traffic flows between the switches. With ISL, an
Ethernet frame is encapsulated with a header that contains a
VLAN ID. Web Links Understanding and Configuring VLAN
Trunking Protocol (VTP) http://www.cisco.com/en/US/tech/
tk389/tk689/technologies_tech_note
09186a0080094c52.shtml
Content 9.1 Trunking 9.1.5 Trunking
implementation To create or configure a VLAN trunk on a Cisco
IOS command-based switch, configure the port first as a trunk
and then specify the trunk encapsulation with the following
commands: Before attempting to configure a VLAN trunk on a
port, determine what encapsulation the port can support. This
can be done using the show port capabilities command. In
the example, notice in the highlighted text that Port 2/1 will
support only the IEEE 802.1Q encapsulation. Verify that
trunking has been configured and verify the settings by using
the show trunk [mod_num/port_num] command from
privileged mode on the switch. Figure shows the trunking modes
available in Fast Ethernet and Gigabit Ethernet. Lab
Activity Lab Exercise: Trunking with ISL This lab is to
create an ISL trunk line between the two switches to allow
communication between paired VLANs. Lab Activity Lab
Exercise: Trunking with 802.1q This lab is to create an 802.1q
trunk line between the two switches to allow communication
between paired VLANs. Lab Activity e-Lab Activity:
Trunking with ISL In this lab, the student will create multiple
VLANs on two separate switches, name the switches, and assign
multiple member ports to the switches. Lab Activity
e-Lab Activity: Trunking with 802.1q In this lab, the student
will create multiple VLANs on two separate switches, name the
switches, and assign multiple member ports to the switches.
Web Links Understanding and Configuring VLAN Trunk
Protocol (VTP) http://www.cisco.com/en/US/tech/ tk389/tk689/
technologies_tech_ note09186a0080094c52.shtml
Content
9.2 VTP 9.2.1 History of VTP VLAN Trunking
Protocol (VTP) was created to solve operational problems in a
switched network with VLANs. Consider the example of a domain
with several interconnected switches that support several
VLANs. To maintain connectivity within VLANs, each VLAN must be
manually configured on each switch. As the organization grows
and additional switches are added to the network, each new
switch must be manually configured with VLAN information. A
single incorrect VLAN assignment could cause two potential
problems: - Cross-connected VLANs due to VLAN
configuration inconsistencies
- VLAN misconfiguration
across mixed media environments such as Ethernet and Fiber
Distributed Data Interface (FDDI)
With VTP, VLAN
configuration is consistently maintained across a common
administrative domain. Additionally, VTP reduces the complexity
of managing and monitoring VLAN networks.
Content
9.2 VTP 9.2.2 VTP concepts The
role of VTP is to maintain VLAN configuration consistency
across a common network administration domain. VTP is a
messaging protocol that uses Layer 2 trunk frames to manage the
addition, deletion, and renaming of VLANs on a single domain.
Further, VTP allows for centralized changes that are
communicated to all other switches in the network. VTP messages
are encapsulated in either Cisco proprietary Inter-Switch Link
(ISL) or IEEE 802.1Q protocol frames, and passed across trunk
links to other devices. In IEEE 802.1Q frames a 4 byte field is
added that tags the frame. Both formats carry the VLAN ID.
While switch ports are normally assigned to only a single VLAN,
trunk ports by default carry frames from all VLANs. Web
Links VLAN Trunking Protocol Basics
http://infocenter.cramsession.com/
techlibrary/gethtml.asp?ID=1676
Content 9.2
VTP 9.2.3 VTP operation A VTP domain is made up of one
or more interconnected devices that share the same VTP domain
name. A switch can be in one VTP domain only. When transmitting
VTP messages to other switches in the network, the VTP message
is encapsulated in a trunking protocol frame such as ISL or
IEEE 802.1Q. Figure shows the generic encapsulation for VTP
within an ISL frame. The VTP header varies, depending upon the
type of VTP message, but generally, four items are found in all
VTP messages: - VTP protocol version: Either Version 1
or 2
- VTP message type: Indicates one of four types
- Management domain name length: Indicates size of the
name that follows
- Management domain name: The name
configured for the management domain
VTP switches
operate in one of three modes: - Server
- Client
- Transparent
VTP servers can
create, modify, and delete VLAN and VLAN configuration
parameters for the entire domain. VTP servers save VLAN
configuration information in the switch NVRAM. VTP servers send
VTP messages out to all trunk ports. VTP clients cannot create,
modify, or delete VLAN information. This mode is useful for
switches lacking memory to store large tables of VLAN
information. The only role of VTP clients is to process VLAN
changes and send VTP messages out all trunk ports. Switches in
VTP transparent mode forward VTP advertisements but ignore
information contained in the message. A transparent switch will
not modify its database when updates are received, nor will the
switch send out an update indicating a change in its VLAN
status. Except for forwarding VTP advertisements, VTP is
disabled on a transparent switch. VLANs detected within the
advertisements serve as notification to the switch that traffic
with the newly defined VLAN IDs may be expected. In Figure ,
Switch C transmits a VTP database entry with additions or
deletions to Switch A and Switch B. The configuration database
has a revision number that is incremented by one. A higher
configuration revision number indicates that the VLAN
information that is being sent is more current then the stored
copy. Any time a switch receives an update that has a higher
configuration revision number the switch will overwrite the
stored information with the new information being sent in the
VTP update. Switch F will not process the update because it is
in a different domain. This overwrite process means that if the
VLAN does not exist in the new database, it is deleted from the
switch. In addition, VTP maintains its own NVRAM. An erase
startup-configuration clears the NVRAM of configuration
commands, but not the VTP database revision number. To set the
configuration revision number back to zero, the switch must be
rebooted. By default, management domains are set to a nonsecure
mode, meaning that the switches interact without using a
password. Adding a password automatically sets the management
domain to secure mode. The same password must be configured on
every switch in the management domain to use secure mode.
Web Links VLAN Trunking Protocol Basics
http://infocenter.cramsession.com/
techlibrary/gethtml.asp?ID=1676
Content 9.2
VTP 9.2.4 VTP implementation With VTP, each switch
advertises on its trunk ports, its management domain,
configuration revision number, the VLANs that it knows about,
and certain parameters for each known VLAN. These advertisement
frames are sent to a multicast address so that all neighboring
devices can receive the frames. However, the frames are not
forwarded by normal bridging procedures. All devices in the
same management domain learn about any new VLANs configured in
the transmitting device. A new VLAN must be created and
configured on one device only in the management domain. All the
other devices in the same management domain automatically learn
the information. Advertisements on factory-default VLANs are
based on media types. User ports should not be configured as
VTP trunks. Each advertisement starts as configuration revision
number 0. As changes are made the configuration revision number
is increased incrementally by one, (n + 1). The revision number
continues to increment until it reaches 2,147,483,648. When it