Chapter 6 – Sections & Objectives
- 6.1 VLAN Segmentation
- Explain the purpose of VLANs in a switched network.
- Explain how a switch forwards frames based on VLAN configuration in a multi-switch environment.
- 6.2 VLAN Implementations
- Configure a switch port to be assigned to a VLAN based on requirements.
- Configure a trunk port on a LAN switch.
- Troubleshoot VLAN and trunk configurations in a switched network.
- 6.3 Inter-VLAN Routing Using Routers
- Describe the two options for configuring Inter-VLAN routing.
- Configure Legacy Inter-VLAN Routing.
- Configure Router-on-a-Stick Inter-VLAN Routing
6.1 VLAN Segmentation
6.1.1 Overview of VLANs
VLAN Definitions
- VLANs allow an administrator to segment networks based on factors such as function, project team, or application, without regard for the physical location of the user or device.
- VLANs enable the implementation of access and security policies according to specific groupings of users.
- A VLAN is a logical partition of a Layer 2 network.
- Multiple partitions can be created, allowing for multiple VLANs to co-exist.
- Each VLAN is a broadcast domain, usually with its own IP network.
- VLANs are mutually isolated, and packets can only pass between them via a router.
- The partitioning of the Layer 2 network takes place inside a Layer 2 device, usually via a switch.
- The hosts grouped within a VLAN are unaware of the VLAN’s existence.
Benefits of VLANs
Types of VLANs
- Data VLAN – user generated traffic
- Default VLAN – all switch ports become part of this VLAN until switch is configured, show vlan brief
- Native VLAN – used for untagged traffic
- Management VLAN – used to access management capabilities
Voice VLANs
- VoIP traffic is time-sensitive and requires:
- Assured bandwidth to ensure voice quality.
- Transmission priority over other types of network traffic.
- Ability to be routed around congested areas on the network.
- Delay of less than 150 ms across the network.
- The voice VLAN feature enables access ports to carry IP voice traffic from an IP phone.
6.1.2 VLANs in a Multi-Switched Environment
VLAN Trunks
- The links between switches S1 and S2, and S1 and S3 are configured to transmit traffic coming from VLANs 10, 20, 30, and 99 across the network. This network could not function without VLAN trunks.
- A VLAN trunk is a point-to-point link that carries more than one VLAN.
- A VLAN trunk is usually established between switches so same-VLAN devices can communicate, even if physically connected to different switches.
- A VLAN trunk is not associated to any VLANs; neither is the trunk ports used to establish the trunk link.
- Cisco IOS supports IEEE802.1q, a popular VLAN trunk protocol.
Controlling Broadcast Domains with VLANs
- VLANs can be used to limit the reach of broadcast frames.
- A VLAN is a broadcast domain of its own.
- A broadcast frame sent by a device in a specific VLAN is forwarded within that VLAN only.
- VLANs help control the reach of broadcast frames and their impact in the network.
- Unicast and multicast frames are forwarded within the originating VLAN.
Tagging Ethernet Frames for VLAN Identification
- Frame tagging is the process of adding a VLAN identification header to the frame.
- It is used to properly transmit multiple VLAN frames through a trunk link.
- Switches tag frames to identify the VLAN to which they belong.
- Different tagging protocols exist; IEEE 802.1Q is a vey popular example.
- The protocol defines the structure of the tagging header added to the frame.
- Switches add VLAN tags to the frames before placing them into trunk links and remove the tags before forwarding frames through non-trunk ports.
- When properly tagged, the frames can transverse any number of switches via trunk links and still be forwarded within the correct VLAN at the destination.
Native VLANs and 802.1Q Tagging
- Control traffic sent on the native VLAN should not be tagged.
- Frames received untagged, remain untagged and are placed in the native VLAN when forwarded.
- If there are no ports associated to the native VLAN and no other trunk links, an untagged frame is dropped.
- When configuring a switch port on a Cisco switch, configure devices so that they do not send tagged frames on the native VLAN.
- In Cisco switches, the native VLAN is VLAN 1, by default.
Voice VLAN Tagging
Activity – Predict Switch Behavior
- Scenario 1: PC 1 sends a broadcast.
