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QUESTION 211
What is the default maximum reservable bandwidth (percentage) by any single flow on an
interface after enabling RSVP?
A. 75 percent
B. 60 percent
C. 56 percent
D. 50 percent
E. 25 percent
Answer: A
Explanation:
You must plan carefully to successfully configure and use RSVP on your network. At a minimum, RSVP must reflect your assessment of bandwidth needs on router interfaces. Consider the following questions as you plan for RSVP configuration:
How much bandwidth should RSVP allow per end-user application flow? You must understand the “feeds and speeds” of your applications. By default, the amount reservable by a single flow can be the entire reservable bandwidth. You can, however, limit individual reservations to smaller amounts using the single flow bandwidth parameter. This value may not exceed the interface reservable amount, and no one flow may reserve more than the amount specified. How much bandwidth is available for RSVP? By default, 75 percent of the bandwidth available on an interface is reservable. If you are using a tunnel interface, RSVP can make a reservation for the tunnel whose bandwidth is the sum of the bandwidths reserved within the tunnel. How much bandwidth must be excluded from RSVP so that it can fairly provide the timely service required by low-volume data conversations? End-to-end controls for data traffic assumes that all sessions will behave so as to avoid congestion dynamically. Real-time demands do not follow this behavior. Determine the bandwidth to set aside so bursty data traffic will not be deprived as a side effect of the RSVP QOS configuration.
QUESTION 212
Which two protocols can have their headers compressed through MQC? (Choose two.)
A. RTP
B. RTSP
C. HTTP
D. TCP
E. UDP
Answer: AD
Explanation:
RTP or TCP IP header compression is a mechanism that compresses the IP header in a data packet before the packet is transmitted. Header compression reduces network overhead and speeds up transmission of RTP and TCP packets.
Cisco IOS software provides a related feature called Express RTP/TCP Header Compression. Before this feature was available, if compression of TCP or RTP headers was enabled, compression was performed in the process-switching path. Compression performed in this manner meant that packets traversing interfaces that had TCP or RTP header compression enabled were queued and passed up the process to be switched. This procedure slowed down transmission of the packet, and therefore some users preferred to fast-switch uncompressed TCP and RTP packets. Now, if TCP or RTP header compression is enabled, it occurs by default in the fast-switched path or the Cisco Express Forwarding-switched (CEF-switched) path, depending on which switching method is enabled on the interface. Furthermore, the number of TCP and RTP header compression connections was increased.
If neither fast-switching nor CEF-switching is enabled, then if TCP or RTP header compression is enabled, it will occur in the process-switched path as before. The Express RTP and TCP Header Compression feature has the following benefits:
1. It reduces network overhead.
2. It speeds up transmission of TCP and RTP packets. The faster speed provides a greater benefit on slower links than faster links.
QUESTION 213
You have a router running BGP for the MPLS network and OSPF for the local LAN network at the
sales office. A route is being learned from the MPLS network that also exists on the OSPF local
network. It is important that the router chooses the local LAN route being learned from the
downstream switch running OSPF rather than the upstream BGP neighbor. Also, if the local OSPF
route goes away, the BGP route needs to be used. What should be configured to make sure that
the router will choose the LAN network as the preferred path?
A. static route needs to be added
B. floating static route needs to be added
C. bgp backdoor command
D. ospf backdoor command
Answer: C
Explanation:
Congestion control
The Frame Relay network uses a simplified protocol at each switching node. It achieves simplicity by omitting link-by-link flow-control. As a result, the offered load has largely determined the performance of Frame Relay networks. When offered load is high, due to the bursts in some services, temporary overload at some Frame Relay nodes causes a collapse in network throughput. Therefore, frame-relay networks require some effective mechanisms to control the congestion. Congestion control in frame-relay networks includes the following elements:
Admission Control provides the principal mechanism used in Frame Relay to ensure the guarantee of resource requirement once accepted. It also serves generally to achieve high network performance. The network decides whether to accept a new connection request, based on the relation of the requested traffic descriptor and the network’s residual capacity. The traffic descriptor consists of a set of parameters communicated to the switching nodes at call set-up time or at service-subscription time, and which characterizes the connection’s statistical properties.
