Lecture 3 - Router-on-a-Stick Inter-VLAN Routing PDF

Lecture 3: Inter-VLAN Routing INFR 1421 Introduction to Networking II Brent MacRae January 2025 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 n...

Lecture 3: Inter-VLAN Routing INFR 1421 Introduction to Networking II Brent MacRae January 2025 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/multilayer switch. ▪ There are three options for inter-VLAN routing: Legacy inter-VLAN routing Router-on-a-Stick Layer 3 switching using SVIs © 2025 Brent MacRae 2 Inter-VLAN Routing Operation Legacy Inter-VLAN Routing ▪ In the past: In this example, the router was configured with two separate Router interfaces were used to route physical interfaces to interact with the different VLANs and perform the routing. 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. © 2025 Brent MacRae 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. Subinterfaces are software-based virtual interfaces that are assigned to physical interfaces. ▪ Each subinterface is assigned an IP address specific to its subnet or VLAN and is also © 2025 Brent MacRae 4 configured to tag frames for that VLAN. Configure Router-on-a-Stick Inter-VLAN Routing Configure Router-on-a Stick: Switch Configuration ▪ To enable inter-VLAN routing using router-on-a stick, start by enabling trunking on the switch port that is connected to the router. © 2025 Brent MacRae 5 Configure Router-on-a-Stick Inter-VLAN Routing Configure Router-on-a Stick: Router Subinterface Configuration ▪ The router-on-a-stick method requires subinterfaces to be configured for each routable VLAN. The subinterfaces must be configured to support VLANs using the encapsulation dot1Q VLAN-ID interface configuration command. © 2025 Brent MacRae 6 Configure Router-on-a-Stick Inter-VLAN Routing Configure Router-on-a Stick: Verifying Subinterfaces ▪ By default, Cisco routers are configured to route traffic between local subinterfaces. As a result, routing does not specifically need to be enabled. ▪ Use the show ip route commands to verify the subinterface configurations. ▪ The show ip route command displays the routing table containing the networks associated with outgoing subinterfaces. © 2025 Brent MacRae 7 Configure Router-on-a-Stick Inter-VLAN Routing Configure Router-on-a Stick: Verifying Routing ▪ Remote VLAN device connectivity can be tested using the ping command. The command sends an ICMP echo request and when a host receives an ICMP echo request, it responds with an ICMP echo reply. ▪ Traceroute is a useful utility for confirming the routed path taken between two devices. © 2025 Brent MacRae 8 Layer 3 Switching Operation and Configuration Introduction to Layer 3 Switching ▪ Multilayer switches provide high-packet processing rates using hardware-based switching (millions of packets per second). ▪ Catalyst multilayer switches support the following types of Layer 3 interfaces: Routed port - A layer 3 interface Switch virtual interface (SVI) - Virtual Interface for inter-VLAN routing ▪ All Layer 3 Cisco Catalyst switches support routing protocols, but several models require enhanced software for specific routing protocol features. © 2025 Brent MacRae 9 Layer 3 Switching Operation and Configuration Inter-VLAN Routing with Switch Virtual Interfaces ▪ In the early days of switched networks, switching was fast and routing was slow. Therefore the layer 2 switching portion was extended as much as possible into the network. ▪ Now routing can be performed at wire speed, and is performed at both the distribution and core layers. ▪ Distribution switches are configured as Layer 3 gateways using Switch Virtual Interfaces (SVIs) or routed ports. ▪ Routed ports are usually implemented between the distribution and core layers. © 2025 Brent MacRae 10 Layer 3 Switching Operation and Configuration Inter-VLAN Routing with Switch Virtual Interfaces (Cont.) ▪ A Switch Virtual Interface (SVI) is a virtual interface that is configured within a multilayer switch: To provide a gateway for a VLAN so that traffic can be routed into or out of that VLAN. To provide Layer 3 IP connectivity to the switch. To support routing protocol and bridging configurations. ▪ Advantages of SVIs: Faster than router-on-a-stick. No need for external links from the switch to the router