Routing over Trunk Links

A more robust and cost effective approach uses trunk links between the switches and routers, instead of multiple physical links. Because trunk links transport multiple VLANs over a single link, only one link to an external router is required. A router connected to a switch by a single trunk link is usually referred to as a router on a stick, or a one-armed router. 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.

This section discusses three types of trunk links that can be used: IEEE 802.1Q, ISL, and ATM LANE.

802.1Q and ISL Trunks

Both IEEE 802.1Q and Inter-Switch Link (ISL) trunks can be used to transport multiple VLANs to a router. Both encapsulation methods use Fast Ethernet or Gigabit Ethernet as the physical media for trunking. Figure 7-3 shows a typical application using a router and ISL trunk links for interVLAN routing. Catalyst A is connected to the router using a trunk link that transports VLANs 1, 2, and 3.

Recall from Chapter 4, "VLANs and Trunking," that 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. Recall also that frames from the native VLAN of an 802.1Q trunk are not tagged with the VLAN number.

Figure 7-3 InterVLAN Routing Using Trunk Links

VLAN 1

Figure 7-3 InterVLAN Routing Using Trunk Links

VLAN 1

As in the previous example, suppose a workstation attached to VLAN 1 on Catalyst A needs to send a packet to a station on VLAN 5 on Catalyst B. The station sends the frame to the router, using the VLAN connected to that station. The packet is sent out over the ISL trunk link, identified with VLAN 1. The router receives the packet and unencapsulates it, examining the identifier. The packet is inbound on VLAN 1 and destined for a station on VLAN 5. The router makes a routing decision and forwards the packet toward Catalyst B, after encapsulating it in the ISL trunk with VLAN identifier 5.

On a router, 802.1Q and ISL trunks are connected by either a Fast or Gigabit Ethernet interface. (EtherChannel is also an option that can be used.) To support various VLANs, individual subinterfaces are configured with either 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. Trunking encapsulations also require some link bandwidth overhead, as either a 30-byte header (ISL) or a 4-byte header (802.1Q) increases each frame.

ATM LANE

As discussed in Chapter 6, "Trunking with ATM LANE," switches can trunk VLANs to other switches over ATM links using the LAN Emulation (LANE) standard. Switches with LANE modules can participate by operating local LANE Clients (LECs) for each VLAN-to-emulated LAN (ELAN) bridge. The LANE modules must also communicate with the other LANE components (LANE Configuration Server [LECS], LANE Server [LES], and BUS) in the network to become active members in the emulated LANs.

If only switches are involved in LANE, each ELAN remains isolated from other ELANs. This isolation follows the principle of VLANs, which require a Layer 3 routing function to move data between VLANs. Routing can be performed between ELANs trunked over LANE by adding an ATM interface to an external router.

The router can use a single ATM link to connect to an ATM switch—not directly to an ATM link on a Catalyst switch. Figure 7-4 shows an example network using a LANE-attached router to route between ELANs. The router interacts with LANE just as a switch does: it must find the LECS; operate a LEC for each ELAN that it supports; locate and join the LES and BUS for each ELAN; and build Data Direct virtual circuits (VCs) to other LECs in the network.

Figure 7-4 InterELAN Routing Using ATM LANE

atm0

Catalyst A atm 0.1: LEC1, LES/BUS 1 atm 0.2: LEC2, LES/BUS 2

atm0

Catalyst B

Catalyst B

atm0

atm0

Router atm 0.1: LEC1 atm 0.2: LEC2

Router atm 0.1: LEC1 atm 0.2: LEC2

A major ATM interface on the router is broken up into logical subinterfaces. The major interface can be configured with an LECS, if required. The individual subinterfaces represent single ELANs and can be configured with LES/BUS pairs or a LEC for the ELAN. Packets coming in over a LEC on one subinterface are processed by the router and forwarded out another LEC on a different subinterface, depending on the Layer 3 addresses and other routing decisions.

Continue reading here: Foundation Summary Bgh

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