Expressway-E Call Flow with DNS External explanation

[sabir.bedi1]

Hi Team ,

Can you help me with accurate call flow for Expressway -E ,Exp-C ?

Assuming i am getting a call from a VC Endpoint from public >Exp- E>Exp-C>CUCM>SX10 ?

Can you brief this call flow ?

Does DNS - External comes in picture before call is hitting the Exp-E?

How does DNS Help resolving the internal domain for public user ?

Please help with brief explanation

[Jens Didriksen]

For this to work you must have the appropriate SRV records in place on your DNS server, which points to your Exp-E.

For arguments sake, lets say your domain is mycompany.com, the host name of your Exp-E is expe.mycompany.com and your end-point URI is sx10@mycompany.com.

So, when the external party calls sx10@mycompany.com, it will do a DNS lookup, starting with the DNS root servers, asking it they know how to get to mycompany.com. See this graphic for a visual explanation: http://royal.pingdom.com/2009/06/08/a-visual-explanation-of-how-dns-lookups-work/ - the query will eventually reach your DNS server which will direct the call to the appropriate device according to the SRV record, which, in this case is your Exp-E.

The Exp-E will look for sx10@mycompany.com, and, thanks your search rules etc, will route the call to the Exp-C which in turn will route it to the CUCM.

So, yes, the DNS stuff has to  happen before anything else can take place, as it is thanks to the DNS records, that external parties can find your Exp-E.

/jens

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T

[sabir.bedi1]

HI Jens Didriksen ,

Thank you for the brief explanation .It is really helpful .Please find below call flow and scenario for my lab set up .Help me in understanding if DNS is resolved in this case ?and How ?

• Acano Server 192.168.1.25
• CUCM 192.168.1.22
• Exp-C   192.168.1.11
• Exp-E   192.168.5.2(DMZ)
• IVR Port Meeting Place DN 2560
• DNS Server 192.168.5.5 

Call Flow :-

For Internal users : -  DX70---SIP---CUCM----SIP-----ACANO

For VRF A  Calling :- VRF A (Polycom) -----SIP----Expressway-E---------- Expressway -C---------Acano

For VRF B  Calling : VRF B(SX10) ---SIP – Expressway -E----------- Expressway -C----------Acano

VRF Users dials 2560@192.168.5.2 to reach Acano Meeting place .

[Jens Didriksen]

DNS is involved in all in- and outbound calls when they go through the Exp-E. (For outbound calls to external sites, it will do the DNS lookup for you as long as it has been configured to do so.

However, your DNS server is on a private IP address, which means it's not reachable by external sites, so any DNS lookups from external sites will fail.

You will need a public facing DNS server as well, which will allow you to use split DNS, which is pretty cool.

This means you have two zones for the same domain, one used by the internal network, the other used by the external network.

So, internal hosts are directed to the internal DNS server for name resolution, whilst external hosts are directed to an external DNS server for name resolution.

So now you can use different SRV records for internal and external clients.

/jens

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[sabir.bedi1]

Hi DNS 192.168.5.5 is in DMZ Zone separate VLAN in my scenario i.e public facing 

but once a user dials 2560@192.168.5.2 please explain if DNS comes in picture or NOT

[Jens Didriksen]

You are bypassing DNS when using alias@ip_address, so DNS is not involved at all.

DNS will only be used when using alias@domain format.

/jens

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[VCsupport17]

Hi Jens,

Thanks for this explanation. But in our case we have these:

Endpoint URI registered on CUCM is 97xx@IP_Address_of_CUCM (9700@10.240.1.10).

Internal domain used for CUCM is mycompany.com for example.

Expressway-E domain is external.com for example.

Let say SRV records and DNS are correctly configured on Expressway-E, how can the external endpoint dial the endpoint registered in the CUCM with URI 9700@10.240.1.10?

Thanks,

Sy

[Jens Didriksen]

For SRV records to actually work, they need to be able to dial alias@domain, i.e. 9700@external.com, the SRV records is pointing to the Exp-E so the call will hit it, then transform the address so it's routed to the Exp-C and from there to the CUCM.

/jens

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