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Which of the following best describes why the notification is needed? To speed STP convergence by having the Root converge quickly. To allow the Root to keep accurate count of the number of topology changes. To trigger the process that causes all switches to use a short timer to help flush the CAM.
Two switches have four parallel Ethernet segments, none of which forms into an EtherChannel. Assuming Two switches have four Ethernet segments connecting them, with the intention of using an EtherChannel. An EtherChannel can still form using these four segments, even though some configuration settings do not match on the corresponding ports on each switch. Which settings do not have to match? Allowed VLAN list c. If Which of the following are valid Blocking b. Listening c.
Learning d. Forwarding e. Disabled f. Loop Guard b. UDLD c. UplinkFast d. BackboneFast e. A trunk between switches lost its physical transmit path in one direction only. Which of the following features protect against the STP problems caused by such an event? And in a network that makes good use of Layer 3 switching, each STP instance might span only three to four switches, making the STP issues much more manageable— but more forgettable in terms of helping you remember things you need to know for the exam.
This chapter reviews the details of IEEE STP uses messaging between switches to stabilize the network into a logical, loop-free topology. To do so, STP causes some interfaces popularly called ports when discussing STP to simply not forward or receive traffic—in other words, the ports are in a blocking state.
The remaining ports, in an STP forwarding state, together provide a loop-free path to every Ethernet segment in the network. Following the table, each of the three steps is explained in more detail. Table Three Major Elect the root switch The switch with the lowest bridge ID wins; the standard bridge ID is 2-byte priority followed by a MAC address unique to that switch. Determine the Designated Port When multiple switches connect to the same segment, this is the for each segment switch that forwards the least cost Hello onto a segment.
Electing a Root Switch Only one switch can be the root of the spanning tree; to select the root, the switches hold an election. If a switch hears a superior Hello—a Hello with a lower bridge ID—it stops claiming to be root by ceasing to originate and send Hellos.
Instead, the switch starts forwarding the superior Hellos received from the superior candidate. Eventually, all switches except the switch with the best bridge ID cease to originate Hellos; that one switch wins the election and becomes the root switch.
As a result, some switch must win the root election. The format of the original Figure shows the original and new format of the bridge IDs. Figure IEEE Priority field. The process proceeds as described in the following list:. The root creates and sends a Hello every Hello timer 2 seconds default. Switches do not forward Hellos out ports that stabilize into a blocking state.
Of all the ports in which a switch receives Hellos, the port with the least calculated cost to the root is the RP. For example, Figure shows the loop network design and details several STP cost circulations. In Figure , SW1 happened to become root, and is originating Hellos of cost 0.
SW3 receives two Hellos, one with cost 0 and one with cost Good design practices dictate using the same STP cost on each end of a point-to-point Ethernet segment, but the values can be different. While the costs shown in Figure might seem a bit contrived, the same result would happen with default port costs if the link from SW1 to SW3 were Fast Ethernet default cost 19 , and the other links were Gigabit Ethernet default cost 4.
Table lists the default port costs according to IEEE Note that the IEEE updated When a switch receives multiple Hellos with equal calculated cost, it uses the following tiebreakers:. Use the lowest port priority of the neighboring switch. The neighboring switch added its own port priority to the Hello before forwarding it. Use the lowest internal port number of the forwarding switch as listed inside the received Hellos.
Note that if the first tiebreaker in this list fails to produce an RP, this switch must have multiple links to the same neighboring switch. The last two tiebreakers simply help decide which of the multiple parallel links to use. By definition, only the DP on that segment should forward frames onto the segment. To win the right to be the DP, a switch must send the Hello with the lowest advertised cost onto the segment.
Similarly, SW4 will forward a Hello with cost 38, as shown in Figure When STP has a stable topology, the following occurs:. The root switch generates a Hello regularly based on the Hello timer. Each switch updates and forwards the Hello out its Designated Ports. For each blocking port, the switch regularly receives a copy of the Hello from the DP on that segment.
The switches do not forward Hellos out blocking interfaces. When some deviation from these events occurs, STP knows that the topology has changed and that convergence needs to take place. For instance, one simple case might be that the root switch loses power; the rest of the switches will not hear any Hello messages, and after the Maxage timer expires default 10 times Hello, or 20 seconds , the switches elect a new root based on the logic described earlier in this chapter.
For a more subtle example, consider Figure , which shows the same loop network as in Figure In this case, however, the link from SW1 to SW2 has just failed.
