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Cisco 300-820: What must administrator configure to allow external clients to discover the Expressway-E server

By: Author Alex Lim

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Question

An administrator must configure the DNS SRV records for Mobile and Remote Access for a company with the domain example.com, the Expressway-E FQDN is ExpE.example.com, and the Expressway-C FQDN is Exp-C.example.com. What must the administrator configure to allow external clients to discover the Expressway-E server?

A. SRV record _collab-edge._tls.example.com that resolves to ExpE.example.com on port 8443
B. SRV record _cisco-uds._tcp.example.com that resolves to ExpE.example.com on port 5061
C. SRV record _cisco-uds._tcp.example.com that resolves to ExpC.example.com on port 8443
D. SRV record _collab-edge._tls.example.com that resolves ta ExpE.example.com on port 5061

Answer

A. SRV record _collab-edge._tls.example.com that resolves to ExpE.example.com on port 8443

Explanation

The correct answer is A. SRV record _collab-edge._tls.example.com that resolves to ExpE.example.com on port 8443.

Mobile and Remote Access (MRA) is a feature of Cisco Expressway that allows external clients, such as Jabber or Cisco IP Phones, to register and communicate with the Cisco Unified Communications Manager (CUCM) without using a VPN. MRA requires two Expressway servers: an Expressway-E server that acts as the edge server and faces the public Internet, and an Expressway-C server that acts as the core server and connects to the internal network.

To enable MRA, the administrator must configure the DNS SRV records for the company domain, which is used by the external clients to discover the Expressway-E server. The DNS SRV records are special records that provide information about the hostname, port number, and protocol of a service offered by a domain.

The DNS SRV record for MRA must have the following format:

_collab-edge._tls.<domain> <ttl> IN SRV <priority> <weight> <port> <target>

Where:

  • _collab-edge._tls is the service name and protocol for MRA.
  • <domain> is the company domain name, such as example.com.
  • <ttl> is the time to live for the record, in seconds.
  • IN SRV indicates that this is an SRV record.
  • <priority> is a number that indicates the preference of the record. Lower numbers have higher priority.
  • <weight> is a number that indicates the relative weight of the record. Higher numbers have higher weight.
  • <port> is the port number used by the service. For MRA, this is 8443.
  • <target> is the hostname of the server that provides the service. For MRA, this is the Expressway-E FQDN.

Therefore, for a company with the domain example.com, the Expressway-E FQDN ExpE.example.com, and the Expressway-C FQDN ExpC.example.com, the DNS SRV record for MRA would be:

_collab-edge._tls.example.com 3600 IN SRV 10 10 8443 ExpE.example.com

This record would allow external clients to discover and connect to ExpE.example.com on port 8443 for MRA.

The other options are incorrect because:

  • B. SRV record _cisco-uds._tcp.example.com that resolves to ExpE.example.com on port 5061. This is false because this record is not used for MRA, but rather for service discovery of CUCM by internal clients. The service name and protocol for CUCM service discovery is _cisco-uds._tcp, and the port number is 5061. The target should be the CUCM FQDN, not the Expressway-E FQDN.
  • C. SRV record _cisco-uds._tcp.example.com that resolves to ExpC.example.com on port 8443. This is false because this record is not used for MRA or CUCM service discovery. The service name and protocol for CUCM service discovery is _cisco-uds._tcp, not _collab-edge._tls. The port number for CUCM service discovery is 5061, not 8443. The target should be the CUCM FQDN, not the Expressway-C FQDN.
  • D. SRV record _collab-edge._tls.example.com that resolves ta ExpE.example.com on port 5061. This is false because this record has an incorrect port number for MRA. The port number for MRA is 8443, not 5061. The port number 5061 is used for SIP over TLS, which is a different protocol than MRA.

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