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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:
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
Which command is used to enable EtherChannel hashing for Layer 3 IP and Layer 4 port-based
A. mpls ip cef
B. port-channel ip cef
C. mpls ip port-channel cef
D. port-channel load balance
E. mpls ip load-balance
F. ip cef EtherChannel channel-id XOR L4
G. ip cef connection exchange
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
Which three combinations are valid LACP configurations that will set up a channel? (Choose
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.
R1 is not learning about the 172.16.10.0 subnet from the BGP neighbor R2 (220.127.116.11).
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
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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