The traffic descriptor consists of three elements:
Committed Information Rate (CIR) – The average rate (in bit/s) at which the network guarantees to transfer information units over a measurement interval T. This T interval is defined as: T = Bc/CIR. Committed Burst Size (BC) – The maximum number of information units transmittable during the interval T. Excess Burst Size (BE) – The maximum number of uncommitted information units (in bits) that the network will attempt to carry during the interval.
Once the network has established a connection, the edge node of the Frame Relay network must monitor the connection’s traffic flow to ensure that the actual usage of network resources does not exceed this specification. Frame Relay defines some restrictions on the user’s information rate. It allows the network to enforce the end user’s information rate and discard information when the subscribed access rate is exceeded.
Explicit congestion notification is proposed as the congestion avoidance policy. It tries to keep the network operating at its desired equilibrium point so that a certain Quality of Service (QoS) for the network can be met. To do so, special congestion control bits have been incorporated into the address field of the Frame Relay:
FECN and BECN. The basic idea is to avoid data accumulation inside the network. FECN means Forward Explicit Congestion Notification. The FECN bit can be set to 1 to indicate that congestion was experienced in the direction of the frame transmission, so it informs the destination that congestion has occurred. BECN means Backwards Explicit Congestion Notification. The BECN bit can be set to 1 to indicate that congestion was experienced in the network in the direction opposite of the frame transmission, so it informs the sender that congestion has occurred.
QUESTION 214
In BGP routing, what does the rule of synchronization mean?
A. A BGP router can only advertise an EBGP learned route, provided that the route is an IGP route in
the routing table.
B. A BGP router can only advertise an IBGP learned route, provided that the route is an IGP route in
the routing table.
C. A BGP router can only advertise an IBGP learned route, provided that the route is an IGP route
that is not in the routing table.
D. A BGP router can only advertise an EBGP learned route, provided that the route is a metric of 0 in
the BGP table.
Answer: B
Explanation:
When an AS provides transit service to other ASs and if there are non-BGP routers in the AS, transit traffic might be dropped if the intermediate non-BGP routers have not learned routes for that traffic via an IGP. The BGP synchronization rule states that if an AS provides transit service to another AS, BGP should not advertise a route until all of the routers within the AS have learned about the route via an IGP. The topology shown in demonstrates the synchronization rule
QUESTION 215
Router 1 is configured for BGP as dual-homed on the Cisco network. Which three BGP attributes
are carried in every BGP update on this router (both IBGP and EBGP)? (Choose three.)
A. origin
B. router-ID
C. AS-path
D. local-preference
E. next-hop
Answer: ACE
Explanation:
There are basically two major types of attribute:
Well Known.
Optional
Well Known:
Well known attributes are must be recognized by each compliant of BGP implementations. Well known attributes are propagated to other neighbors also. Further divided into:
1. Mandatory: It is BGP well known attributes. Mandatory attributes are must be present in all update message passed between BGP peers. It is present in route description. Must be supported and propagated.
2. Discretionary: It is BGP well known attributes. Discretionary attributes may be present on update message.
Must be supported; propagation optional.
Optional:
Optional attributes are recognized by some implementation of BGP & expected that not recognized by everyone. Optional attributes are propagated to their neighbors based on the meanings.
Further divided into:
1. Transitive: Optional transitive attributes don’t have to be supported, but must be passed onto peers. Marked as partial if unsupported by neighbor
2. Non Transitive: Optional non-transitive attributes don’t have to be supported, and can be ignored.
Deleted if unsupported by neighbor
BGP attributes:
1. Weight (Attribute Type Mandatory):
Weight is a Cisco-defined attribute that is local to a router. The weight attribute is not advertised to neighboring routers. If the router learns about more than one route to the same destination, the route with the highest weight is preferred.
2. Local preference (Attribute Type Discretionary):
The local preference attribute is used to prefer an exit point from the local autonomous system. Unlike the weight attribute, the local preference attribute is propagated throughout the local AS. If there are multiple exit points from the AS, the local preference attribute is used to select the exit point for a specific route.
