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A Cisco Catalyst switch operates in a network similar to a traditional bridge. Each VLAN
that you configure on the switch implements address learning, forwarding and filtering
decisions, and loop avoidance mechanisms as if the VLAN were a separate physical bridge.
The Cisco Catalyst switch implements VLANs by restricting traffic forwarding to
destination ports that are in the same VLAN as the originating ports. So when a frame
arrives on a switch port, the switch must retransmit the frame to only the ports that belong
to the same VLAN. In essence, a VLAN that is operating on a switch limits transmission of
unicast, multicast, and broadcast traffic. Traffic originating from a particular VLAN floods
to only the other ports in that VLAN.
A port normally carries only the traffic for the single VLAN to which it belongs. For a
VLAN to span across multiple switches, a trunk is required to connect two switches. A
trunk can carry traffic for multiple VLANs. Figure 2-6 shows a trunk carrying multiple
VLANs between two switches.
Figure 2-6 VLAN Trunk
IP IP
Trunk
Data VLAN
Data VLAN
Voice VLAN
24 Chapter 2: Medium-Sized Switched Network Construction
You configure ports that belong to a VLAN with a membership mode that determines to
which VLAN they belong. Figure 2-7 displays the various VLAN membership modes.
Figure 2-7 VLAN Membership Modes
The VLAN membership mode Characteristics of Cisco Catalyst switch ports are as follows:
■ Static VLAN: An administrator statically configures the assignment of VLANs to
ports.
■ Dynamic VLAN: Cisco Catalyst switches support dynamic VLANs using a VLAN
Membership Policy Server (VMPS). Some Cisco Catalyst switches can be designated
as the VMPS; you can also designate an external server as the VMPS. The VMPS
contains a database that maps MAC addresses to VLAN assignments. When a frame
arrives at a dynamic port on the Cisco Catalyst access switch, the switch queries the
VMPS server for the VLAN assignment based on the source MAC address of the
arriving frame. A dynamic port can belong to only one VLAN at a time. Multiple hosts
can be active on a dynamic port only if they belong to the same VLAN.
■ Voice VLAN: A voice VLAN port is an access port attached to a Cisco IP phone,
configured to use one VLAN for voice traffic and another VLAN for data traffic.
Understanding Trunking with 802.1Q
A trunk is a point-to-point link between one or more Ethernet switch interfaces and another
networking device such as a router or a switch. Ethernet trunks carry the traffic of multiple
VLANs over a single link and allow you to extend the VLANs across an entire network.
Cisco supports IEEE 802.1Q for FastEthernet and Gigabit Ethernet interfaces. In addition,
some Cisco switches support Cisco Inter-Switch Link (ISL) trunks, a prestandard trunking
technology. Figure 2-8 shows an example of trunks interconnecting Cisco Catalyst
switches.
Static VLAN Dynamic VLAN Voice VLAN
Fa0/1 Fa0/2 Fa0/3
MAC = 1111.1111.1111
VMPS
1111.1111.1111 = VLAN 10
VLAN 10
VLAN 55
VLAN 15
VLAN 5
IP
Implementing VLANs and Trunks 25
Figure 2-8 802.1Q Trunks
Ethernet trunk interfaces support different trunking modes. You can configure an interface
as trunking or nontrunking, or you can have it negotiate trunking with the neighboring
interface.
Every 802.1Q port is assigned to a trunk, and all ports on a trunk are in a native VLAN. A
native VLAN is used in IEEE 802.1Q to send untagged frames to any non-802.1Q devices
that might exist on the segment. Every 802.1Q port is assigned an identifier value that is
based on the native VLAN ID (VID) of the port. (The default is VLAN 1.) All untagged
frames are assigned to the VLAN specified in this VID parameter.
802.1Q Frame
IEEE 802.1Q uses an internal tagging mechanism that inserts a four-byte tag field into the
original Ethernet frame between the Source Address and Type or Length fields. Because
802.1Q alters the frame, the trunking device recomputes the frame check sequence (FCS)
on the modified frame.
It is the responsibility of the Ethernet switch to look at the four-byte tag field and determine
where to deliver the frame. An Ether Type of 0x8100 indicates to devices that the frame has
an 802.1Q tag. A tiny part of the four-byte tag field—three bits to be exact—is used to
specify the priority of the frame. The details of this are specified in the IEEE 802.1p
standard. The 802.1Q header contains the 802.1p field, so you must have 802.1Q to have
802.1p. Following the priority bit is a single flag to indicate whether the addressing is Token
Ring. This is because 802.1Q tagging could also be implemented in a Token Ring
environment; the flag will be 0 for an Ethernet frame. The remainder of the tag is used for
the VID. Figure 2-9 shows the 802.1Q frame format.
Cisco Catalyst
Series Switch
802.1Q
Trunk
802.1Q
Trunk
Catalyst
Series Switch
Catalyst
Series Switch
802.1Q
Trunk
802.1Q
Trunk
Catalyst
Series Switch
Catalyst
Series Switch
VLAN 1
VLAN 2 VLAN 3 VLAN 1 VLAN 2
VLAN 3
26 Chapter 2: Medium-Sized Switched Network Construction
Figure 2-9 802.1Q Frame Format
802.1Q Native VLAN
An 802.1Q trunk and its associated trunk ports have a native VLAN value. 802.1Q does not
tag frames for the native VLAN. Therefore, ordinary stations can read the native untagged
frames but cannot read any other frame because the frames are tagged. Figure 2-10 shows
a frame from the native VLAN being distributed across the network trunks untagged.
Figure 2-10 Untagged Frame
Understanding VLAN Trunking Protocol
VLAN Trunking Protocol (VTP) is a Layer 2 messaging protocol that maintains VLAN
configuration consistency by managing the additions, deletions, and name changes of
VLANs across networks. VTP minimizes misconfigurations and configuration
Ether Type (0x8100) PRI VLAN ID
Token Ring Encapsulation Flag
Destination Source Length/Ether Type Data FCS Original Frame
Destination Source Tag Length/Ether Type Data FCS Tagged Frame
802.1Q
Trunk
802.1Q
Trunk
802.1Q
Trunk
802.1Q
Trunk
VLAN 1
VLAN 2 VLAN 3
VLAN 1
VLAN 1 VLAN 2
VLAN 3
Native VLAN–Untagged
Implementing VLANs and Trunks 27
inconsistencies that can cause problems, such as duplicate VLAN names or incorrect
VLAN-type specifications. Figure 2-11 shows how you can use VTP to manage VLANs
between switches.
Figure 2-11 VTP
A VTP domain is one switch or several interconnected switches sharing the same VTP
environment. You can configure a switch to be in only one VTP domain.
By default, a Cisco Catalyst switch is in the no-management-domain state until it receives
an advertisement for a domain over a trunk link or until you configure a management
domain. Configurations made to a VTP server are propagated across trunk links to all the
connected switches in the network.