Ch07 IP Multicast

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2006, Cisco Systems, Inc. All rights reserved.

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Cisco Networking Academy Program

CCNP SWITCH: Implementing IP Switching

Chapter 7: Preparing the Campus for Advanced Services


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The demand is increasing for Internet, intranet, and multimedia applications in which onesender transmits to a group of receivers simultaneously. Examples of these applicationsinclude the following:

Transmitting a corporate message to all employees

Broadcasting video and audio, including interactive video for distance learning andIPTV

Transmitting data from a centralized data warehouse to multiple departments

Communicating stock quotes to brokers

Computing collaboratively

Algorithmic trading floor applications using cloud computing

Subsequently, using IP multicast instead of unicast to distribute this information reduces

the overall network load and bandwidth consumption. This is a result of IP multicastssimultaneous distribution of an IP data frame to a host group that is defined by a singlemulticast IP address, or flow. To accomplish this same distribution with unicast wouldrequire the source to send a unicast frame to each host independently.

Note: Multicast reduces overall network load and bandwidth by distributing traffic tomultiple end users simultaneously.

The following slides discuss IP multicast routing and its inherent advantages, functionality,and configuration in campus network. We begin with an introduction to multicast and thendiscuss the fundamentals of the IP multicast. We also discuss the important Layer 3 andLayer 2 multicast protocols and the design recommendations for deploying multicast inmultilayer switched networks.

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IP multicast addresses specify a set of IP hosts that have joined a groupand are interested in receiving multicast traffic designated for thatparticular group. The table outlines the IP multicast address conventions.

Applications allocate multicast addresses dynamically or statically.Dynamic multicast addressing provides applications with a group addresson demand. Because dynamic multicast addresses have a specificlifetime, applications must request this type of address only for as long asthe address is needed.

Statically allocated multicast addresses are reserved for specific protocolsthat require well-known addresses, such as Open Shortest Path First(OSPF). IANA assigns these well-known addresses, which are calledpermanent host groups and are similar in concept to the well-known TCPand UDP port numbers. The following slides discuss the details of the

multicast addresses listed in the table.

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IANA has reserved addresses in the range 224.0.0.0 to 224.0.0.255(link local destination addresses) to be used by network protocols ona local network segment. Routers do not forward packets in thisaddress range, because these packets are typically sent with a Time-To-Live (TTL) value of 1. Network protocols use these addresses forautomatic router discovery and to communicate important routinginformation. For example, OSPF uses the IP addresses 224.0.0.5and 224.0.0.6 to exchange link-state information. Address 224.0.0.1identifies the all-hosts group. Every multicast-capable host must jointhis group when initializing its IP stack. If you send an ICMP echorequest to this address, all multicast-capable hosts on the networkanswer the request with an ICMP echo reply. Address 224.0.0.2identifies the all-routers group. Multicast routers join this group on allmulticast-enabled interfaces.


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Addresses in the 232.0.0.0 to 232.255.255.255 range are reserved forSource-Specific Multicast (SSM), an extension of the ProtocolIndependent Multicast (PIM) protocol that allows for an efficient data-

delivery mechanism in one-to-many communications. In SSM, forwardingdecisions are based on both group and source addresses, which isreferred to as (S,G). The special notation of (S,G), pronounced S commaG, uses S as the IP address of the source and G as the multicast groupaddress. This unique (S,G) is known within SSM as a channel. SSM alsoremoves address allocation problems because the source address makeseach channel unique. SSM requires that the host be aware of the sourceand the group it wants data from and a method to signal this (S,G)requirement to the router.

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RFC 3180, GLOP Addressing in 233/8, proposes that the 233.0.0.0 to233.255.255.255 address range be reserved for statically definedaddresses by organizations that already have an autonomous system

number reserved. This practice is called GLOP addressing and the term isnot an acronym. The autonomous system number of the domain isembedded into the second and third octets of the 233.0.0.0 to233.255.255.255 address range. For example, the autonomous system62010 is written in hexadecimal format as F23A. Separating the twooctets F2 and 3A results in 242 and 58 in decimal format, respectively.These values result in a subnet of 233.242.58.0/24 that is globallyreserved for autonomous system 62010 to use.

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Multicast-capable routers and multilayer switches create distribution trees that control thepath that IP multicast traffic takes through the network to achieve loop-free forwarding.Reverse Path Forwarding (RPF) is the mechanism that performs an incoming interface

check to determine whether to forward or drop an incoming multicast frame. RPF is a keyconcept in multicast forwarding. This RPF check helps to guarantee that the distributiontree for multicast is loop-free. In addition, RPF enables routers to correctly forwardmulticast traffic down the distribution tree.

