VLANS Chapter 3

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VLANS _Cisco_Kid

BENEFITS OF A VLAN 3.1

User productivity and network adaptability are key drivers for business growth and success. Implementing

VLAN technology enables a network to more flexibly support business goals. The primary benefits of usingVLANs are as follows:

y Security - Groups that have sensitive data are separated from the rest of the network, decreasing thechances of confidential information breaches. Faculty computers are on VLAN 10 and completelyseparated from student and guest data traffic.

y Cost reduction - Cost savings result from less need for expensive network upgrades and more efficientuse of existing bandwidth and uplinks.

y Higher perf ormance - Dividing flat Layer 2 networks into multiple logical workgroups (broadcastdomains) reduces unnecessary traffic on the network and boosts performance.

y Broadcast storm mitigation - Dividing a network into VLANs reduces the number of devices that may

participate in a broadcast storm. As discussed in the "Configure a Switch" chapter, LAN segmentation prevents a broadcast storm from propagating to the whole network. In the figure you can see that

although there are six computers on this network, there are only three broadcast domains: Faculty,Student, and Guest.

y Improved IT staff efficiency - VLANs make it easier to manage the network because users with similarnetwork requirements share the same VLAN. When you provision a new switch, all the policies and procedures already configured for the particular VLAN are implemented when the ports are assigned. It

is also easy for the IT staff to identify the function of a VLAN by giving it an appropriate name. In thefigure, for easy identification VLAN 20 has been named "Student", VLAN 10 could be named

"Faculty", and VLAN 30 "Guest."

y Simpler project or application management - VLANs aggregate users and network devices to support

business or geographic requirements. Having separate functions makes managing a project or workingwith a specialized application easier, for example, an e-learning development platform for faculty. It is

also easier to determine the scope of the effects of upgrading network services.

VLAN ID RANGES

Access VLANs are divided into either a normal range or an extended range.

Normal Range VLANs

y Used in small- and medium-sized business and enterprise networks.

y Identified by a VLAN ID between 1 and 1005.

y IDs 1002 through 1005 are reserved f or Tok en Ring and FDDI VLANs.

y IDs 1 and 1002 to 1005 are automatically created and cannot be removed. You will learn more

about VLAN 1 later in this chapter.

y Configurations are stored within a VLAN database file, called vlan.dat. The vlan.dat file is located

in the flash memory of the switch.

y The VLAN trunking protocol (VTP), which helps manage VLAN configurations between switches,

can only learn normal range VLANs and stores them in the VLAN database file.



Extended Range VLANs

y Enable service providers to extend their infrastructure to a greater number of customers. Some global

enterprises could be large enough to need extended range VLAN IDs.

y Are identified by a VLAN ID between 1006 and 4094.

y Support fewer VLAN features than normal range VLANs.y Are saved in the running configuration file.

y VTP does not learn extended range VLANs.

255 VLANs Configurable

One Cisco Catalyst 2960 switch can support up to 255 normal range and extended range VLANs, although

the number configured affects the performance of the switch hardware. Because an enterprise network mayneed a switch with a lot of ports, Cisco has developed enterprise-level switches that can be joined or stacked

together to create a single switching unit consisting of nine separate switches. Each separate switch can have 48

ports, which totals 432 ports on a single switching unit. In this case, the 255 VLAN limit per single switchcould be a constraint for some enterprise customers.

3 .1.2 Types of VLANs

Today there is essentially one way of implementing VLANs - port-based VLANs. A port-based VLAN is

associated with a port called an access VLAN.

However in the network there are a number of terms for VLANs. Some terms define the type of network traffic

they carry and others define a specific function a VLAN performs. The following describes common VLANterminology:

Data VLAN

A data VLAN is a VLAN that is configured to carry only user-generated traffic. A VLAN could carry voice- based traffic or traffic used to manage the switch, but this traffic would not be part of a data VLAN. It is

common practice to separate voice and management traffic from data traffic. The importance of separating user data from switch management control data and voice traffic is highlighted by the use of a special term used to



identify VLANs that only carry user data - a "data VLAN". A data VLAN is sometimes referred to as a user VLAN.

.