- Scenario 2: PC 2 sends a broadcast.
- Scenario 3: PC 3 sends a broadcast.
6.2 VLAN Implementations
6.2.1 VLAN Assignment
VLAN Ranges on Catalyst Switches
- Cisco Catalyst 2960 and 3560 Series switches support over 4,000 VLANs.
- VLANs are split into two categories:
- Normal range VLANs
- VLAN numbers from 1 to 1,005
- Configurations stored in the vlan.dat (in the flash memory)
- IDs 1002 through 1005 are reserved for Token Ring and Fiber Distributed Data Interface (FDDI) VLANs, automatically created and cannot be removed
- Extended Range VLANs
- VLAN numbers from 1,006 to 4,096
- Configurations stored in the running configuration (NVRAM)
- VLAN Trunking Protocol (VTP) does not learn extended VLANs
- Normal range VLANs
- Normal Range VLANs
Creating a VLAN
Assigning Ports to VLANs
Changing VLAN Port Membership
- Remove VLAN Assignment
- Interface F0/18 was previously assigned to VLAN 20 which is still active, F0/18 reset to VLAN1
Deleting VLANs
- The entire vlan.dat file can be deleted using thedelete flash:vlan.dat privileged EXEC mode command
- Abbreviated command version (delete vlan.dat) can be used if the vlan.dat file has not been moved from its default location
Verifying VLAN Information
6.2.1 VLAN Trunks
Configuring IEEE 802.1q Trunk Links
Resetting the Trunk to Default State
Verifying Trunk Configuration
6.2.3 Troubleshoot VLANs and Trunks
IP Addressing Issues with VLANs
- It is a common practice to associate a VLAN with an IP network.
- Because different IP networks only communicate through a router, all devices within a VLAN must be part of the same IP network to communicate.
- The figure displays that PC1 cannot communicate to the server because it has a wrong IP address configured.
Missing VLANs
- If all the IP address mismatches have been solved, but the device still cannot connect, check if the VLAN exists in the switch.
- If the VLAN to which a port belongs is deleted, the port becomes inactive. All ports belonging to the VLAN that was deleted are unable to communicate with the rest of the network.
- Not functional until the missing VLAN is created using thevlanvlan_id global configuration.
Introduction to Troubleshooting Trunks
Note: To solve a native VLAN mismatch, configure the native VLAN to be the same VLAN on both sides of the link.
Common Problems with Trunks
- Trunking issues are usually associated with incorrect configurations.
- The most common type of trunk configuration errors are:
- Native VLAN mismatches
- Trunk mode mismatches
- Allowed VLANs on trunks
- If a trunk problem is detected, the best practice guidelines recommend to troubleshoot in the order shown above.
Incorrect Port Mode
Incorrect VLAN List
- VLANs must be allowed in the trunk before their frames can be transmitted across the link.
- Use the switchport trunk allowed vlan command to specify which VLANs are allowed in a trunk link.
- Use the show interfaces trunk command to ensure the correct VLANs are permitted in a trunk.
6.3 Inter-VLAN Routing Using Routers
6.3.1 Inter-VLAN Routing Operation
What is Inter-VLAN Routing?
- Layer 2 switches cannot forward traffic between VLANs without the assistance of a router.
- Inter-VLAN routing is a process for forwarding network traffic from one VLAN to another, using a router.
Legacy Inter-VLAN Routing
- In the past:
- Actual routers were used to route between VLANs.
- Each VLAN was connected to a different physical router interface.
- Packets would arrive on the router through one interface, be routed and leave through another.
- Because the router interfaces were connected to VLANs and had IP addresses from that specific VLAN, routing between VLANs was achieved.
- Large networks with large number of VLANs required many router interfaces.
- In this example, the router was configured with two separate physical interfaces to interact with the different VLANs and perform the routing.
Router-on-a-Stick Inter-VLAN Routing
- The router-on-a-stick approach uses only one of the router’s physical interface.
- One of the router’s physical interfaces is configured as a 802.1Q trunk port so it can understand VLAN tags.
- Logical subinterfaces are created; one subinterface per VLAN.
