話と行動の距離はどのぐらいありますか。これは人の心によることです。意志が強い人にとって、行動は目と鼻の先にあるのです。あなたはきっとこのような人でしょう。Ciscoの650-027認定試験に申し込んだ以上、試験に合格しなければならないです。これもあなたの意志が強いことを表示する方法です。JPexamが提供したトレーニング資料はインターネットで最高のものです。Ciscoの650-027認定試験に合格したいのなら、JPexamのCiscoの650-027試験トレーニング資料を利用してください。
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たくさんの人はCisco 644-068認証試験を通ることが難しいと思います。もし弊社の問題集を勉強してそれは簡単になります。弊社はオンラインサービスとアフターサービスとオンラインなどの全面方面を含めてます。オンラインサービスは研究資料模擬练習問題などで、アフターサービスはJPexamが最新の認定問題だけでなく、絶えずに問題集を更新しています。
JPexamが提供した対応性の訓練問題をテストにして初めてCiscoの644-068認定試験に参加する受験者の最もよいな選択でございます。真実試験問題が似てるのを確保することができて一回合格するのは目標にしています。もし試験に失敗したら、弊社が全額で返金いたします。
試験番号：650-027問題集
試験科目：Cisco Digital Media System for SE/FE
最近更新時間：2014-04-30
問題と解答：全35問 650-027 練習問題
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>>詳しい紹介はこちら
試験番号：350-001問題集
試験科目：CCIE Routing and Switching Written Exam #350-001, v4.0
最近更新時間：2014-04-30
問題と解答：全505問 350-001 全真問題集
100％の返金保証。1年間の無料アップデート。
>>詳しい紹介はこちら
試験番号：644-068問題集
試験科目：Advanced Routing and Switching for Field Engineers - ARSFE
最近更新時間：2014-04-30
問題と解答：全50問 644-068 復習資料
100％の返金保証。1年間の無料アップデート。
>>詳しい紹介はこちら
現状に自己満足して、自分の小さな持ち場を守って少ない給料をもらって解雇されるのを待っている人がいないです。こんな生活はとてもつまらないですから。あなたの人生をよりカラフルにしたいのですか。ここで成功へのショートカットを教えてあげます。即ちCiscoの350-001認定試験に受かることです。この認証を持っていたら、あなたは、高レベルのホワイトカラーの生活を送ることができます。実力を持っている人になって、他の人に尊敬されることもできます。JPexamはCiscoの350-001試験トレーニング資料を提供できます。JPexamを利用したら、あなたは美しい夢を実現することができます。さあ、ためらわずにJPexamのCiscoの350-001試験トレーニング資料をショッピングカートに入れましょう。
Ciscoの650-027認定試験を除いて、最近非常に人気がある試験はまたCisco、Cisco、IBM、SAPなどの様々な認定試験があります。しかし、もし650-027認証資格を取りたいなら、JPexamの650-027問題集はあなたを願望を達成させることができます。試験の受験に自信を持たないので諦めることをしないでください。JPexamの試験参考書を利用することを通して自分の目標を達成することができますから。650-027認証資格を入手してから、他のIT認定試験を受験することもできます。JPexamの試験問題集を手にすると、どのような試験でも問題ではありません。
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NO.1 Which statement is true about loop guard?
A. Loop guard only operates on interfaces that are considered point-to-point by the spanning tree.
B. Loop guard only operates on root ports.
C. Loop guard only operates on designated ports.
D. Loop guard only operates on edge ports.
Answer: A
Cisco 学習 350-001 模擬 350-001 関節
Explanation:
Understanding How Loop Guard Works
Unidirectional link failures may cause a root port or alternate port to become designated as root if
BPDUs are absent. Some software failures may introduce temporary loops in the network. Loop
guard checks if a root port or an alternate root port receives BPDUs. If the port is receiving
BPDUs, loop guard puts the port into an inconsistent state until it starts receiving BPDUs again.
Loop guard isolates the failure and lets spanning tree converge to a stable topology without the
failed link or bridge.
You can enable loop guard per port with the set spantree guard loop command.
Note When you are in MST mode, you can set all the ports on a switch with the set spantree
global-defaults loop-guard command.
When you enable loop guard, it is automatically applied to all of the active instances or VLANs to
which that port belongs. When you disable loop guard, it is disabled for the specified ports.
Disabling loop guard moves all loop-inconsistent ports to the listening state.
