2013 Latest Cisco 350-001 Exam Section 6: RPF (5 Questions)
QUESTION NO: 1 The Testking network is using multicasting for corporate video training sessions. All routers in the Testking network are enabled for IP multicast. How are these video streaming multicast packets forwarded by these routers? (Choose all that apply)
A. When a multicast packet arrives at a router, the router performs a Reverse Path forwarding (RPF) check on the packet. If the RPF check succeeds, the packet is forwarded, otherwise, it is dropped.
B. When traffic is flowing down the source tree 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, if the packet has arrived on the interface leading back to source, the RPF check succeeds and the packets is forward. Otherwise, it is dropped.
When traffic is flowing down the source tree the router looks up the source address in the multicast routing table to determine if the packet has arrived on the interface that is on the reverse path back to the source. If the packet has arrived on the interface leading back to the source, the RPF check successfully the packets is forwarded. Otherwise, it is dropped.
D. When traffic is flowing down the source tree the router looks up the source address in the multicast routing table to determine if the packet has arrived on the interface that is on the reverse path back to the source and forward path to the receiver. If the reverse path and forward path is found successfully the packet is forwarded. Otherwise, it is dropped.
E. When a multicast packet arrives at a router, the router does not have to perform an RPF check on the packet. The router looks up the source address in the unicast routing table to determine if the destination path is present. If this succeeds the packet is forwarded. Otherwise, it is dropped.
Answer: A, B
In unicast routing, traffic is routed through the network along a single path from the
considers only the destination address and how to forward the traffic toward that destination. The router scans through its routing table for the destination address and then forwards a single copy of the unicast packet out the correct interface in the direction of the destination. In multicast forwarding, the source is sending traffic to an arbitrary group of hosts that are represented by a multicast group address. The multicast router must determine which direction is the upstream direction (toward the source) and which one is the downstream direction (or directions). If there are multiple downstream paths, the router replicates the packet and forwards it down the appropriate downstream paths (best unicast route metric)-which is not necessarily all paths. Forwarding multicast traffic away from the source, rather than to the receiver, is called Reverse Path Forwarding (RPF). RPF is described in the following section.
ReversePath Forwarding (RPF)
PIM uses the unicast routing information to create a distribution tree along the reverse path from the receivers towards the source. The multicast routers then forward packets along the distribution tree from the source to the receivers. RPF is a key concept in multicast forwarding. It 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 forward a multicast packet only if it is received on the upstream interface. This RPF check helps to guarantee that the distribution tree will be loop-free.
When a multicast packet arrives at a router, the router performs an RPF check on the packet. If the RPF check succeeds, the packet is forwarded. Otherwise, it is dropped. For traffic flowing down a source tree, the RPF check procedure works as follows:
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.
If the packet has arrived on the interface leading back to the source, the RPF check succeeds and the packet is forwarded.
If the RPF check in Step 2 fails, the packet is dropped.
C, D. The RPF lookup is done on the unicast routing table, not the multicast routing table.
E. RPF checks must be done in order to maintain a loop free multicast topology.
QUESTION NO: 2
While troubleshooting an IP multicast issue, you issue the “show ip mroute” command:
Router#show ip mroute 184.108.40.206
IP Multicast Routing table
Flags: D -Dense, S -Sparse, C -Connected, L -Local, P -Pruned
R -RP-bit set, F -Register flag, T -SPT-bit set, J -JOIN SPT
X -Proxy Join Timer Running
Interface state: Interface, next-hop or VCD, State/Mode
(*, 220.127.116.11), 00:09:49/00:04:23 RP 10.1.24.1, flags: SC
Incoming interface: Serial1.708, RPF nbr 10.1.20.2
Outgoing interface list:
Ethernet0, Forward/Sparse, 00:09:50/00:04:12
You are trying to trace this multicast address back to the source of this multicast shared tree. Based on the information above, what is the IP address of the upstream neighbor?
