Unit 9 - VLAN Trunking Protocol - VLAN Trunking Protocol...

VLAN Trunking Protocol
Unit 9:
VLAN Trunking Protocol
CCNA 3 – Version 3.1

VLAN Trunking Protocol
Introduction to VLAN Trunking
•
VLAN trunking uses tagged frames to allow
multiple VLANs to be carried throughout a
large switched network over shared
backbones
•
Two trunking protocols:
–
IEEE 802.1Q (widely implemented)
–
Cisco’s Inter-Switch Link (ISL) - a proprietary
trunking protocol that can be implemented in Cisco
networks
•
Once a network is configured with VTP, many
of the VLAN configuration tasks are automatic

VLAN Trunking Protocol
A Trunk is…
•
A physical and logical connection between two
switches across which network traffic travels
•
A single transmission channel between two points
(usually switching centers)
•
A point-to-point link that supports several VLANs

VLAN Trunking Protocol
Another Look at Trunking

VLAN Trunking Protocol
Before and After Trunking
•
Before : Each switch is uses two physical links so that each port
carries traffic for a single VLAN. Simplest way of implementing
inter-switch VLAN communication, but does not scale well.
•
After : Trunking bundles multiple virtual links over one physical
link by allowing the traffic for several VLANs to travel over a
single cable between the switches (conserves switch ports)

VLAN Trunking Protocol
Trunking Operation
•
Two types of trunking mechanisms:
–
frame filtering (brief look at)
–
frame tagging (focus is here)

VLAN Trunking Protocol
Frame Filtering
(not as common)

VLAN Trunking Protocol
Frame Tagging
(the standard)
•
Frame Tagging
–
adopted as the standard trunking
mechanism by IEEE
–
assigns an identifier to the frames; makes
management easier with faster delivery of
frames
–
Most common tagging schemes for
Ethernet segments are:
•
ISL – Cisco proprietary Inter-Switch Link
protocol.
•
802.1Q – IEEE standard that will be focused on
in this section.

VLAN Trunking Protocol
Frame Tagging & Encapsulation Methods
•
Frame tagging places a unique identifier in
the header of each frame as it is forwarded
throughout the network backbone
–
identifier is examined by each switch before any
broadcasts or transmissions are made to other
switches, routers, or end-station devices.
–
Last 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

VLAN Trunking Protocol
IEEE 802.1Q vs. ISL
•
Both are Ethernet frame tagging
mechanisms
•
Both protocols maintain VLAN
information as traffic flows between the
switches
•
IEEE 802.1Q
–
The VLAN ID (4 byte field) is inserted into
the frame
•
ISL
–
The frame is encapsulated with a header
that contains a VLAN ID

VLAN Trunking Protocol
Trunking Implementation
•
To create or configure a VLAN trunk on
a Cisco IOS command-based switch:
•
Step 1)
–
configure the port first as a trunk
•
Step 2)
–
specify the trunk encapsulation

VLAN Trunking Protocol
Trunking Implementation
•
Before attempting to configure a VLAN trunk on a
port, determine what encapsulation the port can
support with the following command:

VLAN Trunking Protocol
•
To verify that trunking has been
configured and to check the settings:
Trunking Implementation

VLAN Trunking Protocol
Fast Ethernet and Gigabit Ethernet
Trunking Modes