In last week's newsletter, I wrote about configuring VLANs and trunking on Cisco
switches. For a small environment, it is relatively easy to segment the network
into smaller collision domains using VLANs. In a large campus environment with a
large number of switches, it becomes difficult to maintain VLANs across the LAN.
This week's newsletter will discuss how the VLAN Trunking Protocol (VTP) makes
the administration of VLANs easier an environment with multiple Catalyst
switches.
The core principle of VTP is that interconnected switches are
configured to belong to the same VTP domain (sometimes referred to as a VLAN
management domain). The VTP domain is a logical group of switches that will
share VLAN information. Each switch can only belong to a single VTP domain. The
switches in a VTP domain must be adjacent, and the links connecting the switches
must be configured for trunk mode (we learned how to set up trunking last week).
Swtiches that are members of a VTP domain can be set up in one of three
possible modes:
1) VTP Server Maintains the VLAN database. VLAN
information is stored in NVRAM (flash). VLANs can be created, deleted and edited
on the server.
2) VTP Client Maintains the database but does not
store the information in NVRAM. VLANs cannot be created, deleted and edited on
clients.
3) Transparent Does not participate in the VTP domain. VTP
advertisements are forwarded. VLANs can be created, deleted and edited, but are
local to the switch only. Local VLAN information is stored in
NVRAM.
Switches are configured to be VTP servers without a VTP domain by
default. To join a VTP domain, the switch must be manually configured, or by
connecting the switch to a VTP domain by a trunk port (the switch will receive
domain information via a VTP advertisement).
When changes are made to a
VLAN configuration on a VTP server, VTP advertisements are sent out all trunk
ports to propagate the changes to the rest of the domain. VTP advertisements can
be sent over ISL, 802.1q, 802.10 and LANE trunks. VTP traffic is sent over the
management VLAN (VLAN1), so all trunks must be configured to pass
VLAN1.
To track changes to the VLAN configuration, VTP relies on a
revision number. When a VTP domain is initially configured, the revision number
is 0. Each time the VLAN database is edited the revision number is incremented
by 1. If a switch receives a VTP advertisement with a higher revision number,
the information in the advertisement overwrites the information stored in NVRAM.
Before a switch is added to a VTP domain, the VTP revision number should be
cleared for this very reason. You can zero the revision number by changing the
switch to transparent mode and then back to server mode, change the VTP domain
name to a non-existent name and then back to the original, issue a clear config
all command and reboot the switch (note that this will erase the entire switch
configuration).
To configure an IOS based switch to be a VTP server,
issue the following commands:
These commands configure the switch to be a VTP server in the VTP
domain vtpdom. The changes are saved and the revision number is incremented when
the exit command is issued.
To view the VTP configuration on a switch,
the command "show vtp status" is used (on an IOS based switch).
This week
I wrote about the basics of setting up VTP to centralize the management of VLANs
on a network. There is a lot more to setting up VLANs and VTP. The Cisco website
is an excellent source of information. The link below provides information about
VLAN and VTP concepts and configurations.
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