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New Member

DNS SRV behavior for SIP - VCS-Expressway

We have 2 VCS-Expressways facing Internet from 2 Different Countries. (Not Clustered)

Each VCS-E has seperate VCS_Control as Traversal Clients and these Traversal Clients are neighbored to each other.

To aid High Availability for callers from Internet, we have Published the following SRV records in to Internet.

_sip._tcp.example.com

_sips._tcp.example.com

These SRV records point to a A record - vcsgw.example.com

A Record vcsgw.example.com point to 2 IP Addresses of our VCS-E.

When ever someone from Internet call us on SIP, we see 2 SIP Invites from the Source simultaneously on both VCS-E's

Thus calls from internet towards our Internal Endpoints(Registered on H323) fail, stating Loop detected in the VCS-Controls.

Is this the behavior of SIP SRV.

SHould we change SIP SRV's to points 2 A records seperately and each A record point to seperate IP Addresses.

5 REPLIES
Cisco Employee

Re: DNS SRV behavior for SIP - VCS-Expressway

Yes, change the SRV to point to two hosts.

Thanks,
Guy

Sent from Cisco Technical Support iPad App

New Member

DNS SRV behavior for SIP - VCS-Expressway

Thanks Guy. Will do that and see the results.

DNS SRV behavior for SIP - VCS-Expressway

Please always give some additional information, like used software versions, other components involved

(like what is the remote site using which is trying to dial you), which protocolls are used, how does the

dial plan and call flow looks like, ......

I would say there are more dimensions to this.

The proper deployment should look like as Guy said, multiple SRV records pointing to specific A

records which have only one IP assigned to them. So you should fix that first anyhow and sure

if it fixes it its great.

The behavior how the lookup and handling is done is also part of the remote infrastructure trying to

dial you.

In the case of the VCS would at least not point your issue towards the single A record with two IPs (RR / RoundRobin).

I would still expect the VCS to place the call to only one VCS via SIP.

I  tested that in my lab (VCS-E X7.2.2 and I could confirm, first call attempt hits only the first vcs,

a second second call attempt then hits the second VCS, but not at the same time.)

What I could see is that I got one attempt coming in via sip and h323 on the other one due to interworking

on my initiating VCS. So thats at least not wrong, but not SIP and only happened as the alias was not found.

Sure, a loop is not good and shall be prevented, but in general with the VCS

loop protection it should not break the call.

My best guess is that you have some other issues in your dialplan, zones, searchrules, transforms, ...

This will be to complex for a forum discussion as a small detail might get lost which brings in issues

in your deployment or does not really fit your requirements.

What do you think of asking your Cisco partner or some consultant to analyze your needs and reflect that to your deployment.

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New Member

DNS SRV behavior for SIP - VCS-Expressway

Thanks Martin for your detailed queries.

I have vcs at both source and destination and see 2 search hitting my 2 expressways at same time.

We have 7.2.1 in infra and could see only the sip to h323 call is failing.

But the sip to sip call which uses the same srv are working fine.

H323 to h323 and sip to sip is also fine.

DNS SRV behavior for SIP - VCS-Expressway

Check that your interworking is on at least on the vcs where the endpoint is registered.

Check the search history and details if the endpoint was found.

Check how the endpoint is registered, I would set the h323id as a full uri incl the @domain.com

If you dial the e164 number check that you have a proper search rule which strips the

dialed domain matching on the local zone.

You find examples in the basic vcs-c / -e deployment guides.

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