Understanding How Vlans Work - Cisco WS-C6506 Software Manual

Understanding How VLANs Work

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These ports can be used to integrate a VoIP network with POTS devices, PBXs, or the PSTN.
To configure the Cisco VG200, refer to the documentation that shipped with the gateway.
How a Call Is Made
An IP phone connects to a LAN either through a hub port or a switch port. The IP phone boots up and
uses DHCP to get its IP address and the IP address of its TFTP file server. The IP phone uses its IP
address to talk to the TFTP server and gets its configuration file. The configuration file includes the IP
address of the phone's Cisco CallManager(s). The phone then talks with Cisco CallManager and
registers itself. Each time a phone boots up, it might get a different IP address. Cisco CallManager knows
how to associate a consistent user phone number to a particular phone by using the MAC address of the
phone. Cisco CallManager always maintains a table mapping the phone MAC address and phone
number. Each time a phone registers, the table is updated with the new IP address. During the
registration, Cisco CallManager downloads the key pad template and the feature capability for the
phone. It tells the phone which run-time image it should use. The phone then goes to the TFTP server to
get its run-time image. Each phone has a dedicated TCP connection to Cisco CallManager called the
control channel. All control information, such as key pressing, goes from the phone to Cisco
CallManager through this channel. Instructions to generate ring tone, busy tone, and so on comes from
Cisco CallManager to the phone through this channel.
Cisco CallManager stores the IP-address-to-phone-number mapping (and vice versa) in its tables. When
a user wants to call another user, the user keys in the called party's phone number. Cisco CallManager
translates the phone number to an IP address and generates an IP packet version of the ring tone to the
called IP phone through the TCP connection. When the called IP phone receives the packet, it generates
a ring tone. When the user picks up the phone, Cisco CallManager instructs the called IP phone to start
talking with the calling party and removes itself from the loop. From this point on, the call goes between
the two IP phones through the Real-Time Transport Protocol (RTP) which runs over the User Datagram
Protocol (UDP). Because the voice packets are sensitive to delays, TCP is not suitable for voice
transmission because the timeouts and retries increase the delay between the packets. When any change
occurs during the call due to a feature being pressed on one of the phones, or one of the users hanging
up or pressing the flash button, the information goes to Cisco CallManager through the control channel.
If a call is made to a number outside of the IP PBX network, Cisco CallManager routes the call to an
analog or digital trunk gateway which routes it to the PSTN.
Understanding How VLANs Work
This section describes the native VLANs and the auxiliary VLANs. This section uses the following
terminology:
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Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
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One or two T1 digital ports for connecting to the following:
PSTN using FXO emulation
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T1 channel bank using FXS emulation
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PBX through a trunk (tie) line using ear and mouth (E&M) emulation
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Auxiliary VLAN—Separate VLAN for IP phones
Native VLAN—Traditional VLAN for data
Auxiliary VLAN ID—VLAN ID of an auxiliary VLAN
Native VLAN ID—VLAN ID of a native VLAN
Chapter 55 Configuring a VoIP Network
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