2014 Latest Cisco 350-001 Dump Free Download(81-90)!
QUESTION 81
Spanning Tree Protocol calculates path cost based on which of these?
A. interface bandwidth
B. interface delay
C. interface bandwidth and delay
D. hop count
E. bridge priority
Answer: A
Explanation:
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.
Data rate and STP path cost
The table below shows the default cost of an interface for a given data rate.
QUESTION 82
What two features in Cisco switches help prevent Layer 2 loops? (Choose two.)
A. UniDirectional Link Detection
B. Hot Standby Router Protocol
C. Virtual Router Redundancy Protocol
D. PortFast
E. root guard
F. loop guard
Answer: AF
Explanation:
The STP loop guard feature provides additional protection against Layer 2 forwarding loops (STP loops). An STP loop is created when an STP blocking port in a redundant topology erroneously transitions to the forwarding state. This usually happens because one of the ports of a physically redundant topology (not necessarily the STP blocking port) no longer receives STP BPDUs. In its
operation, STP relies on continuous reception or transmission of BPDUs based on the port role. The designated port transmits BPDUs, and the non-designated port receives BPDUs.
QUESTION 83
In PIM-SM what control plane signaling must a multicast source perform before it begins to send
multicast traffic to a group?
A. The source must send a PIM Register message to the rendezvous point (RP).
B. The source must first join the multicast group using IGMP before sending.
C. The source must perform a Request to Send (RTS) and Clear to Send (CTS) handshake with the
PIM designated router (DR).
D. No control plane signaling needs to be performed; the source can simply begin sending on the
local subnet.
Answer: D
Explanation:
The most common type of multicast issue is the RPF Failure. RPF checks are used both at the control and data plane of multicast routing. Control plane involves PIM signaling some PIM messages are subject to RPF checks. For example, PIM (*,G) Joins are sent toward the shortest path to RP. Next, the BSR/RP address in the BSR messages is subject to RPF check as well. Notice that this logic does not apply to PIM Register messages the unicast register packet may arrive on any interface. However, RPF check is performed on the encapsulated multicast source to construct the SPT toward the multicast source.
Data plane RPF checks are performed every time a multicast data packet is received for forwarding. The source IP address in the packet should be reachable via the receiving interface, or the packet is going to be dropped. Theoretically, with PIM Sparse-Mode RPF checks at the control plane level should preclude and eliminate the data-plane RPF failures, but data-plane RPF failures are common during the moments of IGP reconvergence and on multipoint non-broadcast interfaces. PIM Dense Mode is different from SM in the sense that data-plane operations preclude control- plane signaling. One typical irresolvable RPF problem with PIM Dense mode is known as split-horizon forwarding, where packet received on one interface, should be forwarded back out of the same interface in the hub-and-spoke topology. The same problem may occur with PIM Sparse mode, but this type of signaling allows for treating the NBMA interface as a collection of point-to-point links by the virtue of PIM NBMA mode.
QUESTION 84
Which of these statements about PIM join messages in classic PIM-SM is correct?
A. PIM join messages are sent every 60 seconds to refresh the upstream router’s mroute state for
the multicast tree.
B. Routers send a PIM join acknowledgement in response to each PIM join message received from a
downstream router.
C. PIM join messages are only sent when the multicast distribution tree is first being established.
D. PIM join messages are sent every three minutes to refresh the upstream router’s mroute state for
the multicast tree.
Answer: A
Explanation:
PIM Sparse Mode uses an explicit request approach, where a router has to ask for the multicast feed with a PIM Join message. PIM Sparse Mode is indicated when you need more precise control, especially when you have large volumes of IP multicast traffic compared to your bandwidth. PIM Sparse Mode scales rather well, because packets only go where they are needed, and because it creates state in routers only as needed. There can be different RP’s for different multicast groups, which is one way to spread the load. There is usually one RP per multicast group. Redundancy of RP’s is an advanced topic, and requires a little deeper expertise. One way to do this is with the MSDP protocol (possible later article in the series). PIM Join message is sent towards a Source (or for PIM-SM, possibly towards an RP), based on unicast routing. The Join message says in effect “we need a copy of the multicasts over here”. It connects the sender of the Join and intervening routers to any existing multicast tree, all the way back to the target of the Join if necessary. A Prune message says in effect “we no longer need this over here”. A router receiving a Prune sees whether it has any other interfaces requiring the multicast flow, and if not, sends its own Prune message.
One advanced technique is to arrange a separate and perhaps different copy of the unicast routing information just for multicast purposes. This allows “steering” of the Join messages. Multiprotocol BGP, MBGP, for multicast, is one way to do this
All PIM-SM-enabled routers should be configured with the same message interval time. A router will be pruned from a group if a Join message is not received in the message interval. The default value is three minutes.
http://ciscoarticles.com/Cisco-Multicast-Routing-and-Switching/PIM-SM-Version-2-RP-Selection.html
QUESTION 85
The ip pim autorp listener command is used to do which of these?
