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Refer to the exhibit.
A small enterprise connects its office to two ISPs, using separate T1 links. A static route is used
for the default route, pointing to both interfaces with a different administrative distance, so that one
of the default routes is preferred.
Recently the primary link has been upgraded to a new 10 Mb/s Ethernet link.
After a few weeks, they experienced a failure. The link did not pass traffic, but the primary static
route remained active. They lost their Internet connectivity, even though the backup link was
Which two possible solutions can be implemented to avoid this situation in the future? (Choose
A. Implement HSRP link tracking on the branch router R1.
B. Use a track object with an IP SLA probe for the static route on R1.
C. Track the link state of the Ethernet link using a track object on R1.
D. Use a routing protocol between R1 and the upstream ISP.
Interface tracking allows you to specify another interface on the router for the HSRP process to
monitor in order to alter the HSRP priority for a given group.
If the specified interface\’s line protocol goes down, the HSRP priority of this router is reduced,
allowing another HSRP router with higher priority can become active (if it has preemption
To configure HSRP interface tracking, use the standby [group] track interface [priority] command.
When multiple tracked interfaces are down, the priority is reduced by a cumulative amount. If you
explicitly set the decrement value, then the value is decreased by that amount if that interface is
down, and decrements are cumulative. If you do not set an explicit decrement value, then the
value is decreased by 10 for each interface that goes down, and decrements are cumulative.
The following example uses the following configuration, with the default decrement value of 10.
Note: When an HSRP group number is not specified, the default group number is group 0.
ip address 10.1.1.1 255.255.255.0
standby ip 10.1.1.3
standby priority 110
standby track serial0
standby track serial1
The HSRP behavior with this configuration is:
0 interfaces down = no decrease (priority is 110)
1 interface down = decrease by 10 (priority becomes100)
2 interfaces down = decrease by 10 (priority becomes 90)
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:
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.
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
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
Cisco General Networking Theory Quick Reference Sheets
Which three combinations are valid LACP configurations that will set up a channel? (Choose
Which two options does Cisco PfR use to control the entrance link selection with inbound
optimization? (Choose two.)
A. Prepend extra AS hops to the BGP prefix.
B. Advertise more specific BGP prefixes (longer mask).
C. Add (prepend) one or more communities to the prefix that is advertised by BGP.
D. Have BGP dampen the prefix.
Explanation: PfR Entrance Link Selection Control Techniques
The PfR BGP inbound optimization feature introduced the ability to influence inbound traffic. A
network advertises reachability of its inside prefixes to the Internet using eBGP advertisements to
its ISPs. If the same prefix is advertised to more than one ISP, then the network is multihoming.
PfR BGP inbound optimization works best with multihomed networks, but it can also be used with
a network that has multiple connections to the same ISP. To implement BGP inbound
optimization, PfR manipulates eBGP advertisements to influence the best entrance selection for
traffic bound for inside prefixes. The benefit of implementing the best entrance selection is limited
to a network that has more than one ISP connection.
To enforce an entrance link selection, PfR offers the following methods:
BGP Autonomous System Number Prepend When an entrance link goes out-of-policy (OOP) due
to delay, or in images prior to Cisco IOS Releases 15.2(1) T1 and 15.1(2)S, and PfR selects a
best entrance for an inside prefix, extra autonomous system hops are prepended one at a time (up
to a maximum of six) to the inside prefix BGP advertisement over the other entrances. In Cisco
IOS Releases 15.2(1)T1, 15.1(2)S, and later releases, when an entrance link goes out-of policy
(OOP) due to unreachable or loss reasons, and PfR selects a best entrance for an inside prefix,
six extra autonomous system hops are prepended immediately to the inside prefix BGP
advertisement over the other entrances. The extra autonomous system hops on the other
entrances increase the probability that the best entrance will be used for the inside prefix. When
the entrance link is OOP due to unreachable or loss reasons, six extra autonomous system hops
are added immediately to allow the software to quickly move the traffic away from the old entrance
link. This is the default method PfR uses to control an inside prefix, and no user configuration is
BGP Autonomous System Number Community Prepend
When an entrance link goes out-of-policy (OOP) due to delay, or in images prior to Cisco IOS
(1)T1 and 15.1(2)S, and PfR selects a best entrance for an inside prefix, a BGP prepend
community is attached one at a time (up to a maximum of six) to the inside prefix BGP
advertisement from the network to another autonomous system such as an ISP. In Cisco IOS
Releases 15.2(1)T1, 15.1(2)S, and later releases, when an entrance link goes out-of-policy (OOP)
due to unreachable or loss reasons, and PfR selects a best entrance for an inside prefix, six BGP
prepend communities are attached to the inside prefix BGP advertisement. The BGP prepend
community will increase the number of autonomous system hops in the advertisement of the
inside prefix from the ISP to its peers. Autonomous system prepend BGP community is the
preferred method to be used for PfR BGP inbound optimization because there is no risk of the
local ISP filtering the extra autonomous system hops. There are some issues, for example, not all
ISPs support the BGP prepend community, ISP policies may ignore or modify the autonomous
system hops, and a transit ISP may filter the autonomous system path. If you use this method of
inbound optimization and a change is made to an autonomous system, you must issue an
outbound reconfiguration using the “clear ip bgp” command.
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.
What is the first thing that happens when IPv6 is enabled on an interface on a host?
A. A router solicitation is sent on that interface.
B. There is a duplicate address detection on the host interface.
C. The link local address is assigned on the host interface.
D. A neighbor redirect message is sent on the host interface.
Duplicate address detection (DAD) is used to verify that an IPv6 home address is unique on the
LAN before assigning the address to a physical interface (for example, QDIO). z/OS
Communications Server responds to other nodes doing DAD for IP addresses assigned to the
In order to maintain security, with which hop count are IPv6 neighbor discovery packets sent?
Which two multicast address ranges are assigned as source-specific multicast destination
addresses and are reserved for use by source-specific applications and protocols? (Choose two.)
Explanation: Source-specific multicast (SSM) is a method of delivering multicast packets in which
the only packets that are delivered to a receiver are those originating from a specific source
address requested by the receiver. By so limiting the source, SSM reduces demands on the
network and improves security.
SSM requires that the receiver specify the source address and explicitly excludes the use of the (*,
G) join for all multicast groups in RFC 3376, which is possible only in IPv4\’s IGMPv3 and IPv6\’s
Source-specific multicast is best understood in contrast to any-source multicast (ASM). In the
ASM service model a receiver expresses interest in traffic to a multicast address. The multicast
1. discover all multicast sources sending to that address, and
2. route data from all sources to all interested receivers.
This behavior is particularly well suited to groupware applications where
1. all participants in the group want to be aware of all other participants, and
2. the list of participants is not known in advance.
The source discovery burden on the network can become significant when the number of sources
In the SSM service model, in addition to the receiver expressing interest in traffic to a multicast
address, the receiver expresses interest in receiving traffic from only one specific source sending
to that multicast address.
This relieves the network of discovering many multicast sources and reduces the amount of
multicast routing information that the network must maintain.
SSM requires support in last-hop routers and in the receiver\’s operating system. SSM support is
not required in other network components, including routers and even the sending host. Interest in
multicast traffic from a specific source is conveyed from hosts to routers using IGMPv3 as
specified in RFC 4607.
SSM destination addresses must be in the ranges 126.96.36.199/8 for IPv4 or FF3x::/96 for IPv6.
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