Content Overview Early VLANs were
difficult to implement across networks. Most VLANs were
defined on each switch, which meant that defining VLANs over an
extended network was a complicated task. Every switch
manufacturer had a different idea of the best ways to make
their switches VLAN capable, which further complicated matters.
VLAN trunking was developed to solve these problems. VLAN
trunking allows many VLANs to be defined throughout an
organization by adding special tags to frames to identify the
VLAN to which they belong. This tagging allows many VLANs to be
carried across a common backbone, or trunk. VLAN trunking is
standards-based, with the IEEE 802.1Q trunking protocol now
widely implemented. Cisco’s Inter-Switch Link (ISL) is a
proprietary trunking protocol that can be implemented in all
Cisco networks. VLAN trunking uses tagged frames to allow
multiple VLANs to be carried throughout a large switched
network over shared backbones. Manually configuring and
maintaining VLAN Trunking Protocol (VTP) on numerous switches
can be challenging. The benefit of VTP is that, once a network
is configured with VTP, many of the VLAN configuration tasks
are automatic. This module explains VTP implementation in a
VLAN switched LAN environment. VLAN technology provides network
administrators with many advantages. Among other things, VLANs
help control Layer 3 broadcasts, they improve network security,
and they can help logically group network users. However, VLANs
have an important limitation. They operate at Layer 2, which
means that devices on one VLAN cannot communicate with users on
another VLAN without the use of routers and network layer
addresses. Students completing this module should be able to:
- Explain the origins and functions of VLAN
trunking
- Describe how trunking enables the
implementation of VLANs in a large network
- Define IEEE
802.1Q
- Define Cisco ISL
- Configure and verify
a VLAN trunk
- Define VTP
- Explain why VTP was
developed
- Describe the contents of VTP messages
- List and define the three VTP modes
- Configure and
verify VTP on an IOS-based switch
- Explain why routing
is necessary for inter-VLAN communication
- Explain the
difference between physical and logical interfaces
- Define subinterfaces
- Configure inter-VLAN routing
using subinterfaces on a router port
Content
9.1 Trunking 9.1.1 History of trunking The
history of trunking goes back to the origins of radio and
telephony technologies. In radio technologies, a trunk is a
single communications line that carries multiple channels of
radio signals. In the telephony industry, the trunking concept
is associated with the telephone communication path or channel
between two points. One of these two points is usually the
Central Office (CO). Shared trunks may also be created for
redundancy between COs. The concept that had been used by the
telephone and radio industries was then adopted for data
communications. An example of this in a communications network
is a backbone link between an MDF and an IDF. A backbone is
composed of a number of trunks. At present, the same principle
of trunking is applied to network switching technologies. A
trunk is a physical and logical connection between two switches
across which network traffic travels.
Content
9.1 Trunking 9.1.2 Trunking concepts As
mentioned before, a trunk is a physical and logical connection
between two switches across which network traffic travels. It
is a single transmission channel between two points. Those
points are usually switching centers. In the context of a VLAN
switching environment, a trunk is a point-to-point link that
supports several VLANs. The purpose of a trunk is to conserve
ports when creating a link between two devices implementing
VLANs. Figure illustrates two VLANs shared across two switches,
(Sa and Sb). Each switch is using two physical links so that
each port carries traffic for a single VLAN. This is the
simplest way of implementing inter-switch VLAN communication,
but it does not scale well. Adding a third VLAN would require
using two additional ports, one on each connected switch. This
design is also inefficient in terms of load sharing. In
addition, the traffic on some VLANs may not justify a dedicated
link. Trunking will bundle multiple virtual links over one
physical link by allowing the traffic for several VLANs to
travel over a single cable between the switches. A comparison
for trunking is like a Highway Distributor. The roads with
different starting and ending points share a main national
highway for a few kilometers then will divide again to reach
their particular destinations. This method is more cost
effective than building an entire road from start to end for
every existing or new destination. Web Links
Understanding and Configuring VLAN Trunk Protocol (VTP)
http://www.cisco.com/en/US/tech/
tk389/tk689/technologies_tech_note
09186a0080094c52.shtml
Content 9.1 Trunking 9.1.3 Trunking
operation The switching tables at both ends of the trunk can be
used to make port forwarding decisions based on frame
destination MAC addresses. As the number of VLANs traveling
across the trunk increases, the forwarding decisions become
slower and more difficult to manage . The decision process
becomes slower because the larger switching tables take longer
to process. Trunking protocols were developed to effectively
manage the transfer of frames from different VLANs on a single
physical line. The trunking protocols establish agreement for
the distribution of frames to the associated ports at both ends
of the trunk. Currently two types of trunking mechanisms exist,
frame filtering and frame tagging. Frame tagging has been
adopted as the standard trunking mechanism by IEEE. Trunking
protocols that use a frame tagging mechanism assign an
identifier to the frames to make their management easier and to
achieve a faster delivery of the frames. The unique physical
link between the two switches is able to carry traffic for any
VLAN. In order to achieve this, each frame sent on the link is
tagged to identify which VLAN it belongs to. Different tagging
schemes exist. The most common tagging schemes for Ethernet
segments are listed below: - ISL – Cisco
proprietary Inter-Switch Link protocol.
- 802.1Q
– IEEE standard that will be focused on in this
section.
Interactive Media Activity Fill in
the Blanks: Trunking Operation When the student has completed
this activity, the student will how using trunk links can the
number of physical interfaces needed on a switch. Web
Links Understanding and Configuring VLAN Trunk Protocol
(VTP) http://www.cisco.com/en/US/ tech/tk389/
tk689/technologies_tech_ note09186a0080094c52.shtml
Content 9.1 Trunking 9.1.4 VLANs and
trunking Specific protocols, or rules, are used to implement
trunking. Trunking provides an effective method to distribute
VLAN ID information to other switches. Using frame tagging as
the standard trunking mechanism, as opposed to frame filtering,
provides a more scalable solution to VLAN deployment. Frame
tagging is the way to implement VLANs according to IEEE 802.1Q.
VLAN frame tagging is an approach that has been specifically
developed for switched communications. Frame tagging places a
unique identifier in the header of each frame as it is
forwarded throughout the network backbone. The identifier is
understood and examined by each switch before any broadcasts or
transmissions are made to other switches, routers, or
end-station devices. When the frame exits the network backbone,
the switch removes the identifier before the frame is
transmitted to the target end station. Frame tagging functions
at Layer 2 and requires little processing or administrative
overhead. It is important to understand that a trunk link does
not belong to a specific VLAN. The responsibility of a trunk
link is to act as a conduit for VLANs between switches and
routers. ISL is a protocol that maintains VLAN information as