3. AS path (Attribute Type Mandatory):
When a route advertisement passes through an autonomous system, the AS number is added to an ordered list of AS numbers that the route advertisement has traversed.
4. Origin:
The origin attribute indicates how BGP learned about a particular route. The origin attribute can have one of three possible values:
a. IGP The route is interior to the originating AS. This value is set when the network router configuration command is used to inject the route into BGP. b. EGP -The route is learned via the Exterior Gateway Protocol (EGP).
c. Incomplete
The origin of the route is unknown or is learned some other way. An origin of Incomplete occurs when a route is redistributed into BGP.
5. Multi-exit discriminator (Attribute Type – Non Transitive):
The multi-exit discriminator (MED) or metric attribute is used as a suggestion to an external AS regarding the preferred route into the AS that is advertising the metric.
6. Next-hop (Attribute Type Mandatory):
The EBGP next-hop attribute is the IP address that is used to reach the advertising router. For EBGP peers, the next-hop address is the IP address of the connection between the peers.
7. Community (Attribute Type – Transitive):
The community attribute provides a way of grouping destinations, called communities, to which routing decisions (such as acceptance, preference, and redistribution) can be applied. Route maps are used to set the community attribute. The predefined community attributes are as follows:
a. No-export: Do not advertise this route to EBGP peers. b. No-advertise: Do not advertise this route to any peer. c. Internet: Advertise this route to the Internet community; all routers in the network belong to it.
8. Atomic Aggregate (Attribute Type – Discretionary):
Notes that route summarization has been performed.
9. Aggregator (Attribute Type – Transitive):
Identifies the router and AS where summarization was performed.
10. Originator ID (Attribute Type – Non Transitive): Identifies a route reflector.
11. Cluster List (Attribute Type – Non Transitive):
Records the route reflector clusters the route has traversed.
QUESTION 216
In your Cisco EIGRP network, you notice that the neighbor relationship between two of your
routers was recently restarted. Which two of these choices could have made this occur? (Choose
two.)
A. An update packet with init flag set from a known, already established neighbor relationship was
received by one of the routers.
B. The ARP cache was cleared.
C. The counters were cleared.
D. The IP EIGRP neighbor relationship was cleared manually.
Answer: AD
Explanation:
The following are the most common causes of problems with EIGRP neighbor relationships:
Unidirectional link
Uncommon subnet, primary, and secondary address mismatch Mismatched masks
K value mismatches
Mismatched AS numbers
Stuck in active
Layer 2 problem
Access list denying multicast packets
Manual change (summary router, metric change, route filter) According till Ivan Pepelnjak’s book “EIGRP Network Design Solutions” the Init flag is set in the initial update packet when to neighbors discover each other and start their initial topology table exchange. There are two basic purposes for the Init flag. First, it’s a part of the three way handshake that eigrp uses when building an adjacency.
5. Router B comes up on a wire.
6. Router A receives Router B’s hello, and places it in “pending” state. This is a not completely formed adjacency; as long as B is in this state, A won’t send any routing information to it.
7. Router A sends an empty unicast update with the Init bit set.
8. Router B receives this update with the Init bit set, and places Router A in the “pending” state.
9. Router B now transmits an empty update with the Init bit set, unicast, to A. This empty update also contains the acknowledgement for Router A’s Init update (that this ack is piggybacked is an integral part of the three way handshake process).
10.Router A, on receiving this Init update, places Router B in the “neighbor” state, and sends an acknowledgement for the Init update from Router B.
11.Router B receives this ack, and places A in “neighbor” state. The two routers can now exchange routing information, knowing they have full two way connectivity between them. The second use of the Init bit is more esoteric. Suppose you have Routers A and B, running along fine, for many hours. Router A reloads, but comes back up before Router B’s hold timer has expired. When Router B sees A’s hellos, it will assume that A just missed a couple, and everything is fine. But everything isn’t fine-A just lost all of its routing information! How can A signal this state, and as B to resynchronize? A can send an empty update, with the Init bit set. This causes Router B to place A in the “pending” state, and wipe out all the information it’s learned from A (unless, of course, graceful restart is configured/etc.).
