1. The University of Bolton School of Games Computing & Creative Technologies LCT2516 Network Architecture CCNA Exploration LAN Switching and Wireless Chapter 6 Inter-VLAN Routing Martin Stanhope m.stanhope@bolton.ac.uk 1

2. Chapter sections 6.0 Chapter Introduction 6.1 Inter-VLAN Routing 6.2 Configuring Inter-VLAN Routing 6.3 Troubleshooting Inter-VLAN Routing 6.4 Chapter Labs 6.5 Chapter Summary 6.6 Chapter Quiz 2

3. Chapter outline 6.1 Inter-VLAN Routing –6.1.1 Introducing Inter-VLAN Routing –6.1.2 Interfaces and Subinterfaces 6.2 Configuring Inter-VLAN Routing –6.2.1 Configure Inter-VLAN Routing –6.2.2 Configure Router-on-a-Stick Inter-VLAN Routing 6.3 Troubleshooting Inter-VLAN Routing –6.3.1 Switch Configuration Issues –6.3.2 Router Configuration Issues –6.3.3 IP Addressing Issues 3

4. 6.1 Inter-VLAN Routing - 6.1.1 Introducing Inter-VLAN Routing Each VLAN is a unique broadcast domain, so computers on separate VLANs are, by default, not able to communicate. This chapter looks at adding a router to the switch network to allow traffic from one VLAN to be routed to another different VLAN. In a traditional network that uses multiple VLANs to segment the network traffic into logical broadcast domains, routing is performed by connecting different physical router interfaces to different physical switch ports. See animation 6.1.1.2 to see how PC1 on VLAN 10 communicates with PC3 on VLAN 30 via the router. Notice in this solution how the router has two separate physical interfaces, one configured in VLAN10 and the other in VLAN30. 4

5. 6.1 Inter-VLAN Routing - 6.1.1 Introducing Inter-VLAN Routing continued... Traditional inter-VLAN routing requires multiple physical interfaces on both the router and the switch. This scenario can be improved by using a trunk link between the switch and the router. This topology is often referred to as a ‘Router-on-a-Stick’. The router performs the inter-VLAN routing by accepting VLAN tagged traffic on the trunk interface coming from the adjacent switch and internally routing between the VLANs using subinterfaces. The router then forwards the routed traffic-VLAN tagged for the destination VLAN out of the same physical interface. See animation 6.1.1.3. 5

6. 6.1 Inter-VLAN Routing - 6.1.1 Introducing Inter-VLAN Routing continued... Subinterfaces are multiple virtual interfaces, associated with one physical router interface. Subinterfaces are configured in software on a router and are configured for different subnets corresponding to their VLAN assignment to facilitate logical routing before the data frames are VLAN tagged and sent back out of the physical interface. See animation 6.1.1.3 to see how PC1 on VLAN10 communicates with PC3 on VLAN30 via the router, but note how the router is using only one physical interface connected to a trunk link. The single physical router interface is configured as 3 logical subinterfaces which are given IP and Subnet masks settings to put them in each of the VLANs. 6

7. 6.1 Inter-VLAN Routing - 6.1.1 Introducing Inter-VLAN Routing continued... Inter-VLAN Routing can also be achieved by using ‘Multilayer Switches’ instead of a router. ‘Multilayer Switches’ are not covered in the CCNA course. See animation 6.1.1.4. 7

8. 6.1 Inter-VLAN Routing - 6.1.2 Interfaces and Subinterfaces In a traditional network, using multiple router interfaces (one for each subnet), if a host PC needs to send traffic to another network it will send it to the gateway address which is set up as the router interface connected to the host’s subnet. The host only knows the router gateway IP address but to put a frame together it needs to know the MAC address of the router interface acting as its gateway so sends out an ARP request. Once the router sends back an ARP reply to the host PC, the host can finish framing the packet before forwarding it. When the unicast traffic arrives at the router, the router removes the source and destination MAC addresses to examine the destination IP address of the packet. The router uses the information in its routing table to decide on which port to forward the packet. It then sends out an ARP request in order to put the frame together to move the traffic along to its destination. See animation 6.1.2.1. 8

