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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.
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 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 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.
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.
R1 is not learning about the 172.16.10.0 subnet from the BGP neighbor R2 (22.214.171.124).
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.
What triggered the first SPF recalculation?
A. changes in a router LSA, subnet LSA, and external LSA
B. changes in a router LSA, summary network LSA, and external LSA
C. changes in a router LSA, summary network LSA, and summary ASBR LSA
D. changes in a router LSA, summary ASBR LSA, and external LSA
Is built around links, and any IP prefix change in an area will trigger a full SPF. It advertises IP
information in Router and Network LSAs. The routers thus, advertise both the IP prefix information
(or the connected subnet information) and topology information in the same LSAs. This implies
that if an IP address attached to an interface changes, OSPF routers would have to originate a
Router LSA or a Network LSA, which btw also carries the topology information. This would trigger
a full SPF on all routers in that area, since the same LSAs are flooded to convey topological
change information. This can be an issue with an access router or the one sitting at the edge,
since many stub links can change regularly.
Only changes in interarea, external and NSSA routes result in partial SPF calculation (since type
3, 4, 5 and 7 LSAs only advertise IP prefix information) and thus IS-IS
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
What is the flooding scope of an OSPFv3 LSA, if the value of the S2 bit is set to 1 and the S1 bit is
set to 0?
A. link local
B. area wide
C. AS wide
The Type 1 router LSA is now link local and the Type 2 Network LSA is AS Wide
S2 and S1 indicate the LSA\’s flooding scope. Table 9-1 shows the possible values of these two
bits and the associated flooding scopes.
Table 9-1 S bits in the OSPFv3 LSA Link State Type field and their associated flooding scopes
LSA Function Code, the last 13 bits of the LS Type field, corresponds to the OSPFv2 Type field.
Table 9-2 shows the common LSA types used by OSPFv3 and the values of their corresponding
LS Types. If you decode the hex values, you will see that the default U bit of all of them is 0. The S
bits of all LSAs except two indicate area scope. Of the remaining two, AS External LSAs have an
AS flooding scope and Link LSAs have a linklocal flooding scope. Most of the OSPFv3 LSAs have
functional counterparts in OSPFv2; these OSPFv2 LSAs and their types are also shown in Table
Table 9-2 OSPFv3 LSA types and their OSPFv2 counterparts
Which two OSPF LSA types are new in OSPF version 3? (Choose two.)
B. NSSA external
C. Network link
D. Intra-area prefix
E. AS domain
New LSA Types
OSPFv3 carries over the seven basic LSA types we\’re familiar with from OSPFv2. However, the
type 1 and 2 LSAs have been re-purposed, as will be discussed in a bit. OSPFv3 also introduces
two new LSA types: Link and Intra-area Prefix.
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