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New Member

CSS11503 PAT not working as expected


We are using a CSS11503 to loadbalance SIP traffic. There are no SIP proxies involved and we only want to loadbalance the SIP invites. The CSS is connected in a one-armed set-up. We do not want to NAT the source address so we have no group configuration.

We now have a problem because the loadbalancer is doing a PAT on the source port. The clients can't handle this, because the server is responding directly to the clients with the PATed address as destination port.

We have similar setups with the same CSS and very similar configuration, but the behaviour is different. For the other setup the CSS does not do a source PAT. The only difference is that we use a different client. The clients in our case are Acces Servers, on which E1s terminate.

Some of our clients reside in the same subnet as the servers, but as far as I can determine this should not be an issue, since we want the traffic to return directly and only use the CSS to loadbalance the invites. For us this has working before but with different client Access Servers.

The invites into the CSS are as follows:

UDP: CLIENT-IP:5060 -- VIP-IP:5060 

The invites out of CSS towards server:

UDP:  CLIENT-IP:31234 -- SERVER-IP:5060


!Active version: sg0820001

!*************************** GLOBAL ***************************
  udp-ip-fragment enabled

  flow-state 5060 udp flow-disable nat-enable

!************************** SERVICE **************************

service SERVER-1
  protocol udp
  port 5060
  keepalive frequency 10
  keepalive retryperiod 7
  ip address "SERVER-IP"


!*************************** OWNER ***************************

owner SIP

  content SIP-TRAFFIC
    add service SERVER-1
    protocol udp
    port 5060
    application sip
    advanced-balance sip-call-id
    vip address "VIP-IP"

Do you have any suggestions to turn of the source PAT?

Kind Regards,


Everyone's tags (4)
Cisco Employee

Re: CSS11503 PAT not working as expected

I believe this was fixed with this bug :

CSCea28717    CSS is incorreclty nating a RSH source port

I have tested latest 8.x version and there is no source port pating.

You should consider an upgrade to a more recent version.


New Member

Re: CSS11503 PAT not working as expected

Hi Gilles,

Thanks for your time and answer. I have tested with version sg0820407, but I still experience this issue.

I am just wondering whether this is correct behaviour or not, perhaps this happens because of the "application sip" command.

I have tried configuring a group and do source IP nat, and then configure the command "portmap disable" under the group, but even then the source port is natted. But actually NATing the source IP is not a solution for us because it is causing problems on an application level. At one point the application hands the IP communicationover to the real ip addresses of the client and server.

Do you have any other ideas?