STP Terminology
STP terminology can best be explained by examining how a sample network, such as the one shown in Figure 1-19, operates.
| Note | Notice that STP terminology refers to the devices as bridges rather than switches. |
Within an STP network, one switch is elected as the root bridge—it is at the root of the spanning tree. All other switches calculate their best path to the root bridge. Their alternative paths are put in the blocking state. These alternative paths are logically disabled from the perspective of regular traffic, but the switches still communicate with each other on these paths so that the alternative paths can be unblocked in case an error occurs on the best path.
All switches running STP (it is turned on by default in Cisco switches) send out Bridge Protocol Data Units (BPDU). Switches running STP use BPDUs to exchange information with neighboring switches. One of the fields in the BPDU is the bridge identifier (ID); it comprises a 2-octet bridge priority and a 6-octet MAC address. STP uses the bridge ID to elect the root bridge—the switch with the lowest bridge ID is the root bridge. If all bridge priorities are left at their default values, the switch with the lowest MAC address therefore becomes the root bridge. In Figure 1-19, switch Y is elected as the root bridge.
All the ports on the root bridge are called designated ports, and they are all in the forwarding state—that is, they can send and receive data. The STP states are described in the next section.
On all nonroot bridges, one port becomes the root port, and it is also in the forwarding state. The root port is the one with the lowest cost to the root. The cost of each link is by default inversely proportional to the link’s bandwidth, so the port with the fastest total path from the switch to the root bridge is selected as the root port on that switch. In Figure 1-19, port 1 on switch X is the root port for that switch because it is the fastest way to the root bridge.
| Note | If multiple ports on a switch have the same fastest total path costs to the root bridge, STP considers other BPDU fields. STP looks first at the bridge IDs in the received BPDUs (the bridge IDs of the next switch in the path to the root bridge); the port that received the BPDU with the lowest bridge ID becomes the root port. If these bridge IDs are also equal, the port ID breaks the tie; the port with the lower port ID becomes the root port. The port ID field includes a port priority and a port index, which is the port number. Therefore, if the port priorities are the same (for example, if they are left at their default value), the lower port number becomes the root port. |
Each LAN segment must have one designated port. It is on the switch that has the lowest cost to the root bridge (or, if the costs are equal, the port on the switch with the lowest bridge ID is chosen), and it is in the forwarding state. In Figure 1-19, the root bridge has designated ports on both segments, so no more are required.
| Note | The root bridge sends configuration BPDUs on all its ports periodically—every 2 seconds, by default. These configuration BPDUs include STP timers, therefore ensuring that all switches in the network use the same timers. On each LAN segment, the switch that has the designated port forwards the configuration BPDUs to the segment; every switch in the network therefore receives these BPDUs on its root port. |
All ports on a LAN segment that are not root ports or designated ports are called nondesignated ports and transition to the blocking state—they do not send data, so the redundant topology is logically disabled. In Figure 1-19, port 2 on switch X is the nondesignated port, and it is in the blocking state. Blocking ports do, however, listen for BPDUs.
If a failure happens—for example, if a designated port or a root bridge fails—the switches send topology change BPDUs and recalculate the spanning tree. The new spanning tree does not include the failed port or switch, and the ports that were previously blocking might now be in the forwarding state. This is how STP supports the redundancy in a switched network.
STP States
Figure 1-20 illustrates the various STP port states.
When a port initially comes up, it is put in the blocking state, in which it listens for BPDUs and then transitions to the listening state. A blocking port in an operational network can also transition to the listening state if it does not hear any BPDUs for the max-age time (a default of 20 seconds). While in the listening state, the switch can send and receive BPDUs but not data. The root bridge and the various final states of all the ports are determined in this state.
If the port is chosen as the root port on a switch, or as a designated port on a segment, that port transitions to the learning state after the listening state. In the learning state, the port still cannot send data, but it can start to populate its MAC address table if any data is received. The length of time spent in each of the listening and learning states is dictated by the value of the forward-delay parameter, which is 15 seconds by default. After the learning state, the port transitions to the forwarding state, in which it can operate normally. Alternatively, if in the listening state the port is not chosen as a root port or designated port, it becomes a nondesignated port and transitions back to the blocking state.
Several features and enhancements to STP are implemented on Cisco switches to help to reduce the convergence time—the time it takes for all the switches in a network to agree on the network’s topology after that topology has changed.
Rapid STP
Rapid STP (RSTP) is defined by IEEE 802.1w. RSTP incorporates many of the Cisco enhancements to STP, resulting in faster convergence. Switches in an RSTP environment converge quickly by communicating with each other and determining which links can forward, rather than just waiting for the timers to transition the ports among the various states. RSTP ports take on different roles than STP ports. The RSTP roles are root, designated, alternate, backup, and disabled. RSTP port states are also different from STP port states. The RSTP states are discarding, learning, and forwarding. RSTP is compatible with STP. For example, 802.1w alternate and backup port states correspond to the 802.1d blocking port state.
Virtual LANs
As noted earlier, a broadcast domain includes all devices that receive each others’ broadcasts (and multicasts). All the devices connected to one router port are in the same broadcast domain. Routers block broadcasts (destined for all networks) and multicasts by default; routers forward only unicast packets (destined for a specific device) and packets of a special type called directed broadcasts. Typically, you think of a broadcast domain as being a physical wire, a LAN. But a broadcast domain can also be a VLAN, a logical construct that can include multiple physical LAN segments.