If you enable loop guard on a channel and the first link becomes unidirectional, loop guard blocks
the entire channel until the affected port is removed from the channel. Figure 8-6 shows loop
guard in a triangle switch configuration.
Figure 8-6 Triangle Switch Configuration with Loop Guard
Figure 8-6 illustrates the following configuration:
Switches A and B are distribution switches.
Switch C is an access switch.
Loop guard is enabled on ports 3 /1 and 3 / 2 on Switches A, B, and C.
Use loop guard only in topologies where there are blocked ports. Topologies that have no blocked
ports, which are loop free, do not need to enable this feature. Enabling loop guard on a root switch
has no effect but provides protection when a root switch becomes a nonroot switch.
Follow these guidelines when using loop guard:
Do not enable loop guard on PortFast-enabled or dynamic VLAN ports.
Do not enable PortFast on loop guard-enabled ports.
Do not enable loop guard if root guard is enabled.
Do not enable loop guard on ports that are connected to a shared link.
Note: We recommend that you enable loop guard on root ports and alternate root ports on access
switches.
Loop guard interacts with other features as follows:
Loop guard does not affect the functionality of UplinkFast or BackboneFast.
Root guard forces a port to always be designated as the root port. Loop guard is effective only if
the port is a root port or an alternate port. Do not enable loop guard and root guard on a port at
the
same time.
PortFast transitions a port into a forwarding state immediately when a link is established. Because
a PortFast-enabled port will not be a root port or alternate port, loop guard and PortFast cannot be
configured on the same port. Assigning dynamic VLAN membership for the port requires that the
port is PortFast enabled. Do not configure a loop guard-enabled port with dynamic VLAN
membership. If your network has a type-inconsistent port or a PVID-inconsistent port, all BPDUs are
dropped until the misconfiguration is corrected. The port transitions out of the inconsistent state
after the message age expires. Loop guard ignores the message age expiration on type-inconsistent
ports and PVID-inconsistent ports. If the port is already blocked by loop guard, misconfigured BPDUs
that are received on the port make loop guard recover, but the port is moved into the
type-inconsistent state or PVID-inconsistent state.
In high-availability switch configurations, if a port is put into the blocked state by loop guard, it
remains blocked even after a switchover to the redundant supervisor engine. The newly activated
supervisor engine recovers the port only after receiving a BPDU on that port. Loop guard uses the
ports known to spanning tree. Loop guard can take advantage of logical ports provided by the Port
Aggregation Protocol (PAgP). However, to form a channel, all the physical ports grouped in the
channel must have compatible configurations. PAgP enforces uniform configurations of root guard
or loop guard on all the physical ports to form a channel. These caveats apply to loop guard:
-Spanning tree always chooses the first operational port in the channel to send the BPDUs. If that
link becomes unidirectional, loop guard blocks the channel, even if other links in the channel are
functioning properly. -If a set of ports that are already blocked by loop guard are grouped together
to form a channel, spanning tree loses all the state information for those ports and the new channel
port may obtain the forwarding state with a designated role. -If a channel is blocked by loop guard
and the channel breaks, spanning tree loses all the state information. The individual physical ports
may obtain the forwarding state with the designated role, even if one or more of the links that
formed the channel are unidirectional. You can enable UniDirectional Link Detection (UDLD) to help
isolate the link failure. A loop may occur until UDLD detects the failure, but loop guard will not be
able to detect it. Loop guard has no effect on a disabled spanning tree instance or a VLAN.
Reference:
http://www.cisco.com/en/US/docs/switches/lan/catalyst4000/ 8.2glx/configuration/guide/stp_enha.
html#wp1048163
2. Which command is used to enable EtherChannel hashing for Layer 3 IP and Layer 4 port-based
CEF?
A. mpls ip cef
B. port-channel ip cef
C. mpls ip port-channel cef
D. port-channel load balance
E. mpls ip load-balance
F. ip cef EtherChannel channel-id XOR L4
G. ip cef connection exchange
Answer: D
Cisco 350-001 赤本 350-001 学校 350-001
Explanation:
Port-channel load balance is normally used for enable etherchannel hashing for Layer 3 IP and Layer
4 port based CEF.
