Section 15.2.  Example of Hierarchical Switched L2 Topology

15.2. Example of Hierarchical Switched L2 Topology We know that two hosts can be connected to each other with a cross cable; you do not necessarily have to use a device such as a hub or a bridge. You can do the same between bridges and routers. In the examples in this chapter, you will often see such cross-cable links between bridges. Unlike the scenarios in Chapter 14, where simple two-port bridges link directly to hosts located in the connected LANs, a real bridged network normally has a topology that resembles a tree, where hosts are located only (or mainly) at the leaves' node. Figure 15-1. Basic terminology When you have simple scenarios like the ones seen in Chapter 14, you do not usually enable the STP; in fact, most sites could simply use a single flat LAN instead of employing bridges at all. To better understand STP, we need to see what a real-life scenario looks like. Let's take the classic hierarchical bridged and redundant topology in Figure 15-2(a), which is advertised and evangelized by most of the commercial bridge vendors. The figure leaves out details described later in this chapter, such as the bridge ID and priority, port cost, and priority values, to let you focus on the topology and active links selection. In the subsequent figures in this chapter, we will reuse the symbol definitions in the legend at the bottom of the figure. At the leaves of the tree (the bottom of the figure) are the hosts. The hosts are linked to so-called access bridges (commonly called access switches): the bridges that give network connectivity to the hosts. Access bridges are mainly used to forward traffic between the hosts linked to the same bridge, but they also have one or more links to the upper-layer bridges. The access bridges in Figure 15-2 are labeled A1, A2, A3, and A4. Because hosts are always located at the leaves of the topology, you can have as many links to hosts as you like. They will not cause any loops (of course, we assume there are no links between leaves). Because of that, the links to the host are not affected by the STP: none of them needs to be disabled to define a loop-free topology. After all, the ultimate goal of STP it to make the network look like a big single LAN and provide connectivity to all hosts, so why would you disconnect any of the hosts? Figure 15-2. Hierarchical bridged L2 topology A bridge at the distribution layer (D1 and D2 in the figure) is mainly used to bridge traffic between hosts located in some of the access bridges it is directly connected to. For example, D1 will take care of A1 and A2. Note that D1 is also linked to A3 and A4, although currently D1's links are inactive (dotted lines in the figure). In case the link between D2 and A3 fails, the STP will make sure that the topology is updated so that A3 is again part of the tree. For example, the network could enable the link between D1 and A3; we will see how later in this chapter. The two distribution bridges D1 and D2 are also linked to the two core bridges C1 and C2. It should be clear what C1 and C2's job is: to connect D1's subtree to D2's subtree. (An alternative solution would be one with a single, and maybe more powerful, core bridge.) Between the distribution and core layers there are also redundant links so that if, for example, the link C1-D1 failed, C2-D1 would take over. The higher the layer where a bridge is located, the bigger the volume of traffic that is processed (because the subtree is bigger). The figure shows the links of the topology that the STP has selected to define the loop-free topology, and what ports have been assigned the designated and root roles. In this chapter, we will see what the designated and root roles are used for, how they are assigned, and why. Note that the traffic exchanged between any pair of hosts within the L2 network of Figure 15-2(a) uses L2 protocols to travel (i.e., Ethernet). Routing can be implemented at the core or through the core. From the host's perspective, there is no hierarchy at the link layer, only a flat LAN; the overall topology appears to it like Figure 15-2(b).[*] [*] 12-digit MAC addresses (such as 11:22:33:44:55:66) are replaced with simple 2-digit values to make the figure more readable. The use of multiple bridges has a few advantages: It helps segregate traffic. For example, while Host 1 talks to Host 10, Host 11 can talk to Host 20, and Host 21 can talk to Host 40, all without having to receive and discard each other's frames. So the overall bandwidth of the L2 network is increased. But in the worst case, a frame may need to cross the entire tree to get to its destination. For example, Figure 15-3 shows the path of a frame that needs to go from Host 40 to Host 1. Note that the figure also shows the address learned by each bridge port: for example, the notation 1-10 close to a bridge's interface means that the latter has learned the MAC addresses of Hosts 1 through 10. (We saw in Chapter 14 how address learning works.) Large numbers of hosts become easier to manage. You do not need to connect all the hosts to a single giant bridge, which means the hosts can be located in different areas. Cabling is also simpler to take care of. I'll end my overview of L2 bridged topologies here. You would need a whole book to cover bridging protocols and STP in detail, so I'll move ahead with an overview of the algorithm implemented by the STP. In the rest of this chapter, we will use simpler topologies to describe the protocol. However, what we will see works just the same way in bigger and more complex networks like the one in Figure 15-2.