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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.
Refer to the exhibit.
Which statement is correct about the prefix 184.108.40.206/8?
A. The prefix has encountered a routing loop.
B. The prefix is an aggregate with an as-set.
C. The prefix has been aggregated twice, once in AS 100 and once in AS 200.
D. None of these statements is true.
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.
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.
What is the first thing that happens when IPv6 is enabled on an interface on a host?
A. A router solicitation is sent on that interface.
B. There is a duplicate address detection on the host interface.
C. The link local address is assigned on the host interface.
D. A neighbor redirect message is sent on the host interface.
Duplicate address detection (DAD) is used to verify that an IPv6 home address is unique on the
LAN before assigning the address to a physical interface (for example, QDIO). z/OS
Communications Server responds to other nodes doing DAD for IP addresses assigned to the
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.
In order to maintain security, with which hop count are IPv6 neighbor discovery packets sent?
Refer to the exhibit.
This is an MPLS VPN network with OSPF as the PE-CE routing protocol. Which statement is
A. The routing inside the VPN RED will never work correctly.
B. The routing inside the VPN RED can be enabled by configuring virtual links between the PE
C. The routing inside the VPN RED can be enabled by configuring area 0 inside the VRF on the
D. The routing inside the VPN RED will work without any special OSPF configuration.
E. The routing inside the VPN RED will work if the PE routers have a full mesh of sham-links
configured for VRF RED.
Answer: D Explanation:
Which three fields are optional in an OSPFv3 external LSA? (Choose three.)
A. Forwarding Address
B. External Route
C. Reference Link-State ID
E. Prefix Options
As with OSPFv2, the AS-External LSA advertises prefixes external to the OSPF routing domain;
one LSA is required for each external prefix advertised. However, the format of the OSPFv3 As-
External LSA (Figure 9-10) is different from its OSPFv2 counterpart.
Figure 9-10. OSPFv3 AS-External LSA
http://fengnet.com/book/CCIE Professional Development Routing TCPIP Volu
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