CCNP Enterprise Certification Study Guide: Implementing and Operating Cisco Enterprise Network Core Technologies. Ben PiperЧитать онлайн книгу.
P2p Gi0/2 Desg BLK 4 128.3 P2p Gi0/3 Desg BLK 4 128.4 P2p Gi1/0 Root FWD 4 128.5 P2p Gi1/1 Altn BLK 4 128.6 P2p ! Output truncated
Gi1/0 is once again the root port.
Modifying Port Priority
You can change the port priority for Gi1/1 on SW4 as follows:
SW4(config)#int gigabitEthernet 1/1 SW4(config-if)#spanning-tree vlan 1 port-priority ? <0-224> port priority in increments of 32 SW4(config-if)#spanning-tree vlan 1 port-priority 64
On SW2, you can see the designated port ID for Gi1/1 is now 64.6, and it's the root port:
SW2#show spanning-tree vlan 1 interface gi1/1 detail Port 6 (GigabitEthernet1/1) of VLAN0001 is root forwarding Port path cost 4, Port priority 128, Port Identifier 128.6. Designated root has priority 32769, address 0015.fa83.e900 Designated bridge has priority 32769, address 001d.45cf.e800 Designated port id is 64.6, designated path cost 4 Timers: message age 15, forward delay 0, hold 0 Number of transitions to forwarding state: 3 Link type is shared by default BPDU: sent 43, received 1851
The converged topology is shown in Figure 2.6.
Figure 2.6 Converged VLAN 1 topology with SW2 Gi1/0 blocking
Calculating Blocked Ports
Now we come to perhaps the easiest part of Spanning Tree: determining which remaining ports to block. Consider the connections between SW1 and SW4. Both are directly connected to the root, SW3, and have no need of a link to each other. Because SW1 has the higher bridge priority, it will block both its ports, like so:
SW1#show spanning-tree vlan 1 | i Gi2/0|Gi2/1 Gi2/0 Altn BLK 4 128.9 P2p Gi2/1 Altn BLK 4 128.10 P2p
Blocked ports don't forward traffic, but they still receive and process BPDUs. This ensures that every bridge can learn about changes to the Spanning Tree topology. SW4 has a lower bridge priority and has both its ports forwarding:
SW4#show spanning-tree vlan 1 | i Gi2/0|Gi2/1 Gi2/0 Desg FWD 4 128.9 P2p Gi2/1 Desg FWD 4 128.10 P2p
Any non-root ports that aren't blocking are called designated ports. The job of designated ports is to forward BPDUs from the root bridge so that Spanning Tree can reconverge in the event of a topology change.
Port States
In RSTP, a port can have one of three states: learning, forwarding, or discarding. A port eventually settles into either a forwarding or discarding state. The learning state is transitory and only occurs when RSTP is in the process of determining which ports to block.
Discarding—The port processes incoming BPDUs but doesn't send them. IOS lists a port in the discarding state as BLK or blocking. This is the initial state for all ports.
Learning—The port sends and receives BPDUs but doesn't forward data traffic. The switch looks at ingressing Ethernet frames and adds the source MAC addresses to its MAC address table.
Forwarding—Sends and receives BPDUs, and it also passes normal user and control plane traffic.
Port Roles
In addition to the root and designated port roles, RSTP has two additional port roles you need to know: alternate and backup.
Alternate—An alternate port provides an alternate path to the root. If a root port fails, the switch will place the best (lowest cost) alternate port into a forwarding state. This lets the topology reconverge in a matter of milliseconds.
Backup—You're unlikely to ever see a backup port role, except on an exam. Imagine that two ports are connected to a hub and hence are on the same segment. The port with the lowest cost—or if the costs are equal, the port with the lowest designated port priority—will be the designated port. The other port connected to the segment will be the backup.
Link Types
One way that RSTP achieves a fast convergence time is by avoiding putting some ports through the three-step process of discarding, learning, and forwarding. The idea is that by predefining ports that are directly connected to another switch or to an edge device, RSTP can more quickly figure out which ports to block and which to transition to the forwarding state. RSTP defines three link types:
Point-to-point (P2P)
P2P Edge
Shared
Point-to-Point (P2P)
The P2P link type indicates that the link is a link between only two switches. Thus, each switch knows that it needs to negotiate with only one switch on the other end of the link. RSTP will automatically set the P2P link type on any full-duplex link. You can manually set the P2P link type using the interface command spanning-tree link-type point-to-point.
P2P Edge
This type is reserved for edge devices that don't run Spanning Tree, such as computers, servers, and phones. An edge port skips over the learning state and transitions instantly to a forwarding state. RSTP still sends BPDUs on edge ports just in case there's a switch on the other end. But if it receives a BPDU, it will change the port type to a normal Spanning Tree port. If you're familiar with PortFast, this is essentially the same thing. In fact, IOS uses the portfast keyword to configure an edge port, as follows:
SW2(config)#int gi1/3 SW2(config-if)#spanning-tree portfast edge ? trunk Enable portfast edge on the interface even in trunk mode <cr> SW2(config-if)#spanning-tree portfast edge trunk %Warning: portfast should only be enabled on ports connected to a single host. Connecting hubs, concentrators, switches, bridges, etc... to this interface when portfast is enabled, can cause temporary bridging loops. Use with CAUTION
Keep in mind that an edge port can be an access port or a trunk port that's connected to multiple devices. If the port is a single-VLAN access port, you should use the spanning-tree portfast edge interface command. But if it's a trunked interface—such as one connected to a server running virtual machines or a computer plugged in behind an IP phone on a separate voice VLAN—you must use the spanning-tree portfast edge trunk interface command.
Shared
The Shared port type is one you shouldn't expect to see very often. It's for a port that may be connected to multiple devices, usually via a hub. RSTP considers any half-duplex link to be shared. In this case, RSTP falls back to timer-based 802.1D behavior and may take up to 50 seconds to converge.
Multiple Spanning Tree
MST is designed as an alternative to PVST+. MST allows you to reduce the number of Spanning Tree instances required when you have a large number of VLANs. In per-VLAN Spanning Tree, you must have one Spanning Tree instance per VLAN. For example, if you have 500 VLANs, you must have 500 instances of Spanning Tree.
MST, on the other hand, lets you map multiple VLANs to a single Spanning Tree instance. To keep track of instances, MST uses instance numbers. Because VLAN-to-instance mappings are arbitrary, all switches in the topology must agree on the mappings. To deal with this, MST uses regions. An MST region is a collection of MST instances that share three common values:
Region name
Revision