9. 6.1 Inter-VLAN Routing - 6.1.2 Interfaces and Subinterfaces continued... See graphic 6.1.2.2 for router interface configuration information Inter-VLAN routing on networks with multiple VLANs using a router with a restricted number of physical interfaces is an issue so logical subinterfaces are used instead. Subinterfaces are software-based virtual interfaces that are assigned to a single physical interface. The physical interface isn’t assigned an IP address and subnet mask but each of the subinterfaces are. Each subinterface must be assigned to a different subnet by the choice of IP address and subnet mask. Functionally, the router-on-a-stick model for inter-VLAN routing is the same as using the traditional routing model, but instead of using multiple physical interfaces to perform the routing, multiple subinterfaces of a single physical interface are used. See animation 6.1.2.3 PC1 (VLAN10) unicasts to PC3 (VLAN30) 9

10. 6.1 Inter-VLAN Routing - 6.1.2 Interfaces and Subinterfaces continued... If the physical interface is named, for example, fa0/0 then the subinterfaces are named: fa0/0.1, fa0/0.2, fa0/0.3 etc. The subinterfaces don’t have to be numbered sequentially as shown above, the examples given in the online notes often use subinterface numbers matching the VLAN numbers, for example: –fa0/0.10 –fa0/0.20 –fa0/0.30 See graphic 6.1.2.4 for configuration commands 10

11. 6.1 Inter-VLAN Routing - 6.1.2 Interfaces and Subinterfaces continued... Which is best? Multiple physical router interfaces each connected to different switch access ports over multiple links OR 1 physical router interface with multiple subinterfaces connected to a single switch trunk port over a single trunk link? –Subinterfaces are a good solution if there are not enough router interfaces for 1 per vlan. –Subinterfaces offer a lower cost solution. Fewer interfaces and fewer interconnecting cables. –Subinterfaces do have issues though as traffic between multiple VLANs crosses the same trunk link so there is contention for bandwidth. Trouble shooting VLAN problems can also be difficult. See graphic 6.1.2.5 11

12. 6.2 Configure Inter-VLAN Routing - 6.2.1 Configure Inter-VLAN Routing See graphic 6.2.1.1 showing the traditional model of having multiple physical interfaces on the router, 1 for each VLAN and how the switch ports and router interfaces are configured. See graphic 6.2.1.2 to examine the routing table for the network shown in 6.2.1.1 See graphic 6.2.2.1 showing the same network but this time a single physical router interface using multiple subinterfaces (one per VLAN) is used. See graphic 6.2.2.2 to examine the routing table for the network shown in 6.2.2.1 Inter-VLAN operation can be tested using Ping and Tracert. See graphic 6.2.2.3 12

13. 6.3 Troubleshooting Inter-VLAN Routing - 6.3.1 Switch Configuration Issues See graphic 6.3.1.1 and examine the types of faults described in the 3 network topologies shown which relate to issues with... –Switch ports in the wrong VLANs –Switch ports configured as access ports instead of trunk ports –Problems when a connection between a PC and a switch port fails and there is no redundant link as a backup. See graphic 6.3.1.2 for useful troubleshooting commands. 13

14. 6.3 Troubleshooting Inter-VLAN Routing - 6.3.2 Router Configuration Issues See graphic 6.3.2.1 and examine the types of faults described in the 2 network topologies shown which relate to issues with... –Switch ports in the wrong VLANs –Router subinterfaces in the wrong VLAN See graphic 6.3.2.2 for useful troubleshooting commands. 14

15. 6.3 Troubleshooting Inter-VLAN Routing - 6.3.3 IP Addressing Issues For inter-VLAN routing to operate, a router needs to be connected to all VLANs, either by separate physical interfaces or trunked subinterfaces. Each interface, or subinterface, needs to be assigned an IP address that corresponds to the subnet for which it is connected. 15

16. 6.3 Troubleshooting Inter-VLAN Routing - 6.3.3 IP Addressing Issues continued... See graphic 6.3.3.1 and examine the types of faults described in the 3 network topologies shown which relate to issues with... –Incorrect IP address on a router interface –Incorrect IP address on a PC in VLAN 10 –Incorrect subnet mask on a PC See graphic 6.3.3.2 for useful troubleshooting commands. 16