MAC I am receiving no other Hellos. I must be the root now! I am now DP! SW2 ceases to receive Hellos on its RP. Because SW2 is not receiving Hellos over any other path, it begins a new root election by claiming to be root and flooding Hellos out every port.
Because some switches might not directly notice a change in the STP topology, any switch that detects a change in the STP topology has a responsibility to notify the rest of the switches. The TCN goes up the tree to the root. After that, the root notifies all the rest of the switches. The process runs as follows:. Once received, the root sets the TCA flag on the next several Hellos, which are forwarded to all switches in the network, notifying them that a change has occurred. Transitioning from Blocking to Forwarding When STP reconverges to a new, stable topology, some ports that were blocking might have been designated as DP or RP, so these ports need to be in a forwarding state.
However, the transition from blocking to forwarding state cannot be made immediately without the risk of causing loops. To transition to forwarding state but also prevent temporary loops, a switch first puts a formerly blocking port into listening state, and then into learning state, with each state lasting for the length of time defined by the forward delay timer by default, 15 seconds. Table summarizes the key points about all of the Table IEEE Received Frames?
Stable State? Learning No Yes Transitory. Forwarding Yes Yes Stable. For the ports newly designated as RP or DP, The transition from forwarding to blocking can be made immediately. The redundant links would essentially be used for backup purposes. As a result, the traffic load can be balanced across the available links. For instance, in the common building design with distribution and access links in Figure , focus on the left side of the figure. In this case, the access layer switches block on different ports on VLANs 1 and 2, with different root switches.
With different root switches and with default port costs, the access layer switches end up sending VLAN1 traffic over one uplink and VLAN2 traffic over another uplink. Using When using With standard When building networks using a mix of Cisco and non-Cisco switches, along with NOTE MST is covered later in this chapter.
Example begins with SW1 coincidentally becoming the root switch. The root cost of! Next, SW2 is configured with a lower better priority than SW1,! Note that because SW2 is defaulting to use! The output! For VLAN1,! The other VLANs have a base priority! SW3 gets Hellos! This next command also details the breakdown of the priority and system ID. Bridge ID Priority priority sys-id-ext 1 Address e. Interface Role Sts Cost Prio. Root port and the Designated Ports.
The next command was done immediately after changing the port cost on! STP has already! The preceding example shows one way to configure the priority to a lower value to become the root. This command causes the switch to set the priority lower. The optional diameter parameter causes this command to lower the Hello, Forward Delay, and Maxage timers. This command does not get placed into the configuration, but rather it acts as a macro, being expanded into the commands to set priority and the timers.
NOTE When using the primary option, the spanning-tree vlan command sets the priority to 24, if the current root has a priority larger than 24, Also note that this logic applies to when the configuration command is executed; it does not dynamically change the priority if another switch later advertises a better priority. For instance, when the root fails, a switch must wait on the second Maxage timer to expire.
Then, newly forwarding ports spend 15 seconds each in listening and learning states, which makes convergence take 50 seconds for that one switch. This section covers the key optimizations to STP. Table summarizes when each is most useful, and the short version of how they improve convergence time. PortFast Used on access ports that are not Immediately puts the port into forwarding connected to other switches or hubs state once the port is physically working.
PortFast PortFast optimizes convergence by simply ignoring listening and learning states on ports. In effect, convergence happens instantly on ports with PortFast enabled. Of course, if another switch is connected to a port on which PortFast is enabled, loops may occur.
So, PortFast is intended for access links attached to single end-user devices. UplinkFast UplinkFast optimizes convergence when an uplink fails on an access layer switch. For good STP design, access layer switches should not become root or become transit switches.
A transit switch is a switch that forwards frames between other switches. Figure shows the actions taken when UplinkFast is enabled on a switch, and then when the Root Port fails. MAC Src. CDCD As a result of these steps, SW3 can become root if necessary, but it is unlikely to do so given the large root priority value. Also, the very large costs on each link make the switch unlikely to be used as a transit switch. Optimizing Spanning Tree The access switch also clears out the rest of the entries in its own CAM.
BackboneFast BackboneFast optimizes convergence for any generalized topological case, improving convergence when an indirect failure occurs. Any time a switch learns of an STP failure indirectly, the switch must wait for Maxage to expire before trying to change the STP topology. BackboneFast simply causes switches that indirectly learn of a potential STP failure to ask their upstream neighbors if they know about the failure.
