Cisco Switch Configuration

VLAN Definitions

• VLANs segment LAN devices regardless of their physical location.
• In a scenario depicted, IT, HR, and Sales users on different floors connect to the same LAN segment.
• A VLAN (virtual LAN) is a logical partition of a Layer 2 network.
• With VLANs, you can create multiple partitions and allow multiple entities to co-exist.
• Layer 2 partitioning occurs within a Layer 2 device, commonly a switch.
• Each VLAN serves as a broadcast domain that spans multiple physical LAN segments.
• Hosts within the same VLAN do not know of the VLAN's existence.
• VLANs are isolated; data packets traverse them only via a router.

Benefits of VLANs

• VLANs can help with improved security, reduced costs, and better performance.
• Additional benefits of VLANS include smaller broadcast domains, IT efficiency, and application management.

Types of VLANs

• Default VLAN is also known as VLAN 1.
• All switch ports are members of VLAN 1 by default.
• Data VLANs are created for specific groups of users and carry user-generated traffic.
• Native VLAN carries all untagged traffic, specifically traffic not originating from a VLAN port.
• The native VLAN is VLAN 1 by default.
• Management VLAN carries network management traffic like SSH and SNMP, often the default VLAN 1.

Voice VLANs

• Cisco switches support voice VLANs for time-sensitive voice traffic which require assured bandwidth.
• Voice VLANs maintain a delay of less than 150 ms and give transmission priority, routing around congested areas.
• A voice VLAN feature enables access ports to handle user and IP voice traffic.
• An S3 F0/18 interface tags student traffic configured on VLAN 20 and voice traffic on VLAN 150.

VLAN Trunks

• A VLAN trunk is a point-to-point link that carries more than one VLAN.
• They're established between switches to support intra VLAN communication.
• VLAN trunks are not specific to a single VLAN but act as conduits for multiple VLANs between switches and routers.
• Cisco IOS supports IEEE 802.1Q, which is a popular VLAN trunking protocol.
• A trunk can connect a network device and server if equipped with a 802.1Q-capable NIC.
• Links between switches S1 and S2 and S1 and S3 are configured to transmit traffic from VLANs 10, 20, 30, and 99 across the network.

Controlling Broadcast Domains with VLANs

• A switch port forwards a broadcast frame to all ports, excluding the originating port in one broadcast domain.
• VLANs are used to limit the reach of broadcast frames by creating separate broadcast domains.
• VLANs controls broadcast reach in the network.

Tagging Ethernet Frames for VLAN Identification

• Frames are tagged with VLAN information before being forwarded across a trunk link.
• Frame tagging involves adding a VLAN identification header to the frame to properly transmit frames across a trunk link.
• IEEE 802.1Q is a popular protocol that defines the tagging header structure.
• Switches add VLAN tagging information after the Source MAC address field.
• VLAN ID (VID) is part of the 802.1Q VLAN tag.
• Trunk links add tag information before sending frames, removing the tag on non-trunk ports.

VLAN Ranges on Catalyst Switches

• VLANs are split into normal and extended ranges.
• Normal range VLANs are numbered 1 to 1,005.
• Normal range VLAN configurations are stored in vlan.dat file.
• VLAN IDs 1002 to 1005 are reserved for legacy Token Ring and Fiber Distributed Data Interface (FDDI).
• Extended range VLANs are numbered from 1,006 to 4,096.
• Extended range VLAN configurations are stored in the running configuration (NVRAM).
• VLAN Trunking Protocol (VTP) does not learn extended VLANs.
• Cisco Catalyst 2960 and 3560 Series switches support over 4,000 VLANs.

Creating a VLAN

• The command to create a VLAN is vlan 100,102,105-107 in global configuration mode

Assigning Ports to VLANs

• After VLAN creation, the next step is assigning it to a port.
• An access port can belong to only one VLAN at a time.
• To assign a port to a VLAN, use switchport mode access and switchport access vlan vlan-id in the interface configuration.
• The configurations assign traffic on the port to the specified VLAN.
• The IP address and subnet mask configured on the PC should match the subnet for the VLAN.
• The switchport access vlan command forces VLAN creation if it doesn't exist.

Changing VLAN Port Membership

• VLAN configuration can be changed using commands in the interface mode such as no switchport access vlan.
• Even if interface F0/18 was assigned to VLAN 20, it resets the switchport back to the default VLAN1 after using the command.

