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Inter-VLAN Routing

• Layer 2 switches are unable to forward traffic between VLANs without a router
• Inter-VLAN routing forwards network traffic from one VLAN to another, using a router or multilayer switch

Inter-VLAN Routing Options

• Legacy inter-VLAN routing
• Router-on-a-Stick configuration
• Layer 3 switching using SVIs

Legacy Inter-VLAN Routing

• Dedicated router interfaces were utilized to facilitate routing between VLANs
• Each VLAN was connected to a distinct physical router interface
• Packets arriving at the router via one interface would be routed and then directed out through another
• The connection of router interfaces to specific VLANs, along with the assignment of IP addresses from those VLANs, enabled inter-VLAN routing
• Large networks containing numerous VLANs necessitated a large number of router interfaces

Router-on-a-Stick: Preparation

• VLAN trunking and subinterfaces are an alternative to legacy Inter-VLAN routing
• VLAN trunking enables a single physical router interface to route traffic for multiple VLANs
• The router's physical interface must be connected to a trunk link on the adjacent switch
• Subinterfaces are created on the router for each VLAN
• Subinterfaces are software-based virtual interfaces assigned to physical interfaces
• Each subinterface is assigned to tag frames for each VLAN, and has an IP address specific to its subnet or VLAN

Router-on-a-Stick: Configuration

• Inter-VLAN routing uses a router-on-a-stick
• Trunking is enabled on the switch port connected to the router for this setup

Router Subinterface Configuration

• The router-on-a-stick architecture requires configuring subinterfaces for each routable VLAN
• Subinterfaces must be configured to support VLAN using the encapsulation dot1Q VLAN-ID interface configuration

Verifying Subinterfaces

• Cisco routers are set up to route traffic between local subinterfaces by default
• Routing does not need to be specifically enabled
• The show ip route command can be used to check the subinterface configurations
• The show ip route command displays the routing table with networks associated with outgoing subinterfaces

Verifying Routing

• Connectivity to remote VLAN devices can be tested with the ping command
• When a host receives an ICMP echo request, this sends an ICMP request and responds with an ICMP reply
• Traceroute confirms the routed path between two devices

Introduction to Layer 3 Switching

• Multilayer switches provide high packet processing rates using hardware-based switching

Layer 3 Interfaces

• Routed port - A layer 3 interface
• Switch virtual interface (SVI) - Virtual Interface for inter-VLAN routing

Introduction to Layer 3 Switching

• Layer 3 Cisco Catalyst switches support routing protocols
• Some models may need software for specific routing protocol features

Inter-VLAN Routing with Switch Virtual Interfaces

• Layer 2 switching portion was extended more into the network due to slow routing in the early days of switched networks
• Routing can be performed at wire speed, at both distribution and core layers
• Distribution switches are configured as Layer 3 gateways using Switch Virtual Interfaces (SVIs) or routed ports
• Routed ports are implemented between the distribution and core layers

Switch Virtual Interface (SVI)

• Is a virtual interface that is configured within a multilayer switch

Switch Virtual Interface (SVI) benefits

• Provide a gateway for a VLAN so that traffic can be routed into or out of that VLAN
• Provides Layer 3 IP connectivity to the switch
• Supports routing protocols and bridging configurations

Advantages of SVIs

• Faster than router-on-a-stick
• There is no need for external links from the switch for routing
• Not limited to one link, Layer 2 EtherChannels can be used to get more bandwidth

Routing with Routed Ports

• A routed port acts like an interface on a router
• A routed port is a physical port
• It is not associated with a particular VLAN
• It doesn't support subinterfaces
• Routed ports are configured primarily between switches in the core and distribution layer
• To configure a routed port, use the no switchport interface configuration command on the appropriate port
• Routed ports are not supported on Catalyst 2960 Series switches

VTP Overview

• VLAN Trunking Protocol (VTP) enables network administrators to deal with VLANs on a switch configured as a VTP server
• The VTP server distributes and synchronizes VLAN information over trunk links to VTP-enabled switches

VTP Modes

• Server: Manages domain and VLAN configuration, multiple VTP servers can be configured
• Client: Updates local VTP configurations, client switches cannot change VLAN configurations.
• Transparent: Manages local VLAN configurations that are not shared with the VTP network.

VTP Modes Continued

• VTP Server and Client:
  • Participate fully to VTP advertisements -Global configurations are stored in NVRAM -Updates local VTP configurations from other VTP enabled switches
• VTP Transparent:
  • Forwards only VTP advertisements
  • The local VLAN configuration is stored only in NVRAM
  • VLAN configurations are not shared with the VTP network.
  • Does not update,

VTP Advertisements

• Contains VTP domain name and configuration revision number (sent every 5 mins)
• Three types of VTP advertisements:
  • Summary Advertisements
  • Advertisement Request
  • Subset Advertisements

VTP Advertisements Explained

• Advertisement Request- a response to summary adv. message when the summary advertisement contains a higher config. revision # than current value
• Subset Advertisements- contain any VLAN changes and VLAN information

Configuration Revision Number

• Is a 32-bit number that indicates the level of revision for a VTP packet
• VTP devices track the VTP configuration revision number that is assigned to it
• The received information is used to determine if it is more recent than the current version
• The revision number increases: each time a VLAN is added or deleted, or a VLAN name is changed
• The revision number resets to 0 if the VTP domain name changes or the switch is set to transparent mode

Resetting Configuration Revision

• To reset the configuration revision on a switch, change the VTP domain name then change the name back to the original name

VTP Advertisements: Considerations

• The switch will compare configuration revision to its own revision, if the domain name of a received summary advertisement is the same as currently configured
• If its own configuration revision number is higher or the same, the packet is ignored
• If its own revision number is lower, the switch sends a request, for the subset advertisement

Default VTP Configuration

• VTP Version capable: 1 to 3
• VTP version running: 1
• VTP Domain Name: (blank)
• VTP Pruning Mode: Disabled
• VTP Traps Generation: Disabled
• Device ID: (Device specific)
• Configuration last modified by 0.0.0.0 at 3-1-93 00:02:11
• Feature VLAN: (Displays if Feature VLAN is set)
• VTP Operating Mode: Transparent
• Maximum VLANs supported locally: 255
• Number of existing VLANs: (Number of VLANs)
• Configuration Revision: 0

VTP Caveats

• To reset VTP configuration revision number to zero you should:
  • Change the switch’s VTP domain to a nonexistent VTP domain and then change the domain back
  • Change the switch's VTP mode to transparent and then back to the previous VTP mode.

Steps to Configure VTP

• Step 1 - Configure VTP Server
• Step 2 - Configure VTP Domain Name and Password
• Step 3 - Configure the VTP Clients
• Step 4 - Configure VLANs on the VTP Server
• Step 5 - Verify the VTP clients have received the new VLAN information

Configuring VTP Command

• vtp mode server command to configure a switch as a VTP server
• Confirm all switches are configured with the default configuration before issuing to avoid problems with configuration revision numbers.
• Use the show vtp status command to verify VTP status

Configuring VTP Domain Command

• The vtp domain domain-name command configures the domain name
• The VTP client must have the same domain as the VTP server before accepting VTP advertisements
• The 'vtp password password command configures a password
• The show vtp password command is used to verify

VTP client commands

vtp mode client configures VTP clients

• The client must use the same domain name and password as the VTP server