Written by: admin on 12/15/12

So you have a client that has a VoIP system? They have remote users that need to be able to access the phone system from Internet / VPN? How do you configure an ASA to work with this type of a scenario? Or, even better, why isn’t your ASA configuration working to allow this? If you Google this and look at forums, you will find overly-complicated, convoluted tech-talk and people posting their specific Cisco configs for others to look through and help them with their specific issues as opposed to an easy-to-understand generic formula for how to accomplish this relatively common scenario. So, let’s make it simple:

1.) Log into the Cisco ASDM

2.) First, we need to ensure a NAT policy exists for a Public IP to NAT to the internal IP of the VoIP system / server. Click on “Configuration” at the top, then click on “Firewall” down on the bottom menu. Once in the firewall section, highlight “NAT Rules”

3.) Click on the “Add” option on the right side to add a new static NAT rule and choose “add new static NAT rule”

4.) Original Interface is “inside” with a source that is the internal IP of the VoIP System. The translated Interface is the outside interface. Select the “Use IP Address” option and specify an available static public IP from your ISP that you have not used in a NAT policy yet. Then click “Ok.” Essentially, this tells the ASA to statically (always) translate traffic from inside interface from the inside IP of the VoIP system destined for the outside Interface to translate to the static public IP you specified. In turn, the ASA will automatically translate inbound traffic from the outside static public IP specified from the outside interface to the inside interface destined for the internal IP specified.

5.) Now that has been done, click the “Apply” button at the bottom

6.) Now, we need to add port forwarding rules for VoIP traffic. Click on “Configuration” at the top again and then click on “Firewall” down on the bottom menu again. Highlight “Access Rules” option.

7.) Click on the “Add” option on the right side to add a new access rule and choose “add new access rule”

8.) Choose Interface “Outside” because this is going to be a rule that applies to outside traffic traveling to the inside of the network. Action is to permit. Source is anything out on the Internet (alternatively, you can create a network object or group with specific IP addresses or ranges). Destination is going to be the public NATed IP address for the phone system. Service is tcp-udp/sip (sometimes you may have to create separate rules – one for UDP specific and one for TCP specific SIP. SIP port is 5060 by default)

9.) Repeat step 4 for creating any port forwarding rules you need to have in place based on open ports the VoIP provider specifies as needing to be open. Once done, external remote users should be able to configure their VoIP phones to point to the public IP of your phone system and connect to that phone system to make calls!

10.) Save the running config of the ASA.


11.) You may notice VoIP traffic isn’t fully working in some cases… or sometimes, a phone provider may tell you to “disable SIP / ALG options” in the firewall… so what the heck does that mean? Well, they are talking about an ASA’s default config to inspect SIP packets via its global policy map. By default, the ASA will inspect SIP packets and deal with them how they want to before NATing the packets to the right place. This can cause loss of audio, call quality issues, etc. sometimes if a VoIP system is not meant to have SIP inspection turned on in the firewall. To disable SIP inspection in the ASA, you need to navigate back to “Configuration” then “Firewall” then highlight “Policy Rules.”

12.) Once in “Policy Rules” you highlight the default inspection policy by left clicking on it and then choose the “Edit” button at the top. This will open a new window. At the top, click on the “Rul Actions” tab. Scroll down until you see “SIP” option and then UNCHECK the option and hit “Ok” then click the “Apply” button at the bottom. This essentially sends the following command to the ASA:

policy-map global_policy
class inspection_default
no inspect sip

13.) That’s it! Remote VoIP users on the Internet should be able to configure their VoIP phones to point to the public IP of your phone system and connect to that phone system to make calls!

But what about VPN users using softphones or locations already connected with site-to-site tunnels? No worries, friend. Keeping in mind the settings above regarding what the VoIP system provider may tell you in order to configure your firewall, VPN site-to-site traffic is also governed by an Access List, much like the Firewall Rules. Very similar to steps 7, 8, and 9, you add ACE rules to an existing VPN ACL under the “Site-to-site VPN” option along the bottom of the Configuration page. You drill into “Advanced” and then highlight “ACL Manager.” Select the appropriate Access List (depends on if this is the firewall where the VoIP system is vs. the remote firewall, as both firewalls will need to have these rules added).

And, like with the regular firewall access rules, make sure that the firewalls at both locations are configured to PERMIT the VoIP provider specified traffic from that ASA’s source network to the other ASA’s destination network! Voila! Instant cross-VPN VoIP access to the internal phone system!

Need Help Configuring your Cisco ASA 5505? Contact Exigent Technologies for IT consulting services at 877-394-4368.