Trunks on servers Trunks

84 Figure 3-21. When a trunk breaks, it must not fragment a Layer 3 network The same VLAN, number 101, which has IP address 10.1.101.0 , exists on both switches. Then, for diversity, the first router connects to the first switch and the second router connects to the second switch. This design is seriously flawed. Consider what happens when the trunk connecting the two switches fails. Suddenly two distinct unconnected LAN segments have the same IP address range and the same IPX network number. Now both routers provide valid routes to these networks. Thus, no communication will work properly to either segment. This is almost exactly the same problem I described earlier with two routers on the same LAN segment, but here you can see that it happens with VLANs as well. How does one resolve this problem? A couple of different approaches are available. One method connects both routers to both switches, as shown by the addition of the dashed lines in Figure 3-21 . This solution is not always practical, depending on the capabilities of the routers, since it implies that both routers have multiple interfaces on the same network. In fact, the simplest solution is to just run a second trunk between the two switches, as shown with the dotted line. Then you can simply rely on Spanning Tree to activate this link if the primary fails. Furthermore, if you suffer a complete failure of one entire switch, then you lose half of your workstations, but at least the other half continues to work. A failure of one router allows the other to take over transparently, so this is the most acceptable solution. However, in a good hierarchical design, this sort of problem is less likely to arise because each Access switch connects to two different Distribution switches. Thus, the network would need to have multiple simultaneous trunk failures to get into this sort of problem.

3.5.3.3.3 Trunks on servers

Some types of servers support VLAN trunks directly so that you can have a single server with simultaneous presence on several different VLANs, as shown in Figure 3-22 . 85 Figure 3-22. Some servers connect directly to trunk links to access several VLANs simultaneously This is certainly an interesting thing to do, but its important to understand why you would want to do this before trying it. There are different ways to achieve similar results. For example, many servers support multiple network interface cards NIC. Installing two NICs in a server and connecting them to different VLANs via different switch ports has the benefit of simpler configurations on both the switch and the server and provides higher net throughput. Alternatively, if you cant afford to use multiple physical ports for whatever reason, then you could just as easily put the server behind a router and let the traffic route to all of the different user segments. However, this strategy is cost-effective in some cases. For example, if the trunk connection is a Gigabit Ethernet link, it might be significantly less expensive than deploying a router solution, as routers with high-speed interfaces tend to be very expensive. At the same time, Gigabit Ethernet ports on switches can be costly. This strategy may be a convenient way of deploying a server for multiple user VLANs. However, this method does not scale very well. If there will be many such servers, it would likely be less expensive in the long run to build a specialized high-speed server segment behind a router. Because it is a trunk link, the different VLANs will also compete with one another for server bandwidth on this link. In previous chapters I made the point that only network devices should perform network functions. Therefore, I dont like connecting an end device to multiple VLANs, whether it is through a single port or through multiple ports. An end device should have a single connection to the network unless there is a compelling reason to do something more complicated.

3.5.3.4 VLAN Distribution Areas