Cisco 642-885 Exam Study Guide To Pass

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Question No. 1

What are three BGP configuration characteristics of a multihomed customer that is connected to multiple service providers? (Choose three.)

Answer: B, C, E

Question No. 2

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

Answer: A, C, E

Question No. 3

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

Answer: A, C, D, E

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 No. 4

Referring to the topology diagram show in the exhibit,

which three statements are correct regarding the BGP routing updates? (Choose three.)

Answer: A, B, D

Question No. 5

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

Answer: A, D

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.

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