Requirements of internal SRV records in internal DNS for a MRA deploy

[david.alfaro1]

Hello Dears

I hope you are doing well

The purpose of this message is to know your interpretation to understand some details about the deployment of a MRA infrastructure, specifically about the records of the internal DNS server SRV's.

In the document "Mobile and Remote Access Through Cisco Expressway Deployment Guide (X14.3)" we found the following (page 18):

"Create internal DNS records, for both forward and reverse lookups, for all Unified Communications nodes used with MRA. This allows Expressway-C to find the nodes when IP addresses or hostnames are used instead of FQDNs."

In the document "CCNP Collaboration Cloud and Edge Solutions CLCEI 300-820 Official Cert Guide" we find:

"Just as the public DNS SRV records must refer to the Cisco Expressway-E servers, the internal DNS SRV records must refer to all call processing nodes of a Unified CM cluster, as well as with all Unified CM IM and Presence server SRV records." (page 252)

"When a trusted connection between Cisco Jabber and the Cisco Expressway Edge is established, Cisco Jabber tries to register to the services that are enabled on the Cisco Expressway Core, which in this case is the Unified CM. The Cisco Expressway Core sends a DNS SRV record lookup for _cisco-uds._tcp.company.com to the internal DNS server. The internal DNS server responds with the address of the Unified CM. The Cisco Expressway Core then forwards the registration request from the Cisco Jabber client to the Unified CM." (page 254)

And in the document "JABBER MRA DETAILED CALL FLOW" (I attach the document for better reference), there are some points:

• In the point 5b, about the getting the user home cluster step, it says "ExpwC sends http request to cucm" (page 4).
• in the point 6b, about the getting the servers step, it says "ExpwC sends http request to cucm" (page 4).
• in the point 7c, about the getting user devices step, it says "Cucm sends 200 OK with xml file containing the devices associated with the user" (page 4).
• etc, etc. etc.

Now the questions are the following:

1. The customer has a cluster of 9 nodes (publisher + 8 subscribers), however, they want just to register in the internal DNS, in the SRV record of cisco-uds, just 3 nodes (not all of the cluster nodes), because they claim the just want to use one CMG, even though in the documentation says "all of the nodes of the cluster", the question is, is it right what customer wants to do?, what happen if there are another different CMG with different nodes registering devices in the scenario the customer is propounding?
2. The second question is about the document ""JABBER MRA DETAILED CALL FLOW" (attached), in the points mentioned in the lines above, where it says "...sends http request to cucm", "...sends http request to cucm", "Cucm sends...", I mean "...to ucm...to cucm..., cucm sends..., etc", the questions is, what does mean "... to cucm, cucm sends ..."?, who is exactly concerning "...cucm..." to? is it mentioning to the publisher of the CUCM, to any particular subscriber of the CUCM, to which one of all of the cluster nodes of the CUCM?, it is vague to say just "cucm" when we are talking about of 9 nodes in the whole cucm cluster.

I appreciate your attention,

Kind regards

[b.winter]

My understand always was, that those A-records returned by the SRV lookup, will be used for the HTTP requests by Jabber.
Like you define the TFTP-option 150 for the phones, to retrieve the config file.
So it's up to you, to which nodes (A-records) the SRV-lookup will resolve.

But adding to your question 1: This has nothing to do with how you configure and assign the CMG to phones / endpoints.
CMG is used to determine, which nodes are used for registration, call processing.

[Roger Kallberg]

The Expressway C will know about all the nodes in the CM cluster that has the Call Manager service activated as this is discovered when you add the Publisher into the Unified Communication configuration. With this all the CPE nodes in the CM cluster should be resolvable for the cisco-uds SRV record as you cannot control which node Exp-C will communicate with. About question 2 it is any node in the cluster that acts as a CPE.

[Maren Mahoney · Accepted solution]

For question 1:

You will want forward and reverse A records for all nodes. Only call processing nodes, and in your case at least initially only the three call processing nodes you want to use for Jabber registration internally need the _cisco-uds records. It is important to know that the _cisco-uds records are used for Jabber clients to "find" the CUCM cluster at registration, and not for the final registration itself. If the CMG in the config file of the Jabber client indicates that Jabber should register with one of the other call processing nodes (not represented by a _cisco-uds record) Jabber will do so. So even if your customer later decides to use a new CMG that includes call processing nodes that are not represented by a _cisco-uds record, a Jabber client configured with the new CMG would still work.

For Question 2:

I disagree with @Roger Kallberg, though, about Question 2. The get_edge_config from Expressway-E to Expressway-C will cause (as I understand it) the Expressway-C to query its DNS server for _cisco-uds records. If Expressway-C queried for A-records then it could potentially reach out to 'any node' as Roger wrote. But Expressway-C uses the same _cisco-uds records that internal Jabber clients do to 'find' the CUCM cluster for registration. This will cause Expressway-C to reach out to only those nodes.

Maren

[david.alfaro1]

Hello dear Maren,

I truly appreciate your detailed explanation.

Now, I have the following dude.

Regarding that the SRV cisco-uds has the nodes n1,n2 and n3.

The jabber CMG has the nodes n4,n5 and n6.

Once the Jabber sends the sip REGISTER mensage to "userjabber@n4.example.com", who does the  DNS type "A" query, the aforementioned jabber from the remote device in MRA, or the Expressway C? I believe that the Jabber does the query that it pass through both expressway until reach the internal DNS, it is right? or the expressway C continues to do the query like a proxy in behalf of the jabber device?

Kind regards,

[Roger Kallberg]

Not sure if I follow exactly what you wrote, but it is the Exp-C that communicates with CM on behalf of the client that is located outside of the corporate network.

[david.alfaro1]

Hello Roger and to all those who posted in this discussion of course

Thank you a lot again

Really all of the advises were meaningful for me. It is difficult to decide which is the best answer since all are amazing. Unfortunately I am tied to choose only one answer

Kind regards