A. enable a Cisco router to “passively” listen to Auto-RP packets without the router actively sending or
forwarding any of the packets
B. allow Auto-RP packets in groups 224.0.1.39 and 224.0.1.40 to be flooded in dense mode out
interfaces configured with the ip pim sparse-mode command
C. enable the use of Auto-RP on a router
D. configure the router as an Auto-RP mapping agent
Answer: B
Explanation:
The IP Pim autorp listener allows the Group 224.0.0.39 & 224.0.0.40 to be dense flooded. As the RP announces 224.0.0.39 to the mapping agent and the mapping agent announces 224.0.0.40 to all routers part of the group. where it can be applied:
It can be applied when dense mode is not configured, for example if you have Sparse-mode Multicast Network, and you need not to statically define your RP or use Autorp.
QUESTION 86
In order to configure two routers as anycast RPs, which of these requirements, at a minimum,
must be satisfied?
A. Multicast Source Discovery Protocol mesh-groups must be configured between the two anycast
RPs.
B. The RPs must be within the same IGP domain.
C. Multicast Source Discovery Protocol must be configured between the two anycast RPs.
D. The two anycast RPs must be IBGP peers.
Answer: C
Multicast Source Discovery Protocol (MSDP) is a mechanism to connect multiple PIM sparse-mode (SM) domains. MSDP allows multicast sources for a group to be known to all rendezvous point(s) (RPs) in different domains. Each PIM-SM domain uses its own RPs and need not depend on RPs in other domains. An RP runs MSDP over TCP to discover multicast sources in other domains. An RP in a PIM-SM domain has an MSDP peering relationship with MSDP-enabled routers in another domain. The peering relationship occurs over a TCP connection, where primarily a list of sources sending to multicast groups is exchanged. The TCP connections between RPs are achieved by the underlying routing system. The receiving RP uses the source lists to establish a source path. The purpose of this topology is to have domains discover multicast sources in other domains. If the multicast sources are of interest to a domain that has
receivers, multicast data is delivered over the normal, source-tree building mechanism in PIM-SM. MSDP is also used to announce sources sending to a group. These announcements must originate at the domain’s RP.
MSDP depends heavily on (M)BGP for interdomain operation. It is recommended that you run MSDP in RPs in your domain that are RPs for sources sending to global groups to be announced to the internet.
Each MSDP peer receives and forwards the SA message away from the originating RP to achieve “peer- RPF flooding.” The concept of peer-RPF flooding is with respect to forwarding SA messages. The router examines the BGP or MBGP routing table to determine which peer is the next hop toward the originating RP of the SA message. Such a peer is called an “RPF peer” (Reverse-Path Forwarding peer). The router forwards the message to all MSDP peers other than the RPF peer. If the MSDP peer receives the same SA message from a non-RPF peer toward the originating RP, it drops the message. Otherwise, it forwards the message on to all its MSDP peers. When an RP for a domain receives an SA message from an MSDP peer, it determines if it has any group members interested in the group the SA message describes. If the (*,G) entry exists with a nonempty outgoing interface list, the domain is interested in the group, and the RP triggers an (S,G) join toward the source.
QUESTION 87
Which two of these statements correctly describe classic PIM-SM? (Choose two.)
A. The IOS default is for a last-hop router to trigger a switch to the shortest path tree as soon as a new
source is detected on the shared tree.
B. The IOS default is for every one of the routers on the shared tree to trigger a switch to the shortest
path tree as soon as a new source is detected on the shared tree.
C. The default behavior of switching to the shortest path tree as soon as a new source is detected on
the shared tree can be disabled by setting the value in the ip pim spt-threshold command to
“infinity.”
D. The default behavior of switching to the shortest path tree as soon as a new source is detected on
the shared tree can be disabled by setting the value in the ip pim spt-threshold command to
“zero.”
Answer: AC
Explanation:
They are checking you for syntax ip pim spt-threshold command to “infinity” is the right answer.
same source as above:
IP pim spt-threshold [vrf vrf-name] spt-threshold {kbps | infinity} [group-list access-list] To configure when a Protocol Independent Multicast (PIM) leaf router should join the shortest path source tree for the specified group infinity Causes all sources for the specified group to use the shared tree.
http://www.cisco.com/en/US/docs/ios/12_2/ipmulti/command/reference/1rfmult2.html#wp10201
QUESTION 88
In Layer 2 topologies, spanning-tree failures can cause loops in the network. These unblocked
loops can cause network failures because of excessive traffic. Which two Catalyst 6500 features
can be used to limit excessive traffic during spanning-tree loop conditions? (Choose two.)