Default VLAN

All switch ports become a member of the default VLAN after the initial boot up of the switch. Having all theswitch ports participate in the default VLAN makes them all part of the same broadcast domain. This allows

any device connected to any switch port to communicate with other devices on other switch ports. The defaultVLAN for Cisco switches is VLAN 1. VLAN 1 has all the features of any VLAN, except that you cannot

rename it and you can not delete it. By default, Layer 2 control traffic, such as CDP and spanning tree protocoltraffic, are associated with VLAN 1. In the figure, VLAN 1 traffic is forwarded over the VLAN trunks

connecting the S1, S2, and S3 switches. It is a security best practice to change the default VLAN to a VLANother than VLAN 1; this entails configuring all the ports on the switch to be associated with a default VLAN

other than VLAN 1. VLAN trunks support the transmission of traffic from more than one VLAN. AlthoughVLAN trunks are mentioned throughout this section, they are explained in the next section on VLAN trunking.

Note: Some network administrators use the term "default VLAN" to mean a VLAN other than VLAN 1 defined

by the network administrator as the VLAN that all ports are assigned to when they are not in use. In this case,

the only role that VLAN 1 plays is that of handling Layer 2 control traffic for the network.

Native VLAN A native VLAN is assigned to an 802.1Q trunk port. An 802.1Q trunk port supports traffic coming from many

VLANs (tagged traffic) as well as traffic that does not come from a VLAN (untagged traffic). The 802.1Q trunk port places untagged traffic on the native VLAN. In the figure, the native VLAN is VLAN 99. Untagged traffic

is generated by a computer attached to a switch port that is configured with the native VLAN. Native VLANsare set out in the IEEE 802.1Q specification to maintain backward compatibility with untagged traffic common

to legacy LAN scenarios. For our purposes, a native VLAN serves as a common identifier on opposing ends of a trunk link. It is a best practice to use a VLAN other than VLAN 1 as the native VLAN.

Management VLAN

A management VLAN is any VLAN you configure to access the management capabilities of a switch. VLAN 1would serve as the management VLAN if you did not proactively define a unique VLAN to serve as the

management VLAN. You assign the management VLAN an IP address and subnet mask. A switch can bemanaged via HTTP, Telnet, SSH, or SNMP. Since the out-of-the-box configuration of a Cisco switch has

VLAN 1 as the default VLAN, you see that VLAN 1 would be a bad choice as the management VLAN; youwouldn't want an arbitrary user connecting to a switch to default to the management VLAN. Recall that you

configured the management VLAN as VLAN 99 in the Basic Switch Concepts and Configuration chapter.

Voice VLANs Seems the Users PC is connected in series thru the IP Phone

It is easy to appreciate why a separate VLAN is needed to support Voice over IP (VoIP). Imagine you arereceiving an emergency call and suddenly the quality of the transmission degrades so much you cannotunderstand what the caller is saying. VoIP traffic requires:

y Assured bandwidth to ensure voice quality

y Transmission priority over other types of network traffic

y Ability to be routed around congested areas on the network

y Delay of less than 150 milliseconds (ms) across the network



To meet these requirements, the entire network has to be designed to support VoIP. The details of how toconfigure a network to support VoIP are beyond the scope of the course, but it is useful to summarize how a

voice VLAN works between a switch, a Cisco IP phone, and a computer.

In the figure, VLAN 150 is designed to carry voice traffic. The student computer PC5 is attached to the Cisco IP phone, and the phone is attached to switch S3. PC5 is in VLAN 20, which is used for student data. The F0/18

port on S3 is configured to be in voice mode so that it will tell the phone to tag voice frames with VLAN 150.Data frames coming through the Cisco IP phone from PC5 are left untagged. Data destined for PC5 coming

from port F0/18 is tagged with VLAN 20 on the way to the phone, which strips the VLAN tag before the data isforwarded to PC5. Tagging refers to the addition of bytes to a field in the data frame which is used by the

switch to identify which VLAN the data frame should be sent to. You will learn later about how data frames aretagged.

A Cisco Phone is a Switch

The Cisco IP Phone contains an integrated three-port 10/100 switch as shown in the Figure. The ports provide

dedicated connections to these devices:

y Port 1 connects to the switch or other voice-over-IP (VoIP) device.

y Port 2 is an internal 10/100 interface that carries the IP phone traffic.y Port 3 (access port) connects to a PC or other device.

The voice VLAN feature enables switch ports to carry IP voice traffic from an IP phone. When the switch isconnected to an IP Phone, the switch sends messages that instruct the attached IP phone to send voice traffic

tagged with the voice VLAN ID 150. The traffic from the PC attached to the IP Phone passes through the IP phone untagged. When the switch port has been configured with a voice VLAN, the link between the switch

and the IP phone acts as a trunk to carry both the tagged voice traffic and untagged data traffic.

Note: Communication between the switch and IP phone is facilitated by the CDP protocol. This protocol isdiscussed in greater detail in the CCNA Exploration: Routing Protocols and Concepts course.

