Specifications

Engineering Guidelines
130
The IP phones determine the MAC addresses of the L2 ports to which they are connected by
using Spanning Tree Protocol (STP)/Rapid Spanning Tree Protocol (RSTP), or Cisco Discovery
Protocol (CDP). The IP phones then report to the ICP, sending the MAC address of the L2
switch port to which they are connected.
Automatic CESID updating is designed to work in a homogeneously configured network where
all the access L2 switches in a particular subnet (to which IP phones are connected) report
MAC address information by one, and only one, of the following methods:
•STP/RSTP
• CDP
• Both STP/RSTP and CDP
By ensuring one or both protocols are consistently and uniformly enabled on all L2 switches
within a sub-net, the network administrator can guarantee that each IP phone is able to reliably
detect the L2 MAC address and the L2 Port Number of the switch to which it is connected.
The system administrator must define the preferred protocol (STP/RSTP or CDP) to detect
when a phone has moved to a different physical location. This selection is made during system
initialization using the CESID Assignment form in the System Administration Tool.
Figure 13 on page 131 depicts the three preferred network configurations for E911 compatibility.
Note that the access L2 switches are configured uniformly in that they have STP/RSTP, CDP,
or both protocols enabled. Each phone can detect a unique L2 Port MAC Address and L2 Port
Number from the L2 port to which it is connected.
For illustrative purposes the Port Address and Port Number are shown in the format of “A, 1”,
where “A” represents the Port Address and “1” represents the Port Number.
When both STP/RSTP and CDP are enabled, port numbers from STP/RSTP and CDP may not
always match due to vendor-specific implementations, but MAC addresses will always match.
Note: The network port MAC addresses and physical locations must be known before
the IP phones are deployed.