3. Which two options are contained in a VTP subset advertisement? (Choose two.)
A. followers field
B. MD5 digest
C. VLAN information
D. sequence number
Answer: C,D
Cisco 初心者 350-001 350-001 教科書 350-001 認定試験 350-001 認定資格
Explanation:
Subset Advertisements When you add, delete, or change a VLAN in a Catalyst, the server Catalyst
where the changes are made increments the configuration revision and issues a summary
advertisement. One or several subset advertisements follow the summary advertisement. A subset
advertisement contains a list of VLAN information.
If there are several VLANs, more than one subset advertisement can be required in order to
advertise all the VLANs.
Subset Advertisement Packet Format This formatted example shows that each VLAN information
field contains information for a different VLAN. It is ordered so that lowered-valued ISL VLAN IDs
occur first:
Most of the fields in this packet are easy to understand. These are two clarifications:
Code - The format for this is 0x02 for subset advertisement.
Sequence number - This is the sequence of the packet in the stream of packets that follow a
summary advertisement. The sequence starts with 1.
Advertisement Requests
A switch needs a VTP advertisement request in these situations:
The switch has been reset.
The VTP domain name has been changed.
The switch has received a VTP summary advertisement with a higher configuration revision than
its own.
Upon receipt of an advertisement request, a VTP device sends a summary advertisement. One or
more subset advertisements follow the summary advertisement. This is an example:
Code-The format for this is 0x03 for an advertisement request.
Start-Value-This is used in cases in which there are several subset advertisements. If the first (n)
subset advertisement has been received and the subsequent one (n+1) has not been received,
the Catalyst only requests advertisements from the (n+1)th one.
Reference
http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094c52.shtml
4. Which two statements are true about traffic shaping? (Choose two.)
A. Out-of-profile packets are queued.
B. It causes TCP retransmits.
C. Marking/remarking is not supported.
D. It does not respond to BECN and ForeSight Messages.
E. It uses a single/two-bucket mechanism for metering.
Answer: A,C
Cisco 教科書 350-001 内容 350-001 特典 350-001
Reference:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCwQFjAA&url=htt
p%3A%2F%2Fstaffweb.itsligo.ie%2Fstaff%2Fpflynn%2FTelecoms%25203%2FSlides%2FONT%2
520Mod%25204%2520Lesson%25207.ppt&ei=LoDIUfTTGtO3hAeQz4HQCA&usg=AFQjCNGY24
UkAfy8tKIHlzEm9gfoIjv6fg&sig2=t4UIzkZ12wnO2988dEDyug&bvm=bv.48293060,d.ZG4 (slide 6)
5. Which three options are features of VTP version 3? (Choose three.)
A. VTPv3 supports 8K VLANs.
B. VTPv3 supports private VLAN mapping.
C. VTPv3 allows for domain discovery.
D. VTPv3 uses a primary server concept to avoid configuration revision issues.
E. VTPv3 is not compatible with VTPv1 or VTPv2.
F. VTPv3 has a hidden password option.
Answer: B,D,F
Cisco 短期 350-001 問題集 350-001 認定資格 350-001 勉強法
Explanation:
Key Benefits of VTP Version 3 Much work has gone into improving the usability of VTP version 3 in
three major areas: The new version of VTP offers better administrative control over which device is
allowed to update other devices' view of the VLAN topology. The chance of unintended and
disruptive changes is significantly reduced, and availability is increased. The reduced risk of
unintended changes will ease the change process and help speed deployment. Functionality for the
VLAN environment has been significantly expanded. Two enhancements are most beneficial for
today's networks:
-
In addition to supporting the earlier ISL VLAN range from 1 to 1001, the new version supports
the whole IEEE 802.1Q VLAN range up to 4095.
-
In addition to supporting the concept of normal VLANs, VTP version 3 can transfer information
regarding Private VLAN (PVLAN) structures.
The third area of major improvement is support for databases other than VLAN (for example,
MST).
Brief Background on VTP Version 1 and VTP Version 2 VTP version 1 was developed when only 1k
VLANs where available for configuration. A tight internal coupling of the VLAN implementation, the
VLAN pruning feature, and the VTP function itself offered an efficient means of implementation. It
has proved in the field to reliably support Ethernet, Token Ring, and FDDI networks via VTP . The use
of consistent VLAN naming was a requirement for successful use of VMPS (Vlan Membership Policy
Server). VTP ensures the consistency of VLAN names across the VTP domain. Most VMPS
implementations are likely to be migrated to a newer, more flexible and feature-rich method. To add
support for Token Ring, VTP version 1 was enhanced and called VTP version 2. Certain other minor
changes and enhancements were also added at this time. The functional base in VTP version 3 is left
unchanged from VTP version 2, so backward compatibility is built in. It is possible, on a per link basis,
to automatically discover and support VTP version 2 devices.
