Understanding and Troubleshooting IPSec issues

MVP Expert
MVP Expert

IPSec tunnel is not established between two devices. (RAP-Controller, Master-Standby, Master-Local etc..)

  • IPSec uses DES, 3DES, or AES for encryption.
  • IPSec uses RSA for IKE internet key exchange for during peer authentication phase, to ensure the other side is authentic and who they say they are.
  • IPSec tunnel between two devices will be established in two phases.

IKE Phase 1 :

Key Exchange Phase

Establish a secure channel (ISAKMP SA)

Authenticate computer identity using certificates or pre-shared secret  

Secures Phase 2 negotiations



IKE Phase 2 :

Establishes a secure channel between computers intended for the transmission of data (IPsec SA)

Determines encryption and authentication algorithms for user data

Each SA assigned a unique Security Parameter Index (SPI)

Each SA is unidirectional


Both phases create two unidirectional Security Associations (SA). The controller assigns a locally unique Security Parameter Index (SPI) to tag or name that specific entry.

What are the common issues with IPSec ?

  • Firewall blocking connections between peers
    • ESP:  UDP 50
    • ISAKMP/IKE:  UDP 500
    • NAT-T:  UDP 4500
  • NAT
    • NAT can corrupt ESP integrity check. 
    • When used, NAT-T preserves ESP integrity by pre-pending a new IP header as a NAT target
  • Mismatched Preshared Key
    • Used in Phase 1. Peers must have same value
  • Mismatched Security Parameters
    • IPSec peers must agree on encryption and authentication methods for user data
    • Negotiated during Phase 2

Troubleshooting Flow :


  • Enabling debug level for the security log crypto, authmgr, localdb and l2tp processes may help in identifying IPSec issues.
  • These provide detailed visibility in their authentication processes.

The following debugging will help to identify IPSec issues.


  • L2TP debugging:
       #logging level debugging security process l2tp
  • IKE debugging:
       #logging level debugging security process crypto
  • Authmgr debugging:
       #logging level debugging security process authmgr
  • Localdb debugging:
       #logging level debugging security process localdb

IKE negotiation :


IKE authentication Failed :


IKE successful authentication and successful SA :


Step 1 : How to confirm, whether NAT-T traffic session is on or not and what is NAT-T traffic ?

In many cases, Aruba uses NAT-T (NAT Traversal) to protect the IPSec integrity whenever the tunnel passes through a NAT device.

NATT encapsulates IPSec in a UDP 4500 header.

Ensure that the controller sees a NAT-T session from the peer using the above command.


  • If the peer entry is missing, then ensure that external firewalls are not blocking UDP 4500.
  • Ensure that the tunnel map has the correct peer address.
  • Ensure the peer address is a routable and reachable IP address between the peers.


Step 2 : Confirm whether the VPN pool is present and address are available .

  • The tunnel must obtain a Private Inner IP address assigned by the IPSec concentrator
  • Ensure the address pools created and free address are available

Step 3 : Confirm whether the SA is successful or not

  • Confirm the presence of the isakmp sa using the show crypto isakmp sa command.
  • This identifies which peer initiated the connection, and also how the connection was authenticated.
  • The Private IP address is the VPN “inner address” assigned by the controller from the VPN address pool.

  • View details for the SA of interest by extending the crypto command with the ip address of the peer device.
  • This is the “public outer IP address” that is used to route the tunnel across the network.
  • You can use this to first ensure that an SA established for that peer.
  • This indirectly confirms that the pre-shared key matches.
  • If Phase 1 establishes, then the key values matched.

Step 4 : If you don’t get the expected output from the above, check whether the PSK ( Having a valid certificate ) is configured both sides

Step 5 : : If you don’t get the expected output from the above, verify the ISAKMP negotiation

  • Part of the Phase 1 purpose is to negotiate with the peer how they are going to authenticate the tunnel. ISAKMP policies define this.
  • The controller predefines policies as shown in the output. These are used with different purposes as clued by their names.
  • If the peer devices cannot agree on a policy, then the tunnel fails. Ensure both ends have matching policies


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Last update:
‎06-18-2015 05:38 AM
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