for routing. Not limited to one link. Layer 2 EtherChannels can be used to get more bandwidth. © 2025 Brent MacRae 11 Layer 3 Switching Operation and Configuration Routing with Routed Ports ▪ A routed port is a physical port that acts similarly to an interface on a router: It is not associated with a particular VLAN. It does not support subinterfaces. ▪ Routed ports are primarily configured between switches in the core and distribution layer. ▪ Use the no switchport interface command on the appropriate port to configure a routed port. Note: Routed ports are not supported on Catalyst 2960 Series switches. © 2025 Brent MacRae 12 3.2 VTP © 2025 Brent MacRae 13 VTP Concepts and Operation VTP Overview ▪ VLAN trunking protocol (VTP) allows a network administrator to manage VLANs on a switch configured as a VTP server. ▪ The VTP server distributes and synchronizes VLAN information over trunk links to VTP- enabled switches throughout the switched network. © 2025 Brent MacRae 14 VTP Concepts and Operation VTP Modes © 2025 Brent MacRae 15 VTP Concepts and Operation VTP Modes (Cont.) © 2025 Brent MacRae 16 VTP Concepts and Operation VTP Advertisements ▪ Three types of VTP Advertisements: Summary advertisements – contain VTP domain name and configuration revision number (sent every 5 mins). Advertisement request - response to a summary advertisement message when the summary advertisement contains a higher configuration revision number than the current value. Subset advertisements - contain VLAN information including any changes. © 2025 Brent MacRae 17 VTP Concepts and Operation VTP Advertisements ▪ The configuration revision number is a 32-bit number that indicates the level of revision for a VTP packet. ▪ Each VTP device tracks the VTP configuration revision number that is assigned to it. ▪ This information is used to determine whether the received information is more recent than the current version. ▪ The revision number increases by 1 each time you add a VLAN, delete a VLAN, or change a VLAN name. If the VTP domain name is changed or the switch is set to transparent mode, the revision number is reset to 0. © 2025 Brent MacRae 18 VTP Concepts and Operation VTP Advertisements ▪ If the VTP domain name of a received summary advertisement is the same as currently configured, the switch then compares the configuration revision to its own revision. ▪ If its own configuration revision number is higher or equal to the packet’s configuration revision number, the packet is ignored. ▪ If its own configuration revision number is lower, an advertisement request is sent, asking for the subset advertisement message. © 2025 Brent MacRae 19 VTP Concepts and Operation Default VTP configuration © 2025 Brent MacRae 20 VTP Concepts and Operation VTP Caveats © 2025 Brent MacRae 21 VTP Concepts and Operation VTP Caveats ▪ To reset VTP configuration revision number to zero: Change the switch's VTP domain to a nonexistent VTP domain and then change the domain back to the original name. Change the switch's VTP mode to transparent and then back to previous VTP mode. © 2025 Brent MacRae 22 VTP Configuration VTP Configuration Overview ▪ Steps to Configure VTP: Step 1 - Configure the VTP Server Step 2 - Configure the 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. © 2025 Brent MacRae 23 VTP Configuration Step 1 – Configure the VTP Server ▪ Use the vtp mode server command to configure a switch as a VTP server. Confirm all switches are configured with default configuration before issuing this command to avoid problems with configuration revision numbers. ▪ Use the show vtp status to verify. © 2025 Brent MacRae 24 VTP Configuration Step 2 – Configure the VTP Domain Name and Password ▪ Use the vtp domain domain-name command to configure the domain name. VTP client must have same domain name as the VTP server before it will accept VTP advertisements. ▪ Configure a password using the vtp password password command. Use the show vtp password command to verify. © 2025 Brent MacRae 25 VTP Configuration Step 3 – Configure the VTP Clients ▪ Use the vtp mode client command to configure the VTP clients. ▪ Use same domain name and password as VTP server. © 2025 Brent MacRae 26 VTP Configuration Step 4 – Configure VLANs on the VTP Server 1. 2. © 2025 Brent MacRae 27