Figure 1-21 illustrates the VLAN concept. On the left side of the figure, three individual physical LANs are shown, one each for Engineering, Accounting, and Marketing. These LANs contain workstations—E1, E2, A1, A2, M1, and M2—and servers—ES, AS, and MS. Instead of physical LANs, an enterprise can use VLANs, as shown on the right side of the figure. With VLANs, members of each department can be physically located anywhere, yet still be logically connected with their own workgroup. Therefore, in the VLAN configuration, all the devices attached to VLAN E (Engineering) share the same broadcast domain, the devices attached to VLAN A (Accounting) share a separate broadcast domain, and the devices attached to VLAN M (Marketing) share a third broadcast domain. Figure 1-21 also illustrates how VLANs can span multiple switches; the link between the two switches in the figure carries traffic from all three of the VLANs and is called a trunk.
VLAN Membership
Static port membership means that the network administrator configures which VLAN the port belongs to, regardless of the devices attached to it. This means that after you have configured the ports, you must ensure that the devices attaching to the switch are plugged into the correct port, and if they move, you must reconfigure the switch.
Alternatively, you can configure dynamic VLAN membership. Some static configuration is still required, but this time, it is on a separate device called a VLAN Membership Policy Server (VMPS). The VMPS could be a separate server, or it could be a higher-end switch that contains the VMPS information. VMPS information consists of a MAC address–to–VLAN map. As a result, ports are assigned to VLANs based on the MAC address of the device connected to the port. When you move a device from one port to another port (either on the same switch or on another switch in the network), the switch dynamically assigns the new port to the proper VLAN for that device by consulting the VMPS.
Trunks
As mentioned earlier, a port that carries data from multiple VLANs is called a trunk. A trunk port can be on a switch, a router, or a server. A trunk port can use one of two protocols: Inter-Switch Link (ISL) or IEEE 802.1Q.
ISL is a Cisco-proprietary trunking protocol that involves encapsulating the data frame between an ISL header and trailer. The header is 26 bytes long; the trailer is a 4-byte cyclic redundancy check that is added after the data frame. A 15-bit VLAN ID field is included in the header to identify the VLAN that the traffic is for. (Only the lower 10 bits of this field are used, thus supporting 1024 VLANs.)
The 802.1Q protocol is an IEEE standard protocol in which the trunking information is encoded within a Tag field inserted inside the frame header itself. Trunks using the 802.1Q protocol define a native VLAN. Traffic for the native VLAN is not tagged; it is carried across the trunk unchanged. Consequently, end-user stations that don’t understand trunking can communicate with other devices directly over an 802.1Q trunk as long as they are on the native VLAN. The native VLAN must be defined to be the same VLAN on both sides of the trunk. Within the Tag field, the 802.1Q VLAN ID field is 12 bits long, allowing up to 4096 VLANs to be defined. The Tag field also includes a 3-bit 802.1p user priority field; these bits are used as class of service (CoS) bits for QoS marking. (Chapter 4 describes QoS.)
The two types of trunks are not compatible with each other, so both ends of a trunk must be defined with the same trunk type.
| Note | Multiple switch ports can be logically combined so that they appear as one higher-performance port. Cisco does this with its EtherChannel technology, combining multiple Fast Ethernet or Gigabit Ethernet links. Trunks can be implemented on both individual ports and on these EtherChannel ports. |
STP and VLANs
Cisco developed per-VLAN spanning tree (PVST) so that switches can have one instance of STP running per VLAN, allowing redundant physical links within the network to be used for different VLANs and thus reducing the load on individual links. PVST is illustrated in Figure 1-22.
The top diagram in Figure 1-22 shows the physical topology of the network, with switches X and Y redundantly connected. In the lower-left diagram, switch Y has been selected as the root bridge for VLAN A, leaving port 2 on switch X in the blocking state. In contrast, the lower-right diagram shows that switch X has been selected as the root bridge for VLAN B, leaving port 2 on switch Y in the blocking state. With this configuration, traffic is shared across all links, with traffic for VLAN A traveling to the lower LAN on switch Y’s port 2, whereas traffic for VLAN B traveling to the lower LAN goes out switch X’s port 2.
PVST works only over ISL trunks. However, Cisco extended this functionality for 802.1Q trunks with the PVST+ protocol. Before this became available, 802.1Q trunks supported only Common Spanning Tree, with one instance of STP running for all VLANs.
Multiple-Instance STP (MISTP) is an IEEE standard (802.1s) that uses RSTP and allows several VLANs to be grouped into a single spanning-tree instance. Each instance is independent of the other instances so that a link can forward for one group of VLANs while blocking for other VLANs. MISTP therefore allows traffic to be shared across all the links in the network, but it reduces the number of STP instances that would be required if PVST/PVST+ were implemented.
Rapid per-VLAN Spanning Tree Plus (RPVST+) is a Cisco enhancement of RSTP, using PVST+.
Inter-VLAN Routing
A Layer 3 device can be connected to a switched network in two ways: by using multiple physical interfaces or through a single interface configured as a trunk. These two connection methods are shown in Figure 1-23. The diagram on the left illustrates a router with three physical connections to the switch; each physical connection carries traffic from only one VLAN.
The diagram on the right illustrates a router with one physical connection to the switch. The interfaces on the switch and the router have been configured as trunks; therefore, multiple logical connections exist between the two devices. When a router is connected to a switch through a trunk, it is sometimes called a “router on a stick,” because it has only one physical interface (a stick) to the switch.
Each interface between the switch and the Layer 3 device, whether physical interfaces or logical interfaces within a trunk, is in a separate VLAN and therefore in a separate subnet for IP networks.
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