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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.
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
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
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
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:
When you are troubleshooting duplex mismatches, which two errors are typically seen on the full-
duplex end? (Choose two.)
B. FCS errors
C. interface resets
D. late collisions
Refer to the exhibit.
What is the potential issue with this configuration?
A. There is no potential issue; OSPF will work fine in any condition.
B. Sub-optimal routing may occur since there is no area 1 adjacency between the ABRs.
C. This is a wrong OSPF configuration because all routers must be in area 0 only.
D. This is a wrong OSPF configuration because /30 requires 0.0.0.3 wild card.
Refer to the exhibit.
Which path is selected as best path?
A. path 1, because it is learned from IGP B.
path 1, because the metric is the lowest C.
path 2, because it is external
D. path 2, because it has the higher router ID
What action will a BGP route reflector take when it receives a prefix marked with the community
attribute NO ADVERTISE from a client peer?
A. It will advertise the prefix to all other client peers and non-client peers.
B. It will not advertise the prefix to EBGP peers.
C. It will only advertise the prefix to all other IBGP peers.
D. It will not advertise the prefix to any peers.
Latest 400-101 Dumps400-101 Practice Test400-101 Braindumps
Refer to the exhibit.
R1 is not learning about the 172.16.10.0 subnet from the BGP neighbor R2 (126.96.36.199).
What can be done so that R1 will learn about this network?
A. Disable auto-summary on R2.
B. Configure an explicit network command for the 172.16.10.0 subnet on R2.
C. Subnet information cannot be passed between IBGP peers.
D. Disable auto-summary on R1.
By default, BGP does not accept subnets redistributed from IGP. To advertise and carry subnet
routes in BGP, use an explicit network command or the no auto-summary command. If you disable
auto-summarization and have not entered a network command, you will not advertise network
routes for networks with subnet routes unless they contain a summary route.
Which two statements are correct about Nonstop Forwarding? (Choose two.)
A. It allows the standby RP to take control of the device after a hardware or software fault on the
B. It is a Layer 3 function that works with SSO to minimize the amount of time a network is
unavailable to users following a switchover.
C. It is supported by the implementation of EIGRP, OSPF, RIPv2, and BGP protocols.
D. It synchronizes startup configuration, startup variables, and running configuration.
E. The main objective of NSF is to continue forwarding IP packets following a switchover.
F. Layer 2 802.1w or 802.1s must be used, as 802.1d cannot process the Layer 2 changes.
G. Routing protocol tuning parameters must be the same as the NSF parameters, or failover will
Cisco Nonstop Forwarding (NSF) works with the Stateful Switchover (SSO) feature in Cisco IOS
software. NSF works with SSO to minimize the amount of time a network is unavailable to its
users following a switchover. The main objective of Cisco NSF is to continue forwarding IP
packets following a Route Processor (RP) switchover.
Refer to the exhibit.
Which interface(s) will show ip rpf 188.8.131.52 indicate as RPF interface(s)?
A. Ethernet 1/0
B. Ethernet 0/0
C. Both Ethernet 0/0 and Ethernet 1/0
D. RPF will fail
When troubleshooting multicast routing, the primary concern is the source address. Multicast has
a concept of Reverse Path Forwarding check (RPF check). When a multicast packet arrives on an
interface, the RPF process checks to ensure that this incoming interface is the outgoing interface
used by unicast routing to reach the source of the multicast packet. This RPF check process
prevents loops. Multicast routing does not forward a packet unless the source of the packet
passes a reverse path forwarding (RPF) check. Once a packet passes this RPF check, multicast
routing forwards the packet based only upon the destination address.
What is Phantom RP used for?
A. it is used for load balancing in bidirectional PIM
B. it is used for redundancy in bidirectional PIM
C. it is used for redundancy in PIM-SM
D. it is used for load balancing in PIM-SM
In Bidirectional PIM (Bidir-PIM), the RP does not have an actual protocol function. The RP acts as
a routing vector in which all the traffic converges. The RP can be configured as an address that is
not assigned to any particular device called a Phantom RP. This means that the RP address does
not need to reside on a physical router interface, but can just be an address in a subnet. The RP
can also be a physical router, but it is not necessary.
Refer to the exhibit.
Which two statements are correct? (Choose two.)
A. The hexadecimal value of the number of packets that hit the access list is 0x723E6E12.
B. The access list has logging enabled.
C. The packet was discarded.
D. The command ip access-list logging hash-generation is enabled.
E. The Telnet connection is successfully set up.
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