The RLQ asks the neighboring switch if that neighboring switch is still receiving Hellos from the root. If that neighboring switch had a direct link failure, it can tell the original switch via another RLQ that this path to the root is lost. Once known, the switch experiencing the indirect link failure can go ahead and converge without waiting for Maxage to expire. UplinkFast spanning-tree uplinkfast [max-update-rate rate] global. PortChannels When a network design includes multiple parallel segments between the same pair of switches, one switch ends up in a forwarding state on all the links, but the other switch blocks all but one of the ports of those parallel segments.
As a result, only one of the links can be used at any point in time. Most campus designs today use a minimum of two segments per trunk, in a PortChannel, for better availability. Load Balancing Across PortChannels When a switch decides to forward a frame out a PortChannel, the switch must also decide which physical link to use to send each frame.
Load-balancing methods differ depending on the model of switch and software revision. If load balancing is based on only one header field in the frame, a bitmap of the low-order bits is used; if more than one header field is used, an XOR of the low-order bits is used.
For the best balancing effect, the header fields on which balancing is based need to vary among the mix of frames sent over the PortChannel. For instance, for a Layer 2 PortChannel connected to an access layer switch, most of the traffic going from the access layer switch to the distribution layer switch is probably going from clients to the default router.
For packets coming back from a distribution switch toward the access layer switch, many of the frames might have a source address of that same router, with differing destination MAC addresses. So, you could balance based on source MAC at the access layer switch, and based on destination MAC at the distribution layer switch—or balance based on both fields on both switches. The goal is simply to use a balancing method for which the fields in the frames vary. The port-channel load-balance type command sets the type of load balancing.
PortChannel Discovery and Configuration You can explicitly configure interfaces to be in a PortChannel by using the channel-group number mode on interface subcommand. You would simply put the same command under each of the physical interfaces inside the PortChannel, using the same PortChannel number.
You can also use dynamic protocols to allow neighboring switches to figure out which ports should be part of the same PortChannel. To dynami- cally form a PortChannel using PAgP, you still use the channel-group command, with a mode of auto or desirable.
Table lists and describes the modes and their meanings. As you might imagine, they exchange a system ID to determine which ports connect to the same two switches. The two switches then exchange other information about the candidate links for a PortChannel; several items must be identical on the links for them to be dynamically added to the PortChannel, as follows:.
When PAgP or LACP completes the process, a new PortChannel interface exists, and is used as if it were a single port for STP purposes, with balancing taking place based on the global load- balancing method configured on each switch. To do so, RSTP defines new variations on BPDUs between switches, new port states, and new port roles, all with the capability to operate backwardly compatible with The key components of speeding convergence with RSTP takes advantage of a switched network topology by categorizing ports, using a different link type to describe each.
RSTP takes advantage of the fact that STP logic can be simplified in some cases, based on what is attached to each port, thereby allowing faster convergence. Table lists the three RSTP link types. Point-to-point Connects a switch to one other switch; Cisco switches treat FDX links in which Hellos are received as point-to-point links. Shared Connects a switch to a hub; the important factor is that switches are reachable off that port.
Edge Connects a switch to a single end-user device. In most modern LAN designs with no shared hubs, all links would be either the point-to-point a link between two switches or edge link type. RSTP knows that link-type edge means the port is cabled to one device, and the device is not a switch.
In other words, RSTP puts edge links into forwarding state immediately. RSTP takes advantage of point-to-point links which by definition connect a switch to another switch by asking the other switch about its status. For instance, if one switch fails to receive its periodic Hello on a point-to-point link, it will query the neighbor.
The neighbor will reply, stating whether it also lost its path to the root. RSTP also redefines the port states used with Table compares the port states defined by each protocol. Disabled Disabled Discarding. Enabled Blocking Discarding.
Enabled Listening Discarding. Enabled Learning Learning. Enabled Forwarding Forwarding. In RSTP, a discarding state means that the port does not forward frames, receive frames, or learn source MAC addresses, regardless of whether the port was shut down, failed, or simply does not have a reason to forward.
Once RSTP decides to transition from discarding to forwarding state for example, a newly selected RP , it goes immediately to the learning state. From that point on, the process continues just as it does with RSTP no longer needs the listening state because of its active querying to neighbors, which guarantees no loops during convergence.