Sample Configuration

The figure shows sample output. A discussion of the Cisco IOS commands are beyond the scope of this course,

but you can see that the highlighted areas in the sample output show the F0/18 interface configured with aVLAN configured for data (V LAN 20) and a VLAN configured for voice (VLAN 150).



3.1.3 SWITCH PORT MEMBERSHIP MODES

Switch Ports

Switch ports are Layer 2-only interfaces associated with a physical port. Switch ports are used for managing the physical interface and associated Layer 2 protocols. They do not handle routing or bridging. Switch ports

belong to one or more VLANs.

VLAN Switch Port Modes

When you configure a VLAN, you must assign it a number ID, and you can optionally give it a name. The purpose of VLAN implementations is to judiciously associate ports with particular VLANs. You configure the

port to forward a frame to a specific VLAN. As mentioned previously, you can configure a VLAN in voicemode to support voice and data traffic coming from a Cisco IP phone. You can configure a port to belong to a

VLAN by assigning a membership mode that specifies the kind of traffic the port carries and the VLANs towhich it can belong. A port can be configured to support these VLAN types:

y Static VLAN - Ports on a switch are manually assigned to a VLAN. Static VLANs are configured usingthe Cisco CLI. This can also be accomplished with GUI management applications, such as the Cisco

Network Assistant. However, a convenient feature of the CLI is that if you assign an interface to aVLAN that does not exist, the new VLAN is created for you. To see a sample static-VLAN

configuration, click the Static Mode Example button in the figure. When you are done, click the PortModes button in the figure. This configuration will not be examined in detail now. You will see this

configuration later in the chapter.

y Dynamic VLAN - This mode is not widely used in production networks and is not explored in thiscourse. However, it is useful to know what a dynamic VLAN is. A dynamic port VLAN membership is

configured using a special server called a VLAN Membership Policy Server (VMPS). With the VMPS,you assign switch ports to VLANs dynamically, based on the source MAC address of the device

connected to the port. The benefit comes when you move a host from a port on one switch in thenetwork to a port on another switch in the network, the switch dynamically assigns the new port to the

proper VLAN for that host.

y Voice VLAN - A port is configured to be in voice mode so that it can support an IP phone attached to it.Before you configure a voice VLAN on the port, you need to first configure a VLAN for voice and a

VLAN for data. In the figure, VLAN 150 is the voice VLAN, and VLAN 20 is the data VLAN. It isassumed that the network has been configured to ensure that voice traffic can be transmitted with a

priority status over the network. When a phone is first plugged into a switch port that is in voice mode,the switch port sends messages to the phone providing the phone with the appropriate voice VLAN ID

and configuration. The IP phone tags the voice frames with the voice VLAN ID and forwards all voicetraffic through the voice VLAN.



To examine parts of a voice mode configuration, click the Voice Mode Example button in the figure:

y The configuration command mls qos trust cos ensures that voice traffic is identified as priority traffic.Remember that the entire network must be set up to prioritize voice traffic. You cannot just configure

the port with this command.

y The switchport voice vlan 150 command identifies VLAN 150 as the voice VLAN. You can see thisverified in the bottom screen capture: Voice VLAN: 150 (VLAN0150).

y The switchport access vlan 20 command configures VLAN 20 as the access mode (data) VLAN. You

can see this verified in the bottom screen capture: Access Mode VLAN: 20 (VLAN0020).For more details about configuring a voice VLAN, visit this Cisco.com site:

http://www.cisco.com/en/US/docs/switches/lan/catalyst2975/software/release/12.2_46_ex/configuration/guide/swvoip.html.

3.1.4 Controlling Broadcast Domains With VLANs

Network with VLANs

In the figure, the network has been segmented into two VLANs: Faculty as VLAN 10 and Student as VLAN 20.When the broadcast frame is sent from the faculty computer, PC1, to switch S2, the switch forwards that

broadcast frame only to those switch ports configured to support VLAN 10.

In the figure, the ports that make up the connection between switches S2 and S1 (ports F0/1) and between S1and S3 (ports F0/3) have been configured to support all the VLANs in the network. This connection is called a

trunk. You will learn more about trunks later in this chapter.

When S1 receives the broadcast frame on port F0/1, S1 forwards that broadcast frame out the only portconfigured to support VLAN 10, port F0/3. When S3 receives the broadcast frame on port F0/3, it forwards that

broadcast frame out the only port configured to support VLAN 10, port F0/11. The broadcast frame arrives atthe only other computer in the network configured on VLAN 10, faculty computer PC4.