VTP version 3 adds a number of enhancements to VTP version 1 and VTP version 2: Support for a
structured and secure VLAN environment (Private VLAN, or PVLAN) Support for up to 4k VLANs
Feature enhancement beyond support for a single database or VTP instance Protection from
unintended database overrides during insertion of new switches Option of clear text or hidden
password protection Configuration option on a per port base instead of only a global scheme
Optimized resource handling and more efficient transfer of information These new requirements
made a new code foundation necessary. The design goal was to make VTP version 3 a versatile
vehicle. This was not only for the task of transferring a VLAN DB but also for transferring other
databases-for example, the MST database.
Reference
http://www.cisco.com/en/US/prod/collateral/switches/ps5718 /ps708 /solution_guide_c78_508010.
ht ml
6. Which three options are considered in the spanning-tree decision process? (Choose three.)
A. lowest root bridge ID
B. lowest path cost to root bridge
C. lowest sender bridge ID
D. highest port ID
E. highest root bridge ID
F. highest path cost to root bridge
Answer: A,B,C
Cisco 350-001 過去問 350-001 350-001 認定試験
Explanation:
Configuration bridge protocol data units (BPDUs) are sent between switches for each port. Switches
use s four step process to save a copy of the best BPDU seen on every port. When a port receives a
better BPDU, it stops sending them. If the BPDUs stop arriving for 20 seconds (default), it begins
sending them again.
Step 1 Lowest Root Bridge ID (BID) Step 2 Lowest Path Cost to Root Bridge Step 3 Lowest Sender BID
Step 4 Lowest Port ID
Reference Cisco General Networking Theory Quick Reference Sheets
7. In 802.1s, how is the VLAN to instance mapping represented in the BPDU?
A. The VLAN to instance mapping is a normal 16-byte field in the MST BPDU.
B. The VLAN to instance mapping is a normal 12-byte field in the MST BPDU.
C. The VLAN to instance mapping is a 16-byte MD5 signature field in the MST BPDU.
D. The VLAN to instance mapping is a 12-byte MD5 signature field in the MST BPDU.
Answer: C
Cisco 講座 350-001 350-001 通信 350-001 模擬 350-001 特典 350-001 参考書
Explanation:
MST Configuration and MST Region Each switch running MST in the network has a single MST
configuration that consists of these three attributes:
1.An alphanumeric configuration name (32 bytes)
NO.2 A configuration revision number (two bytes)
NO.3 A 4096-element table that associates each of the potential 4096 VLANs supported on the chassis
to a given instance.
In order to be part of a common MST region, a group of switches must share the same configuration
attributes. It is up to the network administrator to properly propagate the configuration throughout
the region. Currently, this step is only possible by the means of the command line interface (CLI) or
through Simple Network Management Protocol (SNMP). Other methods can be envisioned, as the
IEEE specification does not explicitly mention how to accomplish that step. Note: If for any reason
two switches differ on one or more configuration attribute, the switches are part of different
regions. For more information refer to the Region Boundary section of this document.
Region Boundary In order to ensure consistent VLAN-to-instance mapping, it is necessary for the
protocol to be able to exactly identify the boundaries of the regions. For that purpose, the
characteristics of the region are included in the BPDUs. The exact VLANs-to-instance mapping is not
propagated in the BPDU, because the switches only need to know whether they are in the same
region as a neighbor. Therefore, only a digest of the VLANs-toinstance mapping table is sent, along
with the revision number and the name. Once a switch receives a BPDU, the switch extracts the
digest (a numerical value derived from the VLAN-to-instance mapping table through a mathematical
function) and compares this digest with its own computed digest. If the digests differ, the port on
which the BPDU was received is at the boundary of a region. In generic terms, a port is at the
boundary of a region if the designated bridge on its segment is in a different region or if it receives
legacy 802.1d BPDUs. In this diagram, the port on B1 is at the boundary of region A, whereas the
ports on B2 and B3 are internal to region B:
MST Instances
According to the IEEE 802.1s specification, an MST bridge must be able to handle at least these
two instances:
One Internal Spanning Tree (IST)
One or more Multiple Spanning Tree Instance(s) (MSTIs)
The terminology continues to evolve, as 802.1s is actually in a pre-standard phase. It is likely
these names will change in the final release of 802.1s. The Cisco implementation supports 16
instances: one IST (instance 0) and 15 MSTIs.
