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Which two commands are required to enable multicast on a router, knowing that the receivers only
support IGMPv2? (Choose two.)
A. ip pim rp-address
B. ip pim ssm
C. ip pim sparse-mode
D. ip pim passive
Sparse mode logic (pull mode) is the opposite of Dense mode logic (push mode), in Dense mode
it is supposed that in every network there is someone who is requesting the multicast traffic so
PIM-DM routers begin by flooding the multicast traffic out of all their interfaces except those from
where a prune message is received to eliminate the
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 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.
New Topology Change Mechanisms
When an 802.1D bridge detects a topology change, it uses a reliable mechanism to first notify the
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
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
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 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.
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 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
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.
Refer to the exhibit.
Why is AS 65333 in parentheses?
A. It is an external AS.
B. It is a confederation AS.
C. It is the AS of a route reflector.
D. It is our own AS.
E. A route map has been applied to this route.
F. The BGP next hop is unreachable.
Which two orders in the BGP Best Path Selection process are correct? (Choose two.)
A. Higher local preference, then lowest MED, then eBGP over iBGP paths
B. Higher local preference, then highest weight, then lowest router ID
C. Highest weight, then higher local preference, then shortest AS path
D. Lowest origin type, then higher local preference, then lowest router ID
E. Highest weight, then higher local preference, then highest MED
In order to maintain security, with which hop count are IPv6 neighbor discovery packets sent?
Which command will define a VRF with name \’CCIE\’ in IPv6?
A. ip vrf CCIE
B. ipv6 vrf CCIE
C. vrf definition CCIE
D. ipv6 vrf definition CCIE
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