QUESTION 217
Your Cisco network currently runs OSPF and you have a need to policy-route some specific traffic, regardless of what the routing table shows. Which one of these options would enable you to
policy-route the traffic?
A. source IP address and the protocol (such as SSL, HTTPS, SSH)
B. the packet Time to Live and the source IP address
C. type of service header and DSCP value
D. destination IP address
Answer: A
Explanation:
Policy-based routing (PBR) provides a mechanism for expressing and implementing forwarding/routing of data packets based on the policies defined by the network administrators. It provides a more flexible mechanism for routing packets through routers, complementing the existing mechanism provided by routing protocols. Routers forward packets to the destination addresses based on information from static routes or dynamic routing protocols such as Routing Information Protocol (RIP), Open Shortest Path First (OSPF), or Enhanced Interior Gateway Routing Protocol (Enhanced IGRP). Instead of routing by the destination address, policybased routing allows network administrators to determine and implement routing policies to allow or deny paths based on the following:
Identity of a particular end system
Application
Protocol
Size of packets
QUESTION 218
You use OSPF as your network routing protocol. You use the command show ip route and you
see several routes described as O, O IA, O E1, and O E2. What routes are in your area?
A. O IA
B. O E1
C. O E2
D. O
Answer: D
Explanation:
Depending on the point where a network is sourced, there are various types of routes that could be present in an OSPF domain. When there are multiple routes to a particular network in a OSPF domain, the type of the route influences the route that is selected and installed by the router in the routing table. In OSPF, routes that are learned by a router from OSPF sources within the same area are known as intra-area routes. Routes that originate from an OSPF router in a different area are considered as inter-area routes. Certain networks could belong to a domain outside OSPF, which could then be redistributed into the OSPF by an Autonomous System Boundary Router (ASBR). Such routes are considered external routes. They can be further divided into external type-1 or external type-2 routes, depending on how they are advertised while being redistributing on the ASBR. The difference between these two types is the way in which the metric for the route is calculated.
OSPF-running routers use these criteria to select the best route to be installed in the routing table:
1. Intra-area routes.
2. Inter-area routes.
3. External Type-1 routes.
4. External Type-2 routes.
a. If there are multiple routes to a network with the same route type, the OSPF metric calculated as cost based on the bandwidth is used for selecting the best route. The route with the lowest value for cost is chosen as the best route.
b. If there are multiple routes to a network with the same route type and cost, it chooses all the routes to be installed in the routing table, and the router does equal cost load balancing across multiple paths.
QUESTION 219
What are the mandatory, well-known BGP attributes?
A. origin, AS-path, next-hop
B. AS-path, origin, MED
C. AS-path, origin, weight
D. AS-path, weight, MED
Answer: A
Explanation:
BGP Path Attributes
Mandatory Well-Known Attributes
Origin: Specifies the router’s origin
IGP
EGP
Unknown — Route was redistributed
AS-Path: Sequence of AS numbers through which the route is accessible Next-Hop: IP address of the next-hop router
Discretionary Well-Known Attributes
Local P
Used for consistent routing policy with an AS
Atomic Aggregate: Informs the neighbor AS that the originating router aggregated routes Nontransitive Attributes
Multiexit Discriminator: Used to discriminate between multiple entry points into an AS Transitive Attributes
Aggregator: IP address and AS of the router that performed aggregation Community: Used for route tagging
QUESTION 220
Network A has a spanning-tree problem in which the traffic is selecting a longer path. How is the path cost calculated?
A. number of hops
B. priority of the bridge
C. interface bandwidth
D. interface delay
E. None of the above
Answer: C
Explanation:
STP Path Cost Automatically Changes When a Port Speed/Duplex Is Changed STP calculates the path cost based on the media speed (bandwidth) of the links between switches and the port cost of each port forwarding frame. Spanning tree selects the root port based on the path cost. The port with the lowest path cost to the root bridge becomes the root port. The root port is always in the forwarding state.
If the speed/duplex of the port is changed, spanning tree recalculates the path cost automatically. A change in the path cost can change the spanning tree topology.
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