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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.

Answer: C



New Topology Change Mechanisms

When an 802.1D bridge detects a topology change, it uses a reliable mechanism to first notify the

root bridge.

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

bit set.

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

comes up.

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


Answer: B,E


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

Answer: D



When you are troubleshooting duplex mismatches, which two errors are typically seen on the full-

duplex end? (Choose two.)

A. runts

B. FCS errors

C. interface resets

D. late collisions

Answer: A,B



Which three options are considered in the spanning-tree decision process? (Choose three.)

A. lowest root bridge ID

B. lowest path cost to root bridge

C. lowest sender bridge ID

D. highest port ID

E. highest root bridge ID

F. highest path cost to root bridge

Answer: A,B,C


Configuration bridge protocol data units (BPDUs) are sent between switches for each port.

Switches use s four step process to save a copy of the best BPDU seen on every port. When a

port receives a better BPDU, it stops sending them. If the BPDUs stop arriving for 20 seconds

(default), it begins sending them again.

Step 1 Lowest Root Bridge ID (BID)

Step 2 Lowest Path Cost to Root Bridge

Step 3 Lowest Sender BID

Step 4 Lowest Port ID


Cisco General Networking Theory Quick Reference Sheets

Latest 400-101 Dumps400-101 VCE Dumps400-101 Study Guide


Refer to the exhibit.

R1 is not learning about the subnet from the BGP neighbor R2 (

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 subnet on R2.

C. Subnet information cannot be passed between IBGP peers.

D. Disable auto-summary on R1.

Answer: B


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

Answer: B



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


How will EIGRPv6 react if there is an IPv6 subnet mask mismatch between the Global Unicast

addresses on a point-to-point link?

A. EIGRPv6 will form a neighbor relationship.

B. EIGRPv6 will not form a neighbor relationship.

C. EIGRPv6 will form a neighbor relationship, but with the log MSG: “EIGRPv6 neighbor not on a

common subnet.”

D. EIGRPv6 will form a neighbor relationship, but routes learned from that neighbor will not be

installed in the routing table.

Answer: A Explanation:



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)

join message

Answer: D



Refer to the exhibit.

Which interface(s) will show ip rpf indicate as RPF interface(s)?

A. Ethernet 1/0

B. Ethernet 0/0

C. Both Ethernet 0/0 and Ethernet 1/0

D. RPF will fail

Answer: A


When troubleshooting multicast routing, the primary concern is the source address. Multicast has

a concept of Reverse Path Forwarding check (RPF check). When a multicast packet arrives on an

interface, the RPF process checks to ensure that this incoming interface is the outgoing interface

used by unicast routing to reach the source of the multicast packet. This RPF check process

prevents loops. Multicast routing does not forward a packet unless the source of the packet

passes a reverse path forwarding (RPF) check. Once a packet passes this RPF check, multicast

routing forwards the packet based only upon the destination address.



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