Section 6.1: InterVLAN Routing Design
Several options are available when placing a route processor within a switch campus network. These options are primarily based on the type of connectivity between the switches and the router, and location of the route processor.
6.1.1: Routing with Multiple Physical Links
The simplest method of routing between VLANs is to use several physical links between switches and an external router. Each link is configured for a single VLAN, so that there is a link for each VLAN to be routed. Using one VLAN per link offers an intuitive approach to routing between VLANs as routers associate each physical link with a subnetwork, and transport packets between links. Each link is also segmented from the others, unless bridging arrangements are made within the router. This is useful when the switches and router are already available and can be quickly connected, using a small number of VLANs. No configuration is needed, except the usual interface addressing used on the router. However, when the network grows, this method becomes problematic as every additional VLAN requires an additional physical link to the router.
6.1.2: Routing over Trunk Links
A more robust routing approach is to use trunk links between the switches and routers, instead of multiple physical links. Trunk links transport multiple VLANs over a single link; therefore, only one link to an external router is required. A router connected to a switch by a single trunk link is referred to as a router on a stick. However, a router can also connect to several switches using trunk links. This connection provides end-to-end Layer 3 connectivity between blocks of switches. Three types of trunk links can be used: IEEE 802.1Q, and ISL 6.1.2.1: 802.1Q and ISL Trunks
Both IEEE 802.1Q and Inter-Switch Link (ISL), which are encapsulation methods that use Fast Ethernet or Gigabit Ethernet as the physical media for trunking, can be used to transport multiple VLANs to a router. Both 802.1Q and ISL trunk links identify each frame with a VLAN number. As a frame leaves a switch, the frame is encapsulated and identified with its VLAN. When the router receives a frame over a trunk link, the router unencapsulates the frame and associates it with an interface assigned to the VLAN number that identified the frame. Frames from the native VLAN of an 802.1Q trunk are not tagged with the VLAN number.
To support various VLANs, individual subinterfaces are configured with 802.1Q or ISL encapsulation, and a VLAN number. 802.1Q and ISL trunks offer the advantage of scalability because a single link can transport many VLANs; however, some CPU overhead is involved as the router processes the encapsulation. Therefore, the router cannot use its most efficient packet switching method for packet forwarding. Furthermore, trunking encapsulations also require some link bandwidth overhead; ISL adds a 30-byte header to each frame while 802.1Q adds a 4-byte header to each frame.