Ensurepass

QUESTION 31

Which multicast routing protocol supports dense mode, sparse mode and bidirectional mode?

 

A.	DVMRP
B.	MOSPF
C.	PIM
D.	MP-BGP
E.	MSDP

 

Correct Answer: C

 

 

QUESTION 32

To which three IP multicast groups can a multicast MAC address “01-00-5E-4D-62-B1” listen? (Choose three.)

 

A.	231.205.98.177
B.	231.205.99.177
C.	239.77.98.177
D.	239.205.99.177
E.	224.205.98.177
F.	224.205.99.177

 

Correct Answer: ACE

 

 

QUESTION 33

Refer to the exhibit. R5 is configured as the RP for the PIM-SM domain for AS 1020. If R3 and R4 are correctly configured, which Cisco IOS-XE configuration should be done on R5 to configure it as a PIMv3 BSR router?

 

 

A.	ip pim send-rp-announce loopback 0 scope 16 int gi1/3 , gi 1/4 ip pim sparse-mode int gi 1/1 ip pim bsr-border
B.	ip pim rp-candidate loopback 0 int gi1/3 , gi 1/4 ip pim sparse-mode int gi 1/1 ip pim bsr-border
C.	ip pim rp-candidate loopback 0 int gi1/3 , gi 1/4 ip pim sparse-mode ip pim bsr-border
D.	ip pim send-rp-announce loopback 0 scope 16 int gi1/3 , gi 1/4 ip pim sparse-mode ip pim bsr-border

 

Correct Answer: B

 

 

QUESTION 34

Which two features are used to provide high availability multicast? (Choose two.)

 

A.	BFD
B.	NSF/SSO
C.	PIM NSR
D.	PIM triggered join
E.	IGMP triggered report
F.	MSDP

 

Correct Answer: BD

Explanation:

Triggered joins are sent when the primar
y or the secondary RPF information changes. No RPF change prunes are sent for MoFRR streams.

 

mofrr

To perform a fast convergence (multicast-only fast reroute, or MoFRR) of specified routes/flows when a failure is detected on one of multiple equal-cost paths between the router and the source, use the mofrr command under PIM configuration mode.

 

mofrr rib acl_name

no rib acl_name

 

 

QUESTION 35

Which two statements correctly describe the RPF check when a multicast packet arrives at a router? (Choose two.)

 

A.	The router looks up the source address in the unicast routing table to determine if the packet has arrived on the interface that is on the reverse path back to the source
B.	The router looks up the destination address in the unicast routing table to determine if the packet has arrived on the interface that is on the reverse path back to the destination
C.	If the packet has arrived on the interface leading back to the destination, the RPF check passes and the packet is forwarded. If the RPF check fails, the packet is dropped
D.	If the packet has arrived on the interface leading back to the source, the RPF check passes and the packet is forwarded. If the RPF check fails, the packet is dropped

 

Correct Answer: AD

Explanation:

Reverse Path Forwarding (RPF)

RPF is a fundamental concept in multicast routing that enables routers to correctly forward multicast traffic down the distribution tree. RPF makes use of the existing unicast routing table to determine the upstream and downstream neighbors. A router will only forward a multicast packet if it is received on the upstream interface.

 

This RPF check helps to guarantee that the distribution tree will be loop free.

 

RPF Check

When a multicast packet arrives at a router, the router will perform an RPF check on the packet. If the RPF check is successful, the packet will be forwarded. Otherwise it will be dropped.

 

For traffic flowing down a source tree, the RPF check procedure works as follows:

 

Step 1. Router looks up the source address in the unicast routing table to determine if it has arrived on the interface that is on the reverse path back to the source.

Step 2. If packet has arrived on the interface leading back to the source, the RPF check is successful and the packet will be forwarded.

Step 3. If the RPF check in 2 fails, the packet is dropped.

 

 

QUESTION 36

Refer to the topology diagram shown in the exhibit and the partial configurations shown below.

 

 

Once the attack from 209.165.201.144/28 to 209.165.202.128/28 has been detected, which additional configurations are required on the P1 IOS-XR router to implement source-based remote-triggered black hole filtering?

 

!

 

router bgp 123

 

address-family ipv4 unicast

 

redistribute static route-policy test

 

!

 

A.	router static address-family ipv4 unicast 209.165.202.128/28 null0 tag 666 192.0.2.1/32 null0 tag 667 ! route-policy test if tag is 666 then set next-hop 192.0.2.1 endif if tag is 667 then set community (no-export) endif end-policy !
B.	router static address-family ipv4 unicast 209.165.201.144/28 null0 tag 666 192.0.2.1/32 null0 tag 667 ! route-policy test if tag is 666 then set next-hop 192.0.2.1 endif if tag is 667 then set community (no-export) endif end-policy !
C.	router static address-family ipv4 unicast 209.165.201.144/28 null0 tag 666 192.0.2.1/32 null0 ! route-policy test if tag is 666 then set next-hop 192.0.2.1 set community (no-export) endif end-policy
D.	router static address-family ipv4 unicast 209.165.202.128/28 null0 tag 666 192.0.2.1/32 null0 ! route-policy test if tag is 666 then set next-hop 192.0.2.1 set community (no-export) endif end-policy !

 

Correct Answer: C

Explanation:

Source-Based RTBH Filtering

With destination-based black holing, all traffic to a specific destination is dropped after the black hole has been activated, regardless of where it is coming from. Obviously, this could include legitimate traffic destined for the target. Source-based black holes provide the ability to drop traffic at the network edge based on a specific source address or range of source addresses.