- Each subinterface is configured with an IP address from the VLAN it represents.
- VLAN members (hosts) are configured to use the subinterface address as a default gateway.
Router interface configured to operate as a trunk link and is connected to a trunked switch port. The router performs inter-VLAN routing by accepting VLAN-tagged traffic on the trunk interface coming from the adjacent switch, and then, internally routing between the VLANs using subinterfaces. The router then forwards the routed traffic, VLAN-tagged for the destination VLAN, out the same physical interface as it used to receive the traffic.
Identify the Types of Inter-VLAN Routing Activity
- Legacy or Router-on-a-Stick?
- Legacy or Router-on-a-Stick?
6.3.2 Configure Legacy Inter-VLAN Routing
Configure Legacy Inter-VLAN Routing: Preparation
- Legacy inter-VLAN routing requires routers to have multiple physical interfaces.
- Each one of the router’s physical interfaces is connected to a unique VLAN.
- Each interface is also configured with an IP address for the subnet associated with the particular VLAN.
- Network devices use the router as a gateway to access the devices connected to the other VLANs.
Configure Legacy Inter-VLAN Routing: Switch Configuration
Configure Legacy Inter-VLAN Routing: Router Interface Configuration
6.3.3 Configure Router-on-a-Stick Inter-VLAN Routing
Configure Router-on-a Stick: Preparation
- An alternative to legacy inter-VLAN routing is to use VLAN trunking and subinterfaces.
- VLAN trunking allows a single physical router interface to route traffic for multiple VLANs.
- The physical interface of the router must be connected to a trunk link on the adjacent switch.
- On the router, subinterfaces are created for each unique VLAN.
- Each subinterface is assigned an IP address specific to its subnet or VLAN and is also configured to tag frames for that VLAN.
Configure Router-on-a Stick: Switch Configuration
Configure Router-on-a Stick: Router Subinterface Configuration
Configure Router-on-a Stick: Verifying Subinterfaces
Configure Router-on-a Stick: Verifying Routing
- Access to devices on remote VLANs can be tested using thepingcommand.
- Theping command sends an ICMP echo request to the destination address.
- When a host receives an ICMP echo request, it responds with an ICMP echo reply.
- Tracert is a useful utility for confirming the routed path taken between two devices.
6.4 Chapter Summary
- Explain the purpose of VLANs in a switched network.
- Explain how a switch forwards frames based on VLAN configuration in a multi-switch environment.
- Configure a switch port to be assigned to a VLAN based on requirements.
- Configure a trunk port on a LAN switch.
- Troubleshoot VLAN and trunk configurations in a switched network.
- Describe the two options for configuring inter-VLAN routing.
- Configure Legacy Inter-VLAN Routing.
- Configure Router-on-a-Stick Inter-VLAN Routing
Section 6.1
Terms and Commands
- VLAN
- Logical broadcast domain
- Data VLAN
- Default VLAN
- Native VLAN
- Management VLAN
- show vlan brief
- Voice VLAN
- VLAN Trunk
- VLAN Segmentation
- IEEE 802.1Q
- VLAN Tagging
- Canonical Format Identifier (CFI)
- User Priority
- VLAN ID
- Type
- show interfaces int switchport
Section 6.2
Terms and Commands
- Normal Range VLANs
- Extended Range VLANs
- vlan vlan-id
- name vlan-name
- switchport mode access
- switchport access vlan vlan-id
- interface range
- no switchport access vlan vlan-id
- no vlan vlan-id
- delete flash:vlan.dat
- delete vlan.dat
- show vlan
- show interfaces
- show vlan summary
- show interfaces vlan vlan_id
- switchport mode trunk
- switchport trunk allowed vlan vlan_list
- switchport trunk native vlan vlan_id
- no switchport trunk allowed vlan
- no switchport trunk native vlan
- show interfaces switchport
- no switchport access vlan vlan_id
- show interfaces trunk
- show interfaces int_id trunk
Section 6.3
Terms and Commands
- Legacy Inter-VLAN Routing
- Router-on-a-Stick Inter-VLAN Routing
- interface interface_id.subinterface_id
- encapsulation dot1q vlan_id
- IEEE 802.1Q
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