The upstream neighbor is the IP address associated with the Reverse Path Forwarding Neighbor (RPF nbr), which is 10.1.20.2 in this case.
A. 10.1.24.1 is the IP address of the Rendezvous Point in this example, not the upstream neighbor.
E. 18.104.22.168 is the IP address of the IP multicast session.
QUESTION NO: 3
Part of the Testking IP multicast network is shown below: Router TK2 sends a (S, G) Prune message to the LAN segment. Will this cause router TK1 to stop the multicast flow to router TK3?
A. No. After seeing the Prune message from router TK2, Router TK3 will send a Join message to router TK1 to override the Prune.
B. No. After seeing the Prune message from router TK2, Router TK3 will send a Join message to router TK2 to override the Prune.
C. No. After seeing the Prune message from router TK2, Router TK3 will send a Graft message to router TK2 to override the Prune from router TK2.
D. Yes. Router TK3 will need to send a new Join message to re-join the multicast session.
E. It depends on whether the routers are IGMP version 1 or IGMP version 2.
After a prune, the router waits for joins, if none arrive, then the router drops the Group. In this case, router TK1 will hear the Join message from TK3 to prevent the flow of multicast traffic from being cut off to TK3.
B. Router TK2 will send a Join message to the upstream neighbor, which is TK1 in this case, not TK2.
C. No graft messages will be sent in this case.
E. IGMP versions are irrelevant.
QUESTION NO: 4
What is the primary purpose for the RPF check in IP multicast networks?
A. To establish reverse flow path of multicast traffic from the receiver to the source.
B. To prevent multicast traffic looping through the network.
C. To determine interfaces inclusion in the outgoing interface list.
D. To prevent the movement of unauthorized multicast traffic.
Reverse Path Forwarding (RPF) provides loop avoidance. It is an algorithm used to forward multicast packets. The RPF rules are: If a router receives a datagram on an interface that it uses to send unicast packets to the source of that packet, then the packet has arrived on the RPF interface. If the packet arrives on the RPF interface, a router forwards the packet out the interfaces that are present in the outgoing interface list of a multicast routing table entry. If the packet does not arrive on the RPF interface, the packet is silently discarded.
QUESTION NO: 5
Which Multicast Protocols use Reverse Path Forwarding (RPF) information when
sending multicast traffic streams to the receivers within the TestKing network?
B. PIM Sparse Mode
C. PIM Dense Mode
D. Multicast OSPF
E. PIM Sparse-Dense Mode
Answer: A, C
DVMRP uses a technique known as Reverse Path Forwarding. When a router receives a packet, it floods the packet out of allpaths except the one that leads back to the packet’s source. Doing so allows a data stream to reach all LANs (possibly multiple times). If a router is attached to a set of LANs that donot want to receive a particular multicast group, the router can send a “prune” message back up the distribution tree tostopsubsequent packets from traveling where there are no members. Dense-mode PIM uses Reverse Path Forwarding and looks a lot like DVMRP. The most significant difference between DVMRP and dense-mode PIM is that PIM works with
B. Sparse-mode PIM is optimized for environments where there are many multipoint data streams. Each data stream goes to a relatively small number of the LANs in the internetwork. For these types of groups, Reverse Path Forwarding techniques waste bandwidth. Sparse-mode PIM works by defining a Rendezvous Point. When a sender wants to send data, it first sends to the Rendezvous Point.
D. Multicast OSPF (MOSPF) was defined as an extension to the OSPF unicast routing protocol. OSPF works by having each router in a network understand all of the available links in the network. Each OSPF router calculates routes from itself to all possible destinations. MOSPF works by including multicast information in OSPF link state advertisements. An MOSPF router learns which multicast groups areactive on which LANs. MOSPF builds a distribution tree for each source/group pair and computes a tree for active sources sending to the group. The tree state is cached, and trees must be recomputed when alink state change occurs or when the cache times out.
Topic 8: Security (27 Questions)
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