Providing Network Professionals The Edge

SRST: Maximum IP Phones and Line DNs Supported Per Platform

SRST Maximum IP Phones

SRST (Survivable Remote Site Telephony) provides a mechanism for remote site IP phones to continue to operate in case of WAN failure.  IP phones registered to a centralized CUCM cluster will continue to funtion if the WAN link is lost.  SRST allows … [Continue reading]

What is GARP (Gratuitous ARP Protection)?

Gratuitous ARP (protection) This is an example of a typical standards based (beneficial) LAN feature which can be used in some cases by the bad guys for malicious purposes.  Therefore the following is a brief description on the feature and why … [Continue reading]

Configuring Voice VLANs (Catalyst 3750 and 3560)

voice vlan

In many cases clients would like to have their employee's PC connected directly to the 3-port switch on the back of the Cisco IP phone sitting on the desk.  This greatly simplifies deployments of end-user stations since one Ethernet cable drop can … [Continue reading]

Cisco Inline Power PoE Commands

show power inline

Cisco (PoE) switches by default have their PoE (Power over Ethernet) enabled on every port.  This article shares some of the common CLI commands to work with the PoE capabilities of a standard Cisco (PoE enabled) switch. The #show power inline … [Continue reading]

Cisco IP Phone Boot Process

Cisco IP Phone Boot Process

Clearly understanding the Cisco IP phone boot process is a very important part of the troubleshooting process for any Cisco network engineer.  I will describe the Cisco IP phone boot process in a few steps below.   Steps: The PoE … [Continue reading]

Why Globalize and Localize Numbers?


The concept of globalizing numbers and then turning right around and localizing them before presentation to the phone initially seems confusing and non-nonsensical.   However, after further deliberation and understanding this concept begins to make … [Continue reading]

North American Numbering Plan (NANP) and E.164 Recommendations

The following is a map of international calling codes from around the world: Zone 1:  NANP countries and territories Numbers beginning with +1 represent North America and mostly Caribbean island countries, not including Mexico or Central … [Continue reading]

Why do I have to add End-Users in CUCM?

We know we have to add phones within CUCM but why end users? Adding "End Users" can allow the end user to make their own changes within the PBX system such as: -Configure their own speed dials (using a web admin page from anywhere on the … [Continue reading]


The Real Time Transport Protocol (RTP) transports audio payload between devices. The Real Time Transport Control Protocol (RTCP) carries the call statistics between endpoints. Ports RTP uses random even UDP ports between 16,384 and … [Continue reading]

Media Processing

The following are the typical types of media processing which DSPs are designed to carry out: Coding: The process of converting TDM voice to packetized voice using CODECs such as G.711, G.729, etc. Transcoding: The process of converting from … [Continue reading]