FRAG) to 204.250.118.194

23     18:23:01.647  06/29/07  Sev=Info/4      IKE/0x63000021
Retransmitting last packet!

24     18:23:01.647  06/29/07  Sev=Info/4      IKE/0x63000013
SENDING >>> ISAKMP OAK MM *(Retransmission) to 204.250.118.194

25     18:23:01.657  06/29/07  Sev=Info/4      IKE/0x63000013
SENDING >>> ISAKMP OAK MM (FRAG) to 204.250.118.194

26     18:23:01.657  06/29/07  Sev=Info/4      IKE/0x63000013
SENDING >>> ISAKMP OAK MM (FRAG) to 204.250.118.194

27     18:23:01.657  06/29/07  Sev=Info/4      IKE/0x63000013
SENDING >>> ISAKMP OAK MM (FRAG) to 204.250.118.194

28     18:23:04.161  06/29/07  Sev=Info/5      IKE/0x6300002F
Received ISAKMP packet: peer = 204.250.118.194

29     18:23:04.161  06/29/07  Sev=Info/4      IKE/0x63000014
RECEIVING <<< ISAKMP OAK MM (Retransmission) from 204.250.118.194

30     18:23:04.161  06/29/07  Sev=Info/4      IKE/0x63000021
Retransmitting last packet!

31     18:23:04.161  06/29/07  Sev=Info/4      IKE/0x63000013
SENDING >>> ISAKMP OAK MM *(Retransmission) to 204.250.118.194

32     18:23:04.161  06/29/07  Sev=Info/4      IKE/0x63000013
SENDING >>> ISAKMP OAK MM (FRAG) to 204.250.118.194

33     18:23:04.161  06/29/07  Sev=Info/4      IKE/0x63000013
SENDING >>> ISAKMP OAK MM (FRAG) to 204.250.118.194

34     18:23:04.161  06/29/07  Sev=Info/4      IKE/0x63000013
SENDING >>> ISAKMP OAK MM (FRAG) to 204.250.118.194

35     18:23:09.659  06/29/07  Sev=Info/4      IKE/0x63000021
Retransmitting last packet!

36     18:23:09.659  06/29/07  Sev=Info/4      IKE/0x63000013
SENDING >>> ISAKMP OAK MM *(Retransmission) to 204.250.118.194

37     18:23:09.659  06/29/07  Sev=Info/4      IKE/0x63000013
SENDING >>> ISAKMP OAK MM (FRAG) to 204.250.118.194

38     18:23:09.659  06/29/07  Sev=Info/4      IKE/0x63000013
SENDING >>> ISAKMP OAK MM (FRAG) to 204.250.118.194

39     18:23:09.659  06/29/07  Sev=Info/4      IKE/0x63000013
SENDING >>> ISAKMP OAK MM (FRAG) to 204.250.118.194

40     18:23:12.163  06/29/07  Sev=Info/5      IKE/0x6300002F
Received ISAKMP packet: peer = 204.250.118.194

41     18:23:12.163  06/29/07  Sev=Info/4      IKE/0x63000014
RECEIVING <<< ISAKMP OAK MM (Retransmission) from 204.250.118.194

42     18:23:12.163  06/29/07  Sev=Info/4      IKE/0x63000017
Marking IKE SA for deletion  (I_Cookie=645306F0C808F8FF R_Cookie=05A0A1F6DD8D3080) reason = DEL_REASON_PEER_NOT_RESPONDING

43     18:23:12.163  06/29/07  Sev=Info/4      IKE/0x63000013
SENDING >>> ISAKMP OAK INFO *(HASH, DWR) to 204.250.118.194

44     18:23:12.673  06/29/07  Sev=Info/4      IKE/0x6300004A
Discarding IKE SA negotiation (I_Cookie=645306F0C808F8FF R_Cookie=05A0A1F6DD8D3080) reason = DEL_REASON_PEER_NOT_RESPONDING

45     18:23:12.673  06/29/07  Sev=Info/4      CM/0x63100014
Unable to establish Phase 1 SA with server "204.250.118.194" because of "DEL_REASON_PEER_NOT_RESPONDING"

46     18:23:12.673  06/29/07  Sev=Info/5      CM/0x63100025
Initializing CVPNDrv

47     18:23:12.693  06/29/07  Sev=Info/4      IKE/0x63000001
IKE received signal to terminate VPN connection

48     18:23:13.164  06/29/07  Sev=Info/4      IPSEC/0x63700014
Deleted all keys

49     18:23:13.164  06/29/07  Sev=Info/4      IPSEC/0x63700014
Deleted all keys

50     18:23:13.164  06/29/07  Sev=Info/4      IPSEC/0x63700014
Deleted all keys

51     18:23:13.164  06/29/07  Sev=Info/4      IPSEC/0x6370000A
IPSec driver successfully stopped