If the source address (or range of addresses) of the attack can be identified (spoofed or not), it would be better to drop all traffic at the edge based on the source address, regardless of the destination address. This would permit legitimate traffic from other sources to reach the target. Implementation of source-based black hole filtering depends on Unicast Reverse Path Forwarding (uRPF), most often loose mode uRPF. Loose mode uRPF checks the packet and forwards it if there is a route entry for the source IP of the incoming packet in the router forwarding information base (FIB). If the router does not have an FIB entry for the source IP address, or if the entry points to a null interface, the Reverse Path Forwarding (RPF) check fails and the packet is dropped, as shown in Figure 2. Because uRPF validates a source IP address against its FIB entry, dropping traffic from specific source addresses is accomplished by configuring loose mode uRPF on the external interface and ensuring the RPF check fails by inserting a route to the source with a next hop of Null0. This can be done by using a trigger device to send IBGP updates. These updates set the next hop for the source IP to an unused IP address that has a static entry at the edge, setting it to null as shown in Figure 2.

 

 

 

QUESTION 37

Which four statements are correct regarding MSDP configurations and operations? (Choose four.)

 

A.	The MSDP peers are also typically the RPs in respective routing domains.
B.	SA messages are flooded to all other MSDP peers without any restrictions.
C.	On Cisco IOS, IOS-XE, and IOS-XR, the router can be configured to cache the SA messages to reduce the join latency.
D.	SA messages are used to advertise active sources in a domain.
E.	MSDP establishes neighbor relationships with other MSDP peers using TCP port 639.
F.	MSDP peerings on Cisco IOS, IOS-XE, and IOS-XR support MD5 or SHA1 authentication.

 

Correct Answer: ACDE

Explanation:

http://www.cisco.com/en/US/docs/ios/ipmulti/configuration/guide/imc_msdp_im_pim_sm.html

When MSDP is enabled, an RP in a PIM-SM domain maintains MSDP peering relationships with MSDPenabled routers in other domains. This peering relationship occurs over a TCP connection, where primarily a list of sources sending to multicast groups is exchanged. MSDP uses TCP (port 639) for its peering connections. As with BGP, using point-to-point TCP peering means that each peer must be explicitly configured. The TCP connections between RPs, moreover, are achieved by the underlying routing system. The receiving RP uses the source lists to establish a source path. If the multicast sources are of interest to a domain that has receivers, multicast data is delivered over the normal, source- tree building mechanism provided by PIM-SM. MSDP is also used to announce sources sending to a group. These announcements must originate at the RP of the domain.

 

 

QUESTION 38

Which types of multicast distribution tree can PIM-SM use?

 

A.	Only shared tree rooted at the source
B.	Only shared tree rooted at the RP
C.	Only shortest path tree rooted at the RP
D.	Shared tree rooted at the source and shortest path tree switchover
E.	Shared tree rooted at the RP and shortest path tree switchover
F.	Shared tree rooted at the first-hop router and shortest path tree rooted at the RP

 

Correct Answer: E

 

 

QUESTION 39

Which two BGP mechanisms are used to prevent routing loops when using a design with redundant route reflectors? (Choose two.)

 

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C.

A.	Cluster-list
B.	AS-Path
Originator ID
D.	Community
E.	Origin

 

Correct Answer: AC

Explanation:

http://www.cisco.com/en/US/docs/ios_xr_sw/iosxr_r3.7/routing/configuration/guide/rc37bgp.html

 

As the iBGP learned routes are reflected, routing information may loop. The route reflector model has the following mechanisms to avoid routing loops:

 

Originator ID is an optional, nontransitive BGP attribute. It is a 4-byte attributed created by a route reflector.

 

The attribute carries the router ID of the originator of the route in the local autonomous system. Therefore, if a misconfiguration causes routing information to come back to the originator, the information is ignored.

 

Cluster-list is an optional, nontransitive BGP attribute. It is a sequence of cluster IDs that the route has passed. When a route reflector reflects a route from its clients to nonclient peer
s, and vice versa, it appends the local cluster ID to the cluster-list. If the cluster-list is empty, a new cluster-list is created. Using this attribute, a route reflector can identify if routing information is looped back to the same cluster due to misconfiguration. If the local cluster ID is found in the cluster-list, the advertisement is ignored.

 

 

QUESTION 40

Refer to the Cisco IOS DHCPv6 configuration shown in the exhibit. Which statement is correct?

 

 

A.	The configuration is missing a command under interface Gi0/1 to indicate to the attached hosts to use stateful DHCPv6 to obtain their IPv6 addresses
B.	The IPv6 router advertisements indicate to the attached hosts on the Gi0/1 interface to get other information besides their IPv6 address via stateless auto configuration
C.	The IPv6 DHCPv6 server pool configuration is misconfigured
D.	The DNS server address can also be imported from another upstream DHCPv6 server

 

Correct Answer: A

Explanation:

Server Configuration

In Global Configuration Mode

ipv6 unciast-routing

ipv6 dhcp pool <pool name>

address prefix <specify address prefix> lifetime <infinite> <infinite>

dns-server <specify the dns server address>

domain-name <specify the domain name>

exit

In Interface Configuration Mode

ipv6 address <specify IPv6 Address>

ipv6 dhcp server <server name>rapid-commit

Client Configuration

In Global Configuration Mode

enable

configure terminal

ipv6 unicast-routing

In Interface Configuration Mode

ipv6 address dhcp rapid commit

ipv6 enable

exit

 

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