When VLANs are implemented on a switch, the transmission of unicast, multicast, and broadcast traffic from a

host on a particular VLAN are constrained to the devices that are on the VLAN.Controlling Broadcast Domains with Switches and Routers

Breaking up a big broadcast domain into several smaller ones reduces broadcast traffic and improves network

performance. Breaking up domains into VLANs also allows for better information confidentiality within anorganization. Breaking up broadcast domains can be performed either with VLANs (on switches) or with

routers. A router is needed any time devices on different Layer 3 networks need to communicate, regardlesswhether VLANs are used.

Click the Intra-VLAN Communication button and click the Play button to start the animation.

Intra-VLAN Communication In the figure, PC1, wants to communicate with another device, PC4. PC1 and PC4 are both in VLAN 10.Communicating with a device in the same VLAN is called intra-VLAN communication. The following

describes how this process is accomplished:

Step 1. PC1 in VLAN 10 sends its ARP request frame (broadcast) to switch S2. Switches S2 and S1 send theARP request frame out all ports on VLAN 10. Switch S3 sends the ARP request out port F0/11 to PC4 on

VLAN 10.



Step 2. The switches in the network forward the ARP reply frame (unicast) to PC1. PC1 receives the replywhich contains the MAC address of PC4.

Step 3. PC1 now has the destination MAC address of PC4 and uses this to create a unicast frame with PC4's

MAC address as the destination. Switches S2, S1 and S3 deliver the frame to PC4.

Inter-VLAN Communication

In the figure, PC1 in VLAN 10 wants to communicate with PC5 in VLAN 20. Communicating with a device inanother VLAN is called inter-VLAN communication.

Note: There are two connections from switch S1 to the router: one to carry transmissions on VLAN 10, and the

other to carry transmissions on VLAN 20 to the router interface.

The following describes how this process is accomplished:

Step 1. PC1 in VLAN 10 wants to communicate with PC5 in VLAN 20. PC1 sends an ARP request frame for the MAC address of the default gateway R1.

Step 2. The router R1 replies with an ARP reply frame from its interface configured on VLAN 10.

All switches forward the ARP reply frame and PC1 receives it. The ARP reply contains the MAC address of the

default gateway.

Step 3. PC1 then creates an Ethernet frame with the MAC address of the Default Gateway. The frame is sent

from switch S2 to S1.

Step 4. The router R1 sends an ARP request frame on VLAN 20 to determine the MAC address of PC5.

Switches, S1, S2, S3, broadcast the ARP request frame out ports configured for VLAN 20. PC5 on VLAN 20receives the ARP request frame from router R1.

Step 5. PC5 on VLAN 20 sends an ARP reply frame to switch S3. Switches S3 and S1 forward the ARP reply

frame to router R1 with the destination MAC address of interface F0/2 on router R1.

Step 6. Router R1 sends the frame received from PC1 though S1 and S3 to PC5 (on VLAN 20).Controlling Broadcast Domains with VLANs and Layer 3 Forwarding



LAYER 3 FORWAR DING WITH A LAYER 3 SWITCH In the last chapter, you learned about some of the differences between Layer 2 and Layer 3 switches. The figure

shows the Catalyst 3750G-24PS switch, one of many Cisco switches that supports Layer 3 routing. The iconthat represents a Layer 3 switch is shown. A discussion of Layer 3 switching is beyond the scope of this course,

but a brief description of the switch virtual interface (SVI) technology that allows a Layer 3 switch to routetransmissions between VLANs is helpful.

SVI SVI is a logical interface configured for a specific VLAN. You need to configure an SVI for a VLAN if you

want to route between VLANs or to provide IP host connectivity to the switch. By default, an SVI is created forthe default VLAN (VLAN 1) to permit remote switch administration.

Layer 3 Forwarding

A Layer 3 switch has the ability to route transmissions between VLANs. The procedure is the same as describedfor the inter-VLAN communication using a separate router, except that the SVIs act as the router interfaces for

routing the data between VLANs. The animation describes this process.

In the animation, PC1 wants to communicate with PC5. The following steps outline the communication throughthe Layer 3 switch S1:

Step 1. PC1 sends an ARP request broadcast on VLAN10. S2 forwards the ARP request out all ports configuredfor VLAN 10.

Step 2. Switch S1 forwards the ARP request out all ports configured for VLAN 10, including the SVI for VLAN 10. Switch S3 forwards the ARP request out all ports configured for VLAN 10.

Step 3. The SVI for VLAN 10 in switch S1 knows the location of VLAN 20. The SVI for VLAN 10 in switch

S1 sends an ARP reply back to PC1 with this information.

