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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 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.
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 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.
MST Configuration and MST Region
Each switch running MST in the network has a single MST configuration that consists of these
1. An alphanumeric configuration name (32 bytes)
2. A configuration revision number (two bytes)
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
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
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
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:
According to the IEEE 802.1s specification, an MST bridge must be able to handle at least these
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
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.
Refer to the exhibit.
After a link flap in the network, which two EIGRP neighbors will not be queried for alternative
paths? (Choose two.)
Both 192.168.3.7 and 192.168.3.8 are in an EIGRP Stub area
The Enhanced Interior Gateway Routing Protocol (EIGRP) Stub Routing feature improves network
stability, reduces resource utilization, and simplifies stub router configuration.
Stub routing is commonly used in a hub and spoke network topology. In a hub and spoke network,
one or more end (stub) networks are connected to a remote router (the spoke) that is connected to
one or more distribution routers (the hub). The remote router is adjacent only to one or more
distribution routers. The only route for IP traffic to follow into the remote router is through a
distribution router. This type of configuration is commonly used in WAN topologies where the
distribution router is directly connected to a WAN. The distribution router can be connected to
many more remote routers. Often, the distribution router will be connected to 100 or more remote
routers. In a hub and spoke topology, the remote router must forward all nonlocal traffic to a
distribution router, so it becomes unnecessary for the remote router to hold a complete routing
table. Generally, the distribution router need not send anything more than a default route to the
When using the EIGRP Stub Routing feature, you need to configure the distribution and remote
routers to use EIGRP, and to configure only the remote router as a stub. Only specified routes are
propagated from the remote (stub) router. The router responds to queries for summaries,
connected routes, redistributed static routes, external routes, and internal routes with the message
“inaccessible.” A router that is configured as a stub will send a special peer information packet to
all neighboring routers to report its status as a stub router. Any neighbor that receives a packet
informing it of the stub status will not query the stub router for any routes, and a router that has a
stub peer will not query that peer. The stub router will depend on the distribution router to send the
proper updates to all peers.
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
How is RPF used in multicast routing?
A. to prevent multicast packets from looping
B. to prevent PIM packets from looping
C. to instruct PIM where to send a (*, G) or (S, G) join message
D. to prevent multicast packets from looping and to instruct PIM where to send a (*, G) or (S, G)
Refer to the exhibit.
What does the incoming interface of the above (*, G) entry indicate?
A. the interface closest to the source, according to the unicast routing table
B. the interface where an IGMP join has been received
C. the interface with the highest IP address
D. the last interface to hear a PIM (*, G) join
E. the interface closest to the RP, according to the unicast routing table
A source tree is the simplest form of distribution tree. The source host of the multicast traffic is
located at the root of the tree, and the receivers are located at the ends of the branches. Multicast
traffic travels from the source host down the tree toward the receivers. The forwarding decision on
which interface a multicast packet should be transmitted out is based on the multicast forwarding
table. This table consists of a series of multicast state entries that are cached in the router. State
entries for a source tree use the notation (S, G) pronounced S comma G. The letters represents
the IP address of the source, and G represents the group address.
Shared trees differ from source trees in that the root of the tree is a common point somewhere in
This common point is referred to as the rendezvous point (RP). The RP is the point at which
receivers join to learn of active sources. Multicast sources must transmit their traffic to the RP.
When receivers join a multicast group on a shared tree, the root of the tree is always the RP, and
multicast traffic is transmitted from the RP down toward the receivers. Therefore, the RP acts as a
go-between for the sources and receivers. An RP can be the root for all multicast groups in the
network, or different ranges of multicast groups can be associated with different RPs.
Multicast forwarding entries for a shared tree use the notation (*, G), which is pronounced star
comma G. This is because all sources for a particular group share the same tree. (The multicast
groups go to the same RP.)
Therefore, the * or wildcard represents all sources.
Additional Information from Microsoft
Multicast traffic from source 184.108.40.206 (for example) uses the RPT, meaning the source sends it
to the RP rather than to the multicast group (the router would denote this by having a (*, G) entry
rather than a (S, G) entry). Before sending this traffic, Router 1 checks its unicast routing table to
see if packets from the RP are arriving on the correct interface. In this case they are, because they
arrive on interface I1, and the packets are forwarded.
Apart from interdomain multicast routing, what else is MSDP used for?
A. Source Specific Multicast and IGMPv2
B. Announcing multicast sources to BGP speakers
C. Anycast RP
D. Intradomain multicast routing
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