show vtp status
Cisco switches "show vtp status" Field Descriptions has a MD5 digest field that is a 16-byte
checksum of the
VTP configuration as shown below
Router# show vtp status
VTP Version: 3 (capable)
Configuration Revision: 1
Maximum VLANs supported locally: 1005
Number of existing VLANs: 37
VTP Operating Mode: Server
VTP Domain Name: [smartports]
VTP Pruning Mode: Disabled
VTP V2 Mode: Enabled VTP Traps Generation: Disabled MD5 digest : 0x26 0xEE 0x0D 0x84 0x73
0x0E 0x1B 0x69 Configuration last modified by 172.20.52.19 at 7-25-08 14:33:43 Local updater ID is
172.20.52.19 on interface Gi5 / 2 (first layer3 interface fou) VTP version running: 2
Reference
http://www.cisco.com/en/US/tech/tk389/tk621 /technologies_white_paper09186a0080094cfc.shtm
l http://www.cisco.com/en/US/docs/ios-xml/ios/lanswitch/command/lsw-cr-book.pdf
8. Which statement is true about TCN propagation?
A. The originator of the TCN immediately floods this information through the network.
B. The TCN propagation is a two step process.
C. A TCN is generated and sent to the root bridge.
D. The root bridge must flood this information throughout the network.
Answer: C
Cisco 教本 350-001 練習 350-001 350-001 問題集
Explanation:
Explanation New Topology Change Mechanisms When an 802.1D bridge detects a topology change,
it uses a reliable mechanism to first notify the root bridge. This is shown in this diagram:
Once the root bridge is aware of a change in the topology of the network, it sets the TC flag on the
BPDUs it sends out, which are then relayed to all the bridges in the network. When a bridge
receives a BPDU with the TC flag bit set, it reduces its bridging-table aging time to forward delay
seconds. This ensures a relatively quick flush of stale information. Refer to Understanding
Spanning-Tree Protocol Topology Changes for more information on this process. This topology
change mechanism is deeply remodeled in RSTP . Both the detection of a topology change and its
propagation through the network evolve.
Topology Change Detection
In RSTP , only non-edge ports that move to the forwarding state cause a topology change. This
means that a loss of connectivity is not considered as a topology change any more, contrary to
802.1D (that is, a port that moves to blocking no longer generates a TC). When a RSTP bridge
detects a topology change, these occur:
It starts the TC While timer with a value equal to twice the hello-time for all its non-edge
designated ports and its root port, if necessary.
It flushes the MAC addresses associated with all these ports.
Note: As long as the TC While timer runs on a port, the BPDUs sent out of that port have the TC
bit set.
BPDUs are also sent on the root port while the timer is active.
Topology Change Propagation
When a bridge receives a BPDU with the TC bit set from a neighbor, these occur:
It clears the MAC addresses learned on all its ports, except the one that receives the topology
change.
It starts the TC While timer and sends BPDUs with TC set on all its designated ports and root port
(RSTP no longer uses the specific TCN BPDU, unless a legacy bridge needs to be notified).
This way, the TCN floods very quickly across the whole network. The TC propagation is now a one
step process. In fact, the initiator of the topology change floods this information throughout the
network, as opposed to 802.1D where only the root did. This mechanism is much faster than the
802.1D equivalent. There is no need to wait for the root bridge to be notified and then maintain the
topology change state for the whole network for <max age plus forward delay> seconds.
In just a few seconds, or a small multiple of hello-times, most of the entries in the CAM tables of the
entire network (VLAN) flush. This approach results in potentially more temporary flooding, but on
the other hand it clears potential stale information that prevents rapid connectivity restitution.
Reference
http://www.cisco.com/en/US/tech/tk389/tk621 /technologies_white_paper09186a0080094cfa.shtm
l
9. While you are troubleshooting network performance issues, you notice that a switch is
periodically flooding all unicast traffic. Further investigation reveals that periodically the switch is
also having spikes in CPU utilization, causing the MAC address table to be flushed and relearned.
What is the most likely cause of this issue?