Step 4. PC1 sends data, destined for PC5, as a unicast frame through switch S2 to the SVI for VLAN 10 inswitch S1.

Step 5. The SVI for VLAN 20 sends an ARP request broadcast out all switch ports configured for VLAN 20.

Switch S3 sends that ARP request broadcast out all switch ports configured for VLAN 20.

Step 6. PC5 on VLAN 20 sends an ARP reply. Switch S3 sends that ARP reply to S1. Switch S1 forwards theARP reply to the SVI for VLAN 20.

Step 7. The SVI for VLAN 20 forwards the data, sent from PC1, in a unicast frame to PC5 using the destination

address it learned from the ARP reply in step 6.

3.2.1 VLAN TRUNK S

What is a Trunk?

It is hard to describe VLANs without mentioning VLAN trunks. You learned about controlling network broadcasts with VLAN segmentation, and you saw how VLAN trunks transmitted traffic to different parts of

the network configured in one VLAN. In the figure, the links between switches S1 and S2, and S1 and S3, areconfigured to transmit traffic coming from VLAN 10, 20, 30, and 99. This network simply could not function

without VLAN trunks. You will find that most networks that you encounter are configured with VLAN trunks.This section brings together the knowledge you already have on VLAN trunking and provides the details you

need to be able to configure VLAN trunking in a network.



Definition of a VLAN Trunk

A trunk is a point-to-point link between two network devices that carries more than one VLAN. A VLAN trunk allows you to extend the VLANs across an entire network. Cisco supports IEEE 802.1Q for coordinating trunks

on Fast Ethernet and Gigabit Ethernet interfaces. You will learn about 802.1Q later in this section.

A VLAN trunk does not belong to a specific VLAN, rather it is a conduit for VLANs between switches androuters.

What Problem Does a Trunk Solve?

In the figure, you see the standard topology used in this chapter, except instead of the VLAN trunk that you are

used to seeing between switches S1 and S2, there is a separate link for each subnet. There are four separate linksconnecting switches S1 and S2, leaving three fewer ports to allocate to end-user devices. Each time a new

subnetwork is considered, a new link is needed for each switch in the network.

In the figure, the network topology shows a VLAN trunk connecting switches S1 and S2 with a single physicallink. This is the way a network should be configured. WHY NOT USE 4 PHYSICAL CONNECTIONS FOR

ADDED SPEED?(MY QUESTION)

3.2.1 VLAN TRUNK S: FRAME TAGGING

802.1Q Frame Tagging

Remember that switches are Layer 2 devices. They only use the Ethernet frame header information to forward packets. The frame header does not contain information about which VLAN the frame should belong to.

Subsequently, when Ethernet frames are placed on a trunk they need additional information about the VLANsthey belong to. This is accomplished by using the 802.1Q encapsulation header. This header adds a tag to the

original Ethernet frame specifying the VLAN to which the frame belongs.

Frame tagging has been mentioned a number of times. The first time was in reference to the voice modeconfiguration on a switch port. There you learned that once configured, a Cisco phone (which includes a small

switch) tags voice frames with a VLAN ID. You also learned that VLAN IDs can be in a normal range, 1-1005,and an extended range, 1006-4094. How do VLAN IDs get inserted into a frame?



VLAN Frame Tagging Overview

Before exploring the details of an 802.1Q frame, it is helpful to understand what a switch does when it forwards

a frame out a trunk link. When the switch receives a frame on a port configured in access mode with a staticVLAN, the switch takes apart the frame and inserts a VLAN tag, recalculates the FCS and sends the tagged

frame out a trunk port.

Note: An animation of the trunking operation is presented later in this section.

VLAN Tag Field Details

The VLAN tag field consists of an EtherType field, a tag control information field,and the FCS field.

EtherType field

Set to the hexadecimal value of 0x8100. This value is called the tag protocol ID (TPID) value. With theEtherType field set to the TPID value, the switch receiving the frame knows to look for information in the tag

control information field.

Tag control inf ormation field

The tag control information field contains:

3 bits of user priority - Used by the 802.1p standard, which specifies how to provide expedited transmission ofLayer 2 frames. A description of the IEEE 802.1p is beyond the scope of this course; however, you learned a

little about it earlier in the discussion on voice VLANs.

1 bit of Canonical Format Identifier (CFI) - Enables Token Ring frames to be carried across Ethernet links

easily.

12 bits of VLAN ID (VID) - VLAN identification numbers; supports up to 4096 VLAN IDs.

FCS field

After the switch inserts the EtherType and tag control information fields, it recalculates the FCS values andinserts it into the frame.



