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Refer to the exhibit.
R1 has an EBGP session to ISP 1 and an EBGP session to ISP 2. R1 receives the same prefixes
through both links.
Which configuration should be applied so that the link between R1 and ISP 2 will be preferred for
outgoing traffic (R1 to ISP 2)?
A. Increase local preference on R1 for received routes
B. Decrease local preference on R1 for received routes
C. Increase MED on ISP 2 for received routes
D. Decrease MED on ISP 2 for received routes
Local preference is an indication to the AS about which path has preference to exit the AS in order
to reach a certain network. A path with higher local preference is preferred more. The default value
of preference is 100.
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 are effects of connecting a network segment that is running 802.1D to a network
segment that is running 802.1w? (Choose two.)
A. The entire network switches to 802.1D and generates BPDUs to determine root bridge status. B.
A migration delay of three seconds occurs when the port that is connected to the 802.1D bridge
C. The entire network reconverges and a unique root bridge for the 802.1D segment, and a root
bridge for the 802.1w segment, is chosen.
D. The first hop 802.1w switch that is connected to the 802.1D runs entirely in 802.1D compatibility
mode and converts the BPDUs to either 802.1D or 802.1w BPDUs to the 802.1D or 802.1w
segments of the network.
E. Classic 802.1D timers, such as forward delay and max-age, will only be used as a backup, and
will not be necessary if point-to-point links and edge ports are properly identified and set by the
Each port maintains a variable that defines the protocol to run on the corresponding segment. A
migration delay timer of three seconds also starts when the port comes up. When this timer runs,
the current STP or RSTP mode associated to the port is locked. As soon as the migration delay
expires, the port adapts to the mode that corresponds to the next BPDU it receives. If the port
changes its mode of operation as a result of a BPDU received, the migration delay restarts.
802.1D works by the concept that the protocol had to wait for the network to converge before it
transitioned a port into the forwarding state. With Rapid Spanning Tree it does not have to rely on
any timers, the only variables that that it relies on is edge ports and link types.
Any uplink port that has an alternate port to the root can be directly placed into the forwarding
state (This is the Rapid convergence that you speak of “restored quickly when RSTP is already in
use?”). This is what happened when you disconnected the primary look; the port that was ALT,
moved to FWD immediately, but the switch also still needs to create a BDU with the TC bit set to
notify the rest of the network that a topology has occurred and all non-edge designated ports will
transition to BLK, LRN, and then FWD to ensure there are no loops in the rest of the network. This
is why if you have a host on a switchport, and you know for a fact that it is only one host, enable
portfast to configure the port as an edgeport so that it does not have to transition to all the STP
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
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 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:
Refer to the exhibit.
R1 is not learning about the 172.16.10.0 subnet from the BGP neighbor R2 (184.108.40.206).
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 tunneling techniques support IPv6 multicasting? (Choose two.)
When IPv6 multicast is supported (over a 6to4 tunnel), an IPv6 multicast routing protocol must be
Restrictions for Implementing IPv6 Multicast
IPv6 multicast for Cisco IOS software uses MLD version 2. This version of MLD is fully backward-
compatible with MLD version 1 (described in RFC 2710). Hosts that support only MLD version 1
will interoperate with a router running MLD version 2. Mixed LANs with both MLD version 1 and
MLD version 2 hosts are likewise supported.
IPv6 multicast is supported only over IPv4 tunnels in Cisco IOS Release 12.3(2)T, Cisco IOS
(18)S, and Cisco IOS Release 12.0(26)S.
When the bidirectional (bidir) range is used in a network, all routers in that network must be able to
understand the bidirectional range in the bootstrap message (BSM).
IPv6 multicast routing is disabled by default when the ipv6 unicast-routing command is configured.
On Cisco Catalyst 6500 and Cisco 7600 series routers, the ipv6 multicast-routing also must be
enabled in order to use IPv6 unicast routing
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
For which routes does LDP advertise a label binding?
A. all routes in the routing table
B. only the IGP and BGP routes in the routing table
C. only the BGP routes in the routing table
D. only the IGP routes in the routing table
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