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there is no danger of a loop because the switch has no redundant paths in which to forward a frame
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Figure 19-3: A physical star with switches In a mesh topology, however, the switch repeats the frame to all other redundantly connected switches, causing the frame to be repeated indefinitely (or until all bandwidth is consumed) This dilemma is the same looping problem in a redundant (full or hybrid physical mesh) topology, like the one shown in Figure 19-4
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Figure 19-4: A full mesh without using STP Enter STP It automatically determines where redundant links are present and blocks data traffic on those ports until the primary link fails (in which case, it automatically opens the redundant ports) Figure 19-5 shows the mesh topology using STP
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Figure 19-5: A full mesh using STP Now that you know why STP is used, let's concentrate on how it works
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How STP Works
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STP works by determining what the "best" path is through the network and allowing data traffic to be sent only along that path This functionality is fairly similar to the main functionality of dynamic routing protocols, but, luckily, STP is simpler than most routing protocols
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Understanding basic STP functionality requires an understanding of some new terms and rules Let's begin with the basic terms:
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Port states STP defines several port states: blocking, listening, learning, forwarding, and disabled For right now, you need to be concerned with only three: blocking (which disables data traffic on the port but still allows STP messages to be transmitted and received), forwarding (which allows all traffic), and disabled (which allows no traffic) The disabled state might be used when a link is not present on the port or the port has been administratively disabled Root bridge (RB) The root bridge is the center of the STP topology The basic idea is that a path through the root bridge is the best path to use to get from one side of the network to the other All ports on the root bridge are always in a forwarding state Designated bridge (DB) The designated bridge is the one with the shortest path (lowest cost) to a given network segment This bridge is also sometimes called a parent switch Bridge Protocol Data Unit (BPDU) or Configuration Bridge Protocol Data Unit (CBPDU) STP sends these frames to communicate with other bridges in the STP topology Their purpose is to carry information about the STP topology to all other bridges that are participating in the same topology BPDUs are sent periodically by all switches in the STP topology, but they are not propagated past the first switch to hear them Root port (RP) This port is determined to be the best path to the root bridge This port is always in a forwarding state Port cost The cost of a port indicates how much STP prefers a particular path out a given port STP uses this cost to determine which port is the root port and which bridge on a given segment should be the designated bridge
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STP uses the Spanning Tree Algorithm (STA) to determine what the best path throughout the network is STA relies on a few simple rules:
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The root bridge places all ports in a forwarding state (except those that are disabled administratively or not connected) The bridge with the lowest cost to a given segment is considered the designated bridge for that segment The designated bridge for a given segment places the port connected to that segment in a forwarding state All bridges have a single port that is considered to have the lowest cost (shortest path) to the root bridge This port is considered to be the root port and is placed in a forwarding state All other ports are placed in a blocking state
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Now that you understand the rules and terms, let's look at how STP determines the optimal topology and maintains that topology Figure 19-6 shows a sample network topology
Figure 19-6: The network for the STP example First, a root bridge must be selected This task is performed through a process known as an election When the switches are powered up, they have no knowledge of the other switches or topology of the network Therefore, the switch doesn't know which one is the best switch for the root bridge role Therefore, it simply assumes that it is the best switch for the root bridge role, and it begins advertising itself (using BPDUs) as the root bridge out all nondisabled ports The BPDUs contain only a few pieces of information:
The root bridge's bridge ID The bridge ID is usually the MAC address for the bridge The root bridge's root priority This priority can be set by the administrator to assure that one bridge is likely to become the root This 16-bit field is set to 32,768 by default, or 0x8000 (hex) The bridge's bridge ID This ID identifies the bridge that is sending this BPDU, usually the MAC address of the sending bridge The cost to the root bridge from the advertising bridge This cost is known as the path cost It tells the receiving bridge how "far" from the root bridge the advertising bridge is The sending bridge's port ID and port cost This information tells the receiving bridge which port on the sending bridge this advertisement was sent from, and what the cost is on that port By default, the switch usually sets this number to a value equaling 1000 divided by the speed of the port in megabits per second For example, the default cost for most 10Base-T interfaces is 100 Various timers These timers (max age, hello, and forward delay) determine how quickly the STP topology responds to link failures They are discussed individually later in this section of the chapter
Using the example shown in Figure 19-6, let's see how this election process occurs Table 191 shows the STP configuration Table 19-1: Bridge Configuration for the Example
Fender Bridge ID Bridge priority Port cost, Port 1 Port cost, Port 2 Port cost, Port 3 Port cost, Port 4 001 12 (0xC) 10 100 100 100
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