[cisco-voip] session target dns

[Ed Leatherman]

Thanks Anthony, That was spot on what I was trying to figure out. I've been
using server-groups up until now (and will continue on the CUCM facing
side), the service provider is forcing the change on the side facing them.

Loren: That's an interesting idea to lock in the host resolution on the
CUBE itself, but in this case I think it might set me up for an outage if
the service provider changes their IP Addressing. Maybe I can get them to
commit to telling me before they change those..

On Mon, Mar 5, 2018 at 2:31 PM, Anthony Holloway <
avholloway+cisco-voip at gmail.com> wrote:

> Loren,
>
> Just out of curiosity, why didn't you just use session server groups?
> Based on the config you shared, it looks like it would achieve the same
> thing, but with less config, and not adding in the DNS stack within IOS.
>
> Ed,
>
> *Note, you cannot use DNS in server groups, so it's one or the other.
>
> I also think it's important to know that the IOS code is written such that
> it will look for SRV records first, and then fallback to looking for an A
> (host) record once the DNS timeouts.
>
> E.g.,
>
> You enter "session target dns:collab.domain.com"
>
> IOS looks for _sip._udp.collab.domain.com SRV record first, timesout,
> then looks for collab.domain.com host record second.
>
> *Note that the outgoing session transport on IOS is UDP by default, unless
> you change it to TCP with the command "session transport tcp" at the "voice
> service voip" level, or at the dial-peer level.  So, having a _sip._tcp SRV
> record on your CUBE would create a confusing scenario.  Contrast this with
> the incoming connection, which can be either.  However, SRV records, like
> Loren is showing, are for outbound connection establishments.
>
> I have not done an extensive amount of testing here, but I would be
> curious to know if IOS handles the TTL for the DNS record correctly, or if
> it queries DNS for every setup like how that one defect was hitting CUCM
> SIP trunks for a while there.  I looked for "TTL" in the CVP Config guide,
> but it didn't say.
>
> On Mon, Mar 5, 2018 at 11:19 AM Loren Hillukka <lchillukka at hotmail.com>
> wrote:
>
>> You can have your gw query your DNS server, and you have to add SRV
>> records to your central DNS server (like with the jabber entries required
>> to get jabber sign-in to work).
>>
>> Here’s the example of doing local DNS to static entries on the gateway
>> itself, from the CVP 10 config guide.  CVP is where I first started doing
>> dns srv on the local gateway, as I preferred breaking the call center
>> myself instead of having the AD/DNS teams do it for me without me knowing
>> ;-)
>>
>> ===========
>>
>> You can also configure the Gateway statically instead of using DNS. The
>> following example shows how both the A and SRV type records could be
>> configured:
>>
>> ip host cvp4cc2.cisco.com 10.4.33.132
>>
>> ip host cvp4cc3.cisco.com 10.4.33.133
>>
>> ip host cvp4cc1.cisco.com 10.4.33.131
>>
>> For SIP/TCP:
>>
>> ip host _sip._tcp.cvp.cisco.com srv 50 50 5060 <50%2050%205060>
>> cvp4cc3.cisco.com
>>
>> ip host _sip._tcp.cvp.cisco.com srv 50 50 5060 <50%2050%205060>
>> cvp4cc2.cisco.com
>>
>> ip host _sip._tcp.cvp.cisco.com srv 50 50 5060 <50%2050%205060>
>> cvp4cc1.cisco.com
>>
>> For SIP/UDP:
>>
>> ip host _sip._udp.cvp.cisco.com srv 50 50 5060 <50%2050%205060>
>> cvp4cc3.cisco.com
>>
>> ip host _sip._udp.cvp.cisco.com srv 50 50 5060 <50%2050%205060>
>> cvp4cc2.cisco.com
>>
>> ip host _sip._udp.cvp.cisco.com srv 50 50 5060 <50%2050%205060>
>> cvp4cc1.cisco.com
>>
>>  ============
>>
>> Then your dial-peer would have session target dns:cvp.cisco.com which
>> would point to the SRV record, which would use the weight/priority values
>> to choose the final host, and resolve the selected host to an IP using the
>> normal "ip host name x.x.x.x" entry
>>
>>
>> Loren
>>
>> On Mar 5, 2018, at 10:15 AM, Ed Leatherman <ealeatherman at gmail.com>
>> wrote:
>>
>> Hi everyone,
>>
>> Hopefully a quick question - in a dial-peer on CUBE (16.3.5) how does
>> session target dns: resolve to an IP? I've never used DNS as target before
>> for this.
>>
>> Does CUBE just do a query for the A record by default, or does it do a
>> SRV query by default? I have a SIP provider that wants to start using SRV
>> for their SBC(s) and I'm researching how to setup my end in IOS. If it
>> doesn't query SRV default, where do I toggle that behavior?
>>
>> The command reference just says  "Configures the host device housing the
>> domain name system (DNS) server that resolves the name of the dial peer to
>> receive calls."
>>
>> I've found the knob to tell it what SRV format to use in the sip-ua
>> section.
>>
>> Thanks!
>>
>>
>>
>> --
>> Ed Leatherman
>>
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>


-- 
Ed Leatherman
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