Backup Port A port that is attached to the same link-type shared link as another port on the same switch, but the other port is the DP for that segment. The Backup Port is ready to take over if the DP fails. The concept and general operation is identical to UplinkFast, although RSTP might converge more quickly via its active messaging between switches. The Backup Port role has no equivalent with Cisco-proprietary features; it simply provides protection against losing the DP attached to a shared link when the switch has another physical port attached to the same shared LAN.
Alternatively, you can simply enable In global configuration mode, issue the spanning-tree mode rapid-pvst command. Then, optionally, on an interface VLAN, physical, or PortChannel , configure the spanning-tree link-type point-to-point command. This configures the port for fast changeover to the forwarding state. The following are some of the main benefits of The key to MST configuration is to configure the same parameters on all the switches in the region.
When participating in STP with the external switches, the MST region is made to appear as if it is a single switch; the right side of Figure depicts the STP view of the left side of the figure, as seen by the external switches. Protecting STP The final section in this chapter covers four switch configuration tools that protect STP from different types of problems or attacks, depending on whether a port is a trunk or an access port.
Protecting STP Root Guard and BPDU Guard: Protecting Access Ports Network designers probably do not intend for end users to connect a switch to an access port that is intended for attaching end-user devices.
However, it happens—for instance, someone just may need a few more ports in the meeting room down the hall, so they figure they could just plug a small, cheap switch into the wall socket. The STP topology can be changed based on one of these unexpected and undesired switches being added to the network. For instance, this newly added and unexpected switch might have the lowest bridge ID and become the root. Both features can be used together.
Their base operations are as follows:. Upon receipt of superior BPDUs, this switch puts the port in a loop-inconsistent state, ceasing forwarding and receiving frames until the superior BPDUs cease. With BPDU Guard, the port does not recover from the err-disabled state unless additional configuration is added. You can tell the switch to change from err-disabled state back to an up state after a certain amount of time. Both features prevent switch ports from errantly moving from a blocking to a forwarding state when a unidirectional link exists in the network.
Unidirectional links are simply links for which one of the two transmission paths on the link has failed, but not both. This can happen as a result of miscabling, cutting one fiber cable, unplugging one fiber, GBIC problems, or other reasons. Although UDLD was developed for fiber links because of the unidirectional nature of fiber optic cabling—and therefore the much greater likelihood of a unidirectional link failure in a fiber cable—this feature also supports copper links.
Because STP monitors incoming BPDUs to know when to reconverge the network, adjacent switches on a unidirectional link could both become forwarding, causing a loop, as shown in Figure I must Tx be the DP.
Let me transition to forwarding! Figure shows the fiber link between SW1 and SW2 with both cables. SW2 then transitions to forwarding state, and now all trunks on all switches are forwarding. UDLD uses two modes to attack the unidirectional link problem. The switch whose transmit interface did not fail is placed into an err-disabled state.
If the other switch does not reply to the repeated additional messages, both sides become err-disabled. In all cases, the formerly blocking port that would now cause a loop is prevented from migrating to a forwarding state. With both types of UDLD, the switch can be configured to automatically transition out of err-disabled state.
With Loop Guard, the switch automatically puts the port back into its former STP state when the original Hellos are received again. Table lists the protocols mentioned in this chapter and their respective standards documents. Forward 15 sec Time that switch leaves a port in listening state and learning state; also Delay used as the short CAM timeout timer. Maxage 20 sec Time without hearing a Hello before believing that the root has failed.
Table lists some of the key IOS commands related to the topics in this chapter. The command syntax for switch commands was taken from the Catalyst Multilayer Switch Command Reference, Also refer to Table for several other commands.
Part II: IP. Complete mastery of IP addressing and subnetting is required for any candidate to have a reasonable chance at passing both the CCIE written and lab exam. For the CCIE exam, understanding these topics is required to answer much deeper questions—for instance, a question might ask for the interpretation of the output of a show ip bgp command and a configuration snippet to decide what routes would be summarized into a new prefix.
To answer such questions, the basic concepts and math behind subnetting need to be very familiar. In what subnet does address What is the smallest summary route that could be advertised out a WAN link connecting Router1 to the rest of the network, if subnets not listed here were allowed to be included in the summary?
Which one of the answers lists the smallest summary route s that could be advertised by R1 without also including subnets not listed in this question?
Which two of the following VLSM subnets, when taken as a pair, overlap? Which of the following protocols or tools includes a feature like route summarization, plus administrative rules for global address assignment, with a goal of reducing the size of Internet routing tables?
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