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25.04.2014
Refer to Cisco Technical Tips Conventions for information on conventions used in this document.
IPSec VPN tunnels can also be configured using GRE (Generic Routing Encapsulation) Tunnels with IPsec encryption.
ISAKMP (Internet Security Association and Key Management Protocol) and IPSec are essential to building and encrypting the VPN tunnel. To help make this an easy-to-follow exercise, we have split it into two required steps to get the Site-to-Site IPSec Dynamic IP Endpoint VPN Tunnel to work.
Our example setup consists of the headquarter router R1 which is assigned a static public IP address, and two remote routers, R2 & R3. Next step is to create an access-list and define the traffic we would like the router to pass through each  VPN tunnel. The Crypto Map is the last step of our setup and connects the previously defined ISAKMP and IPSec configuration together. First we create a crypto map named VPN which will be applied to the public interface of our headquarter router, and connect it with the dynamic crypto maps we named as hq-vpn.
Adding additional remote sites in the future is as easy as simply adding more dynamic crypto maps, incrementing the index number and specifying the match address extended access-lists for each remote network.


At this point, we have completed the IPSec VPN configuration on our headquarter router and we can move to the remote endpoint routers. Our remote routers connect to the Internet and are assigned a dynamic IP address which changes periodically by the ISP. Network Address Translation (NAT) is most likely to be configured to provide Internet access to internal hosts. Site to Site VPN networks with Dynamic remote Public IP addresses can only be brought up by the remote sites. Issuing the show crypto session command at the headquarter router will reveal all remote routers public IP addresses. GRE tunnels greatly simply the configuration and administration of VPN tunnels and are covered in our Configuring Point-to-Point GRE VPN Tunnels article. ISAKMP, also called IKE (Internet Key Exchange), is the negotiation protocol that allows two hosts to agree on how to build an IPsec security association.
This means that if we have five different remote sites and configured five different ISAKMP Phase 1 policies (one for each remote router), when our router tries to negotiate a VPN tunnel with each site it will send all five policies and use the first match that is accepted by both ends. This tells our headquarter router that the remote routers have dynamic public IP addresses and ensures it will try to negotiate and establish a VPN tunnel with any router that requests it.


We will need one dynamic crypto map for each remote endpoint, which means a total of two crypto maps for our setup. The configuration is similar for each dynamic crypto map, with only the instance number (10 , 11) and match address (VPN1-TRAFFIC , VPN2-TRAFFIC) changing.
When configuring a Site-to-Site VPN tunnel, it is imperative to instruct the router not to perform NAT (deny NAT) on packets destined to the remote VPN networks. The time required to bring up the VPN Tunnel is sometimes slightly more than 2 seconds, causing the first ping to timeout. This is usually a good shortcut when trying to figure out the public IP address of your remote routers. The goal is to securely connect both remote sites with our headquarters and allow full communication, without any restrictions.



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