A. loop guard
B. storm control
C. storm suppression
D. broadcast suppression
E. BPDU guard
Answer: BD
Explanation:
Traffic Storm Control
A traffic storm occurs when packets flood the LAN, creating excessive traffic and degrading network performance. The traffic storm control feature prevents LAN ports from being disrupted by a broadcast, multicast, or unicast traffic storm on physical interfaces. Traffic storm control (also called traffic suppression) monitors incoming traffic levels over a 1-second traffic storm control interval and, during the interval, compares the traffic level with the traffic storm control level that you configure. The traffic storm control level is a percentage of the total available bandwidth of the port. Each port has a single traffic storm control level that is used for all types of traffic (broadcast, multicast, and unicast).
Traffic storm control monitors the level of each traffic type for which you enable traffic storm control in 1-second traffic storm control intervals. Within an interval, when the ingress traffic for which traffic storm control is enabled reaches the traffic storm control level that is configured on the port, traffic storm control drops the traffic until the traffic storm control interval ends.
Broadcast suppression Broadcast suppression prevents the switched ports on a LAN from being disrupted by a broadcast storm on one of the ports. A LAN broadcast storm occurs when the broadcast or multicast packets flood the LAN, creating excessive traffic and degrading the network performance. Errors in the protocol-stack implementation or in the network configuration can cause a broadcast storm.
Broadcast suppression uses filtering that measures the broadcast activity on a LAN over a time period (15264 nsec to ~1 sec) that varies based on the type of line card and speed setting on the port, and compares the measurement with a predefined threshold. If the threshold is reached, further broadcast activity is suppressed for the duration of a specified time period. Broadcast suppression is disabled by default.
http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/gui de/storm.html\
http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/catos/8.x/configuration/guide/bcasts up.html
QUESTION 89
Why does RSTP have a better convergence time than 802.1D?
A. it is newer
B. it has smaller timers
C. it has less overhead
D. it is not timer-based
Answer: D
Explanation:
RSTP identifies certain links as point to point. When a point-to-point link fails, the alternate link can transition to the forwarding state.
Although STP provides basic loop prevention functionality, it does not provide fast network convergence when there are topology changes. STP’s process to determine network state transitions is slower than RSTP’s because it is timer-based. A device must reinitialize every time a topology change occurs. The device must start in the listening state and transition to the learning state and eventually to a forwarding or blocking state.
When default values are used for the maximum age (20 seconds) and forward delay (15 seconds), it takes 50 seconds for the device to converge. RSTP converges faster because it uses a handshake mechanism based on point-to-point links instead of the timer-based process used by STP. An RSTP domain running switch has the following components:
A root port, which is the “best path” to the root device. A designated port, indicating that the switch is the designated bridge for the other switch connecting to this port.
An alternate port, which provides an alternate root port. A backup port, which provides an alternate designated port. Port assignments change through messages exchanged throughout the domain. An RSTP device generates configuration messages once every hello time interval. If an RSTP device does not receive a configuration message from its neighbor after an interval of three hello times, it determines it has lost connection with that neighbor. When a root port or a designated port fails on a device, the device generates a configuration message with the proposal bit set. Once its neighbor device receives this message, it verifies that this configuration message is better than the one saved for that port and then it starts a synchronizing operation to ensure that all of its ports are in sync with the new information.
Similar waves of proposal agreement handshake messages propagate toward the leaves of the network, restoring the connectivity very quickly after a topology change (in a well-designed network that uses RSTP, network convergence can take as little as 0.5 seconds). If a device does not receive an agreement to a proposal message it has sent, it returns to the original IEEE 802.D convention. RSTP was originally defined in the IEEE 802.1w draft specification and later incorporated into the IEEE 802.1D-2004 specification.
QUESTION 90
Under which two circumstances would an RSTP bridge flush its CAM table? (Choose two.)
A. upon a port state change
B. upon receiving a topology change notification
C. when transitioning from discarding to forwarding
D. when transitioning from forwarding to discarding
E. only when changing from listening to discarding
F. when CAM resources have been completely used up
Answer: BC
Explanation:
First, the goal of RSTP is fast re-convergence. Since ports are assumed to transition to forwarding relatively fast, simply increasing MAC address aging speed is not enough. Thus, when a topology change is detected, RSTP instructs the bridge to flush all MAC address table entries. With Ethernet, this process results in unconstrained flooding until the moment MAC addresses are re-learned. The bridge detecting a topology change sets the TC (Topology Change) bit in all outgoing BPDUs and starts sending BPDUs with the TC bit set upstream through the root port as well. This marking lasts for TCWhile=2xHelloTime seconds and allows the detecting bridge the start the flooding process.
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