In unicast routing, traffic is routed through the network along the path from the singlesource to the single destination host. A router that is forwarding unicast packets does notconsider the source address, by default; the router considers only the destination addressand how to forward the traffic toward the destination with the exception of specializedCEF features. Upon receipt of unicast packets, the router scans through its routing tablefor the destination address and then forwards a single copy of the unicast packet out thecorrect interface to the destination.

In multicast forwarding, the source is sending traffic to an arbitrary group of hosts that isrepresented by a single multicast group address. When a multicast router receives amulticast packet, it determines which direction is the upstream direction (toward thesource) and which one is the downstream direction (toward the receivers). A routerforwards a multicast packet only if the packet is received on the correct upstreaminterface determined by the RPF process.

Note, although routers build a separate IP multicast routing table, the RFP check isdependent on the unicast IP routing table for determining the correct upstream anddownstream interfaces.

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The simplest form of a multicast distribution tree is a source tree withits root at the source and its branches forming a tree through thenetwork to the receivers. Because this tree uses the shortest paththrough the network, it is also referred to as a shortest path tree(SPT). The figure shows an example of an SPT for group 224.1.1.1rooted at the source, Host A, and connecting two receivers, Hosts Band C.

Using the (S,G) notation, the SPT for the example shown in the figureis (192.168.1.1, 224.1.1.1).

The (S,G) notation implies that a separate SPT exists for eachsource sending to each group. For example, if host B is also sendingtraffic to group 224.1.1.1 and Hosts A and C are receivers, a separate(S,G) SPT would exist with a notation of (192.168.2.2, 224.1.1.1).


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Both source trees and shared trees avoid multicast traffic loops.Routing devices replicate the multicast packets only where the treebranches.

Members of multicast groups join or leave at any time; as a result,the distribution trees update dynamically. When all the activereceivers on a particular branch stop requesting traffic for a particularmulticast group, the routers prune that branch from the tree and stopforwarding traffic down it. If one receiver on that branch becomesactive and requests the multicast traffic, the router dynamicallymodifies the distribution tree and starts forwarding traffic again.

Source trees have the advantage of creating the optimal pathbetween the source and the receivers. This advantage guaranteesthe minimum amount of network latency for forwarding multicast

traffic. However, this optimization requires additional overheadbecause the routers maintain path information for each source. In anetwork that has thousands of sources and thousands of groups, thisoverhead quickly becomes a resource issue on routers or multilayerswitches. Memory consumption and troubleshooting complexity fromthe size of the multicast routing table are factors that networkdesigners need to take into consideration when designing multicastnetworks.

Shared trees have the advantage of requiring the minimum amountof state information in each router. This advantage lowers the overallmemory requirements and complexity for a network that allows only


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shared trees. The disadvantage of shared trees is that, under certaincircumstances, the paths between the source and receivers might not bethe optimal paths, which can introduce additional latency in packet delivery.As a result, shared trees might overuse some links and leave others

unused, whereas source-based trees (where sources are distributed)usually distribute traffic across a set of links. For example, in the figure onthe right, the shortest path between Host A (Source 1) and Host B (areceiver) is between Router A and Router C. Because Router D is the rootfor a shared tree, the traffic traverses Routers A, B, D, and then C. Networkdesigners need to carefully consider the placement of the RP whenimplementing a shared treeonly environment.

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PIM dense mode (PIM-DM) multicast routing protocols rely onperiodic flooding of the network with multicast traffic to set up andmaintain the distribution tree. PIM relies on its neighbor information toform the distribution tree. PIM-DM uses a source distribution tree toforward multicast traffic, which is built by respective routers as soonas any multicast source begins transmitting. The figure illustrates anexample of PIM-DM.

PIM Dense Mode is obsolete and no longer deployed in campusnetworks. This discussion is included for the purposes of historicalcompleteness.

PIM-DM assumes that the multicast group members are denselydistributed throughout the network and that bandwidth is plentiful,meaning that almost all hosts on the network belong to the group.

When a router configured for PIM-DM receives a multicast packet,the router performs the RPF check to validate the correct interface forthe source and then forwards the packet to all the interfacesconfigured for multicasting until pruning and truncating occurs. Alldownstream routers receive the multicast packet until the multicasttraffic times out. PIM-DM sends a pruning message upstream onlyunder the following conditions:

Traffic arrives on a non-RPF, point-to-point interface; this is thenext-hop router interface that does not have a best route towardthe multicast source.

A leaf router without receivers sends a prune message, as


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shown in the figure, and the router, which does not have members orreceivers, sends the prune message to the upstream router.

A non-leaf router receives a prune message from all of its neighbors.

In summary, PIM-DM works