Native VLANs and 802.1Q Trunk ing

Now that you know more about how a switch tags a frame with the correct VLAN, it is time to explore how thenative VLAN supports the switch in handling tagged and untagged frames that arrive on an 802.1Q trunk port.

Tagged Frames on the Native VLAN

Some devices that support trunking tag native VLAN traffic as a default behavior. Control traffic sent on the

native VLAN should be untagged. If an 802.1Q trunk port receives a tagged frame on the native VLAN, it dropsthe frame. Consequently, when configuring a switch port on a Cisco switch, you need to identify these devices

and configure them so that they do not send tagged frames on the native VLAN. Devices from other vendorsthat support tagged frames on the native VLAN include IP phones, servers, routers, and non-Cisco switches.

Untagged Frames on the Native VLAN

W hen a Cisco switch trunk port receives untagged frames it forwards those frames to the native VLAN. As

you may recall, the default native VLAN is VLAN 1. When you configure an 802.1Q trunk port, a default PortVLAN ID (PVID) is assigned the value of the native VLAN ID. All untagged traffic coming in or out of the

802.1Q port is forwarded based on the PVID value. For example, if VLAN 99 is configured as the native

VLAN, the PVID is 99 and all untagged traffic is forward to VLAN 99. If the native VLAN has not beenreconfigured, the PVID value is set to VLAN 1.In this example, VLAN 99 will be configured as the native VLAN on port F0/1 on switch S1. This example

shows how to reconfigure the native VLAN from its default setting of VLAN 1.

Starting in privileged EXEC mode, the figure describes how to configure the native VLAN on port F0/1 onswitch S1 as an IEEE 802.1Q trunk with native VLAN 99.

Using the show interfaces interface-id switchport command, you can quickly verify that you have correctlyreconfigured the native VLAN from VLAN 1 to VLAN 99. The highlighted output in the screen capture

indicates that the configuration was successful.



SWITCH1#show interfaces f0/1 switchport (or any fa0/#) WILL GIVE THE VERIFICATION OF CORR ECT

TRUNK SETUP.

3.2.2 A TRUNK IN ACTION

A Trunk in Action

You have learned how a switch handles untagged traffic on a trunk link. You now know that frames traversing atrunk are tagged with the VLAN ID of the access port the frame arrived on. In the figure, PC1 on VLAN 10 and

PC3 on VLAN 30 send broadcast frames to switch S2. Switch S2 tags these frames with the appropriate VLANID and then forwards the frames over the trunk to switch S1. Switch S1 reads the VLAN ID on the frames and

broadcasts them to each port configured to support VLAN 10 and VLAN 30. Switch S3 receives these framesand strips off the VLAN IDs and forwards them as untagged frames to PC4 on VLAN 10 and PC6 on VLAN

30. SEE THE ANIMATION 3.2.2

3.2.3 TRUNK MODES

IEEE, Not ISL(Trunk types) Although a Cisco switch can be configured to support two types of trunk ports, IEEE 802.1Q and ISL, today

only 802.1Q is used. However, legacy networks may still use ISL, and it is useful to learn about each type of

trunk port.

y An IEEE 802.1Q trunk port supports simultaneous tagged and untagged traffic. An 802.1Q trunk port isassigned a default PVID, and all untagged traffic travels on the port default PVID. All untagged traffic

and tagged traffic with a null VLAN ID are assumed to belong to the port default PVID. A packet with aVLAN ID equal to the outgoing port default PVID is sent untagged. All other traffic is sent with a

VLAN tag.

y In an ISL trunk port, all received packets are expected to be encapsulated with an ISL header, and all

transmitted packets are sent with an ISL header. Native (non-tagged) frames received from an ISL trunk port are dropped. ISL is no longer a recommended trunk port mode, and it is not supported on a number

of Cisco switches. ISL drops untagged frames

DTP(Trunk ing protocol)

Dynamic Trunking Protocol (DTP) is a Cisco proprietary protocol. Switches from other vendors do not support

DTP. DTP is automatically enabled on a switch port when certain trunking modes are configured on the switch port.

DTP manages trunk negotiation only if the port on the other switch is configured in a trunk mode that supportsDTP. DTP supports both ISL and 802.1Q trunks. This course focuses on the 802.1Q implementation of DTP. A

detailed discussion on DTP is beyond the scope of this course; however, you will enable it in the labs and

activities associated with the chapter. Switches do not need DTP to do trunking, and some Cisco switches androuters do not support DTP. To learn about DTP support on Cisco switches, visit:http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a008017f86a.shtml.