A. a routing protocol that is flooding updates
B. a flapping port that is generating BPDUs with the TCN bit set
C. STP is not running on the switch
D. a user that is downloading the output of the show-tech command
E. a corrupted switch CAM table
Answer: B
Cisco 試験 350-001 350-001 過去 350-001 過去問 350-001 クラムメディア 350-001 練習
Explanation:
Spanning-Tree Protocol Topology Changes Another common issue caused by flooding is
Spanning-Tree Protocol (STP) Topology Change Notification (TCN). TCN is designed to correct
forwarding tables after the forwarding topology has changed. This is necessary to avoid a
connectivity outage, as after a topology change some destinations previously accessible via
particular ports might become accessible via different ports. TCN operates by shortening the
forwarding table aging time, such that if the address is not relearned, it will age out and flooding
will occur. TCNs are triggered by a port that is transitioning to or from the forwarding state. After
the TCN, even if the particular destination MAC address has aged out, flooding should not happen
for long in most cases since the address will be relearned. The issue might arise when TCNs are
occurring repeatedly with short intervals. The switches will constantly be fast-aging their forwarding
tables so flooding will be nearly constant. Normally, a TCN is rare in a well-configured network.
When the port on a switch goes up or down, there is eventually a TCN once the STP state of the port
is changing to or from forwarding. When the port is flapping, repetitive TCNs and flooding occurs.
Ports with the STP portfast feature enabled will not cause TCNs when going to or from the
forwarding state. Configuration of portfast on all end-device ports (such as printers, PCs, servers,
and so on) should limit TCNs to a low amount. Refer to this document for more information on TCNs:
Understanding Spanning-Tree Protocol Topology Changes Note: In MSFC IOS, there is an
optimization that will trigger VLAN interfaces to repopulate their ARP tables when there is a TCN in
the respective VLAN. This limits flooding in case of TCNs, as there will be an ARP broadcast and the
host MAC address will be relearned as the hosts reply to ARP .
Reference
http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801d080
8. shtml
10. Your network is suffering from regular outages. After troubleshooting, you learn that the
transmit lead of a fiber uplink was damaged. Which two features can prevent the same issues in the
future? (Choose two.)
A. root guard
B. loop guard
C. BPDU guard
D. UDLD
E. BPDU skew detection
Answer: B,D
Cisco 350-001 問題 350-001
Explanation:
STP Loop Guard The STP loop guard feature provides additional protection against Layer 2
forwarding loops (STP loops). An STP loop is created when an STP blocking port in a redundant
topology erroneously transitions to the forwarding state. This usually happens because one of the
ports of a physically redundant topology (not necessarily the STP blocking port) no longer receives
STP BPDUs. In its operation, STP relies on continuous reception or transmission of BPDUs based on
the port role. The designated port transmits BPDUs, and the non-designated port receives BPDUs.
When one of the ports in a physically redundant topology no longer receives BPDUs, the STP
conceives that the topology is loop free. Eventually, the blocking port from the alternate or backup
port becomes designated and moves to a forwarding state. This situation creates a loop. The loop
guard feature makes additional checks. If BPDUs are not received on a non-designated port, and
loop guard is enabled, that port is moved into the STP loop-inconsistent blocking state, instead of
the listening / learning / forwarding state. Without the loop guard feature, the port assumes the
designated port role. The port moves to the STP forwarding state and creates a loop.
Loop Guard versus UDLD Loop guard and Unidirectional Link Detection (UDLD) functionality overlap,
partly in the sense that both protect against STP failures caused by unidirectional links. However,
these two features differ in functionality and how they approach the problem. This table describes
loop guard and UDLD functionality:
Based on the various design considerations, you can choose either UDLD or the loop guard feature.
In regards to STP , the most noticeable difference between the two features is the absence of
protection in UDLD against STP failures caused by problems in software. As a result, the designated
switch does not send BPDUs. However, this type of failure is (by an order of magnitude) more rare
than failures caused by unidirectional links. In return, UDLD might be more flexible in the case of
unidirectional links on EtherChannel. In this case, UDLD disables only failed links, and the channel
should remain functional with the links that remain. In such a failure, the loop guard puts it into
loop-inconsistent state in order to block the whole channel.
Additionally, loop guard does not work on shared links or in situations where the link has been
unidirectional since the link-up. In the last case, the port never receives BPDU and becomes
designated. Because this behavior could be normal, this particular case is not covered by loop guard.
UDLD provides protection against such a scenario. As described, the highest level of protection is
provided when you enable UDLD and loop guard.
Reference
http://www.cisco.com/en/US/tech/tk389/tk621 /technologies_tech_note09186a0080094640.
shtml#loop_guard_vs_uld