Deleting VLANs

• Delete VLANS using the no vlan vlan-id global configuration mode command.
• Use the delete vlan.dat privileged EXEC mode command to delete the entire vlan.dat file.
• This command is used only if the vlan.dat file hasn't moved from its original location.

Verifying VLAN Information

• VLAN configurations are validated using the show vlan and show interfaces commands.
• The show vlan [brief | id vlan-id | name vlan-name | summary] command varies depending on the parameter used.
  • brief, displays one line for each VLAN showing the name, status, and ports.
  • id vlan-id, displays information about a VLAN identified by its ID number (range: 1 to 4094).
  • name vlan-name, displays information for a single VLAN via its name.
  • summary, displays information about a single VLAN with its name, which is an ASCII string from 1 to 32 characters.
• The show interfaces [interface-id | vlan vlan-id] | switchport command:
  • interface-id lists port types, modules, numbers, and channels (range is 1 to 16).
  • vlan vlan-id is VLAN identification and whose range is 1 to 4094.
  • switchport shows the administrative/operational status of a switching port.

Configuring IEEE 802.1q Trunk Links

• A VLAN trunk is an OSI Layer 2 link that carries traffic for all VLANs (unless restricted).
• switchport mode trunk puts the port into permanent trunking mode.
• switchport trunk allowed vlan specifies which VLANs are allowed on the trunk.

Resetting the Trunk to Default State

• The trunk is reset to the default state using these commands: no switchport trunk allowed vlan, no switchport trunk native vlan.

IP Addressing Issues with VLANs

• It's common to associate a VLAN with an IP network.
• Different IP networks must communicate through a router.
• All devices within a VLAN must be part of the same IP network.

Missing VLANs

• You must check the device configuration to ensure the device is correctly assigned to a VLAN.
• If the VLAN to which the port belongs is deleted, the port becomes inactive.

Common Problems with Trunks

• Trunking issues usually stem from incorrect configurations.
• Common trunk configuration errors include native VLAN mismatches, trunk mode mismatches, and allowed VLANs on trunks.
  • Native VLAN Mismatches will cause pose a security risk and create unintended results.
    • For example, one port uses VLAN 99 while the other uses VLAN 100.
  • Trunk Mode Mismatches will cause a loss of network connectivity.
    • For example, one side of the trunk is configured as an access port.
  • Allowed VLANs on Trunks will cause unexpected traffic or no traffic.
    • The list of allowed VLANs does not support current VLAN trunking requirements.

Incorrect Port Mode

• In an example, PC4 can't reach the Web server because the S3 trunk port is configured as an access port.

Incorrect VLAN List

• In an example, PC5 cannot reach the Student Email server because the switchport trunk allowed vlan command reveals S1 isn't allowing VLAN 20.

Introduction to DTP

• Ethernet trunk interfaces support different trunking modes. An interface is set to negotiate trunking or non-trunking with the neighbor interface.
• Trunk negotiation is managed by DTP.
• Dynamic Trunking Protocol (DTP) is a Cisco proprietary protocol.
• DTP is automatically enabled on Catalyst 2960X and Catalyst 3650 Series switches
• You can enable trunking via the switchport mode trunk and switchport nonegotiate commands.

DTP Defaults

• The default configuration for Cisco Catalyst switches is dynamic auto.
• The link between interfaces configured to ignore DTP and stay in trunking mode becomes a trunk.
• Setting the link between S1 and S3 with dynamic auto (default) sets an access link.
• It’s recommended to statically configure with switchport mode trunk.

Negotiated Interface Modes

• The different trunking modes include switchport mode access, switchport mode trunk,switchport mode dynamic auto, switchport mode dynamic desirable, and switchport nonegotiate.
• switchport mode access puts the interface into permanent non-trunking mode and negotiates to convert link to a non-trunk link.
• switchport mode trunk puts the interface into permanent trunking mode and negotiates to convert link to a trunk link.
• switchport mode dynamic auto interface becomes a trunk if the neighboring interface is set to trunk or desirable mode.
• switchport mode dynamic desirable turns the interface into a trunk if the neighboring interface is trunk, desirable, or dynamic auto mode.
• switchport nonegotiate prevents the interface from generating DTP frames.

This lesson covers trunk link establishment, DTP, and VLAN configuration on Cisco switches. It includes switchport modes, VLAN ranges, and essential steps for VLAN setup. It also touches on removing VLANs and the switchport mode access command.