Trunk ing Modes

A switch port on a Cisco switch supports a number of trunking modes. The trunking mode defines how the port

negotiates using DTP to set up a trunk link with its peer port. The following provides a brief description of theavailable trunking modes and how DTP is implemented in each.



³On´ (default)Always on...

The switch port periodically sends DTP frames, called advertisements, to the remote port. The command used is

³switchport mode trunk ́ . The local switch port advertises to the remote port that it is dynamically changing

to a trunking state. The local port then, regardless of what DTP information the remote port sends as a responseto the advertisement, changes to a trunking state. The local port is considered to be in an unconditional (always

on) trunking state.

³Dynamic auto´

The switch port periodically sends DTP frames to the remote port. The command used is switchport mode

dynamic auto . The local switch port advertises to the remote switch port that it is able to trunk but does not

request to go to the trunking state. After a DTP negotiation, the local port ends up in trunking state only if theremote port trunk mode has been configured to be on or desirable. If both ports on the switches are set to auto,

they do not negotiate to be in a trunking state. They negotiate to be in the access (non-trunk) mode state. Theother remote Switchport must be in ³ON´ or ³DYNAMIC DESIRABLE´mode to become connected. Two

ports both set with ³Dynamic Auto´ will not negotiate a Trunk port connection

³Dynamic desirable´

DTP frames are sent periodically to the remote port. The command used is switchport mode dynamic

desirable. The local switch port advertises to the remote switch port that it is able to trunk and asks the remote

switch port to go to the trunking state. If the local port detects that the remote has been configured in on, desirable, or auto mode, the local port ends up in trunking state. If the remote switch port is in the nonegotiate

mode, the local switch port remains as a nontrunking port.

Turn off DTP

You can turn off DTP for the trunk so that the local port does not send out DTP frames to the remote port. Use

the command ³switchport nonegotiate´. The local port is then considered to be in an unconditional trunkingstate. Use this feature when you need to configure a trunk with a switch from another switch vendor.

A Trunk Mode Example

In the figure, the F0/1 ports on switches S1 and S2 are configured with trunk mode on. The F0/3 ports onswitches S1 and S3 are configured to be in auto trunk mode. When the switch configurations are completed and

the switches are fully configured, which link will be a trunk?

Click the Which link will be configured as a trunk? button in the figure.

The link between switches S1 and S2 becomes a trunk because the F0/1 ports on switches S1 and S2 areconfigured to ignore all DTP advertisements and come up and stay in trunk port mode. The F0/3 ports on

switches S1 and S3 are set to auto, so they negotiate to be in the default state, the access (non-trunk) mode state.This results in an inactive trunk link. When you configure a trunk port to be in trunk port mode, there is no

ambiguity about which state the trunk is in-it is always on. It is also easy to remember which state the trunk ports are in-if the port is supposed to be a trunk, trunk mode is on.

Note: The default switchport mode for an interface on a Catalyst 2950 switch is dynamic desirable, but the

default switchport mode for an interface on a Catalyst 2960 switch is dynamic auto. If S1 and S3 were Catalyst2950 switches with interface F0/3 in default switchport mode, the link between S1 and S3 would become an

active trunk.



For information on which Cisco switches support 802.1Q, ISL, and DTP, visit:

http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a008017f86a.shtml#topic1.

For information on how to support ISL on legacy networks, visit:http://www.cisco.com/en/US/tech/tk389/tk689/tsd_technology_support_troubleshooting_technotes_list.html.

HER E¶S A NICE CHART FOR TRUNK SWITCHPORTING INTERACTION R ESULTS;

What AR E THEY TALKING ABOUT IN ³ACCESS´ MODE?

3.3.1 CONFIGUR ING VLANS AND TRUNK S OVERVIEW & ASSIGN A SWITCHPOR T ADD A VLAN; SW1# conf t

SW1(config)# vlan #20(id of vlan)SW1(config vlan)# name practice (vlan name)

SW1(config vlan)# endSW1# show vlan brief

SW1# conf tSW1(config)# int fa0/18

SW1(config if) # switchport mode access ( setting fa0/18 to vlan 20)SW1(config if) # switchport access vlan 20 (vlan 20 would be created even as soon as command is set)

SW1(config if)# endSW1# show vlan brief

For details on the show vlan command output fields, visit:http://www.cisco.com/en/US/docs/ios/lanswitch/command/reference/lsw_s2.html#wp1011412.

For details on the show interfaces command output fields, visit:http://www.cisco.com/en/US/docs/ios/12_0/interface/command/reference/irshowin.html#wp1017387.



REMOVE A VLAN: Ports and whole VLAN

SW1(config)# int fa0/18

SW1(config if) # no switchport mode access ( removing fa0/18 from vlan)

Delete VLANs SW1# conf t

SW1(config)# no vlan20 [(SW1(config)# no vlan#]

The figure provides an example of using the global configuration command no vlan vlan-id to remove VLAN20 from the system. The show vlan brief command verifies that VLAN 20 is no longer in the vlan.dat file.

Alternatively, the entire vlan.dat file can be deleted using the command delete flash:vlan.dat from privileged

EXEC mode. After the switch is reloaded, the previously configured VLANs will no longer be present. Thiseffectively places the switch into is "factory default" concerning VLAN configurations.

Note: Before deleting a VLAN, be sure to first reassign all member ports to a different VLAN. Any ports that

are not moved to an active VLAN are unable to communicate with other stations after you delete the VLAN.

3.3.4 Configure an 802.1Q Trunk To configure a trunk on a switch port, use the switchport mode trunk command. When you enter trunk mode,the interface changes to permanent trunking mode, and the port enters into a DTP negotiation to convert the link

into a trunk link even if the interface connecting to it does not agree to the change. In this course, you willconfigure a trunk using only the switchport mode trunk command. The Cisco IOS command syntax to specify a

native VLAN other than VLAN 1 is shown in the figure. In the example, you configure VLAN 99 as the nativeVLAN.

You are familiar with this topology. The VLANs 10, 20, and 30 will support the Faculty, Student, and Guest

computers, PC1, PC2, and PC3. The F0/1 port on switch S1 will be configured as a trunk port and will forwardtraffic for VLANs 10, 20, and 30. VLAN 99 will be configured as the native VLAN.

Enter global configuration mode.

S1#configure terminalS1(config)#interface interface id

S1(config-if)#switchport mode trunk S1(config-if)#switchport trunk native vlan vlan id

S1(config-if)#end

Verify Trunk Configuration

The figure displays the configuration of switch port F0/1 on switch S1.The command used is the show interfaces interface-ID switchport command .

³S1# show interfaces fa0/1 switchport ³



The first highlighted area shows that port F0/1 has its administrative mode set to Trunk-the port is in trunkingmode. The next highlighted area verifies that the native VLAN is VLAN 99, the management VLAN. At the

bottom of the output, the last highlighted area shows that the enabled trunking VLANs are VLANs 10, 20, and30.

Managing a Trunk Configuration In the figure, the commands to reset the allowed VLANs and the native VLAN of the trunk to the default state

are shown. The command to reset the switch port to an access port and, in effect, deleting the trunk port is alsoshown.

In the figure, the commands used to reset all trunking characteristics of a trunking interface to the default

settings are highlighted in the sample output. The show interfaces f0/1 switchport command reveals that thetrunk has been reconfigured to a default state.

In the figure, the sample output shows the commands used to remove the trunk feature from the F0/1 switch

port on switch S1. The show interfaces f0/1 switchport command reveals that the F0/1 interface is now in staticaccess mode.



3.4.1COMMON PROBLEMS WITH TRUNK S

COMMON PROBLEMS WITH TRUNK S

In this topic, you learn about common VLAN and trunking issues, which usually are associated with incorrectconfigurations. When you are configuring VLANs and trunks on a switched infrastructure, these types of

configuration errors are most common in the following order:

y Native VLAN mismatches - Trunk ports are configured with different native VLANs, for example, if one port has defined VLAN 99 as the native VLAN and the other trunk port has defined VLAN 100 asthe native VLAN. This configuration error generates console notifications, causes control and

management traffic to be misdirected and, as you have learned, poses a security risk.

y Trunk mode mismatches - One trunk port is configured with trunk mode "off" and the other with trunk mode "on". This configuration error causes the trunk link to stop working.

y VLANs and IP Subnets -

End user devices configured with incorrect IP addresses will not have network connectivity. Each VLAN is a logically separate IP subnetwork. Devices within the VLAN must be

configured with the correct IP settings.

y Allowed VLANs on trunks - The list of allowed VLANs on a trunk has not been updated with thecurrent VLAN trunking requirements. In this situation, unexpected traffic or no traffic is being sent over

the trunk.If you have discovered an issue with a VLAN or trunk and do not know what the problem is, start your

troubleshooting by examining the trunks for a native VLAN mismatch and then work down the list. The rest of this topic examines how to fix the common problems with trunks. The next topic presents how to identify and

solve incorrectly configured VLAN and IP subnets.

VLANS Chapter 3

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Transcript of VLANS Chapter 3