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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.
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 options are contained in a VTP subset advertisement? (Choose two.)
A. followers field
B. MD5 digest
C. VLAN information
D. sequence number
When you add, delete, or change a VLAN in a Catalyst, the server Catalyst where the changes are
made increments the configuration revision and issues a summary advertisement. One or several
subset advertisements follow the summary advertisement. A subset advertisement contains a list
of VLAN information.
If there are several VLANs, more than one subset advertisement can be required in order to
advertise all the VLANs.
Subset Advertisement Packet Format
This formatted example shows that each VLAN information field contains information for a different
VLAN. It is ordered so that lowered-valued ISL VLAN IDs occur first:
Most of the fields in this packet are easy to understand. These are two clarifications:
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 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.
A packet from RTD with destination RTG, is reaching RTB. What is the path this packet will take
from RTB to reach RTG?
A. RTB – RTA – RTG
B. RTB – RTD – RTC – RTA – RTG
C. RTB – RTF – RTE – RTA – RTG
D. RTB will not be able to reach RTG since the OSPF configuration is wrong.
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.
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
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