As technologies such as ISL became more common, network designers began to use trunk links to connect routers to a campus backbone. Figure 11-3 illustrates an example of this approach.
Although any trunking technology such as ISL, 802.1Q, 802.10, LAN Emulation (LANE), or MPOA can be used, Ethernet-based approaches are most common (ISL and 802.1Q). Figure 11-3 uses ISL running over Fast Ethernet. The solid lines refer to the single physical link running between the top Catalyst and the router. The dashed lines refer to the multiple logical links running over this physical link.
The primary advantage of using a trunk link is a reduction in router and switch ports. Not only can this save money, it can reduce configuration complexity. Consequently, the trunk-connected router approach can scale to a much larger number of VLANs than the one-link-per-VLAN design.
However, there are disadvantages to the trunk-connected router configuration, including the following:
• Inadequate bandwidth for each VLAN
• Additional overhead on the router
• Older versions of the IOS only support a limited set of features on ISL interfaces
With regard to inadequate bandwidth for each VLAN, consider, for example, the use of a Fast Ethernet link where all VLANs must share 100 Mbps of bandwidth. A single VLAN could easily consume the entire capacity of the router or the link (especially if there is a broadcast storm or Spanning Tree problem).
With regard to the additional overhead on the router caused by using a trunk-connected router, not only must the router perform normal routing and data forwarding duties, it must handle the additional encapsulation used by the trunking protocol. Take ISL running on a 7500 router as an example. Cisco's software-based routers have a number of different switching modes, a term that Cisco uses to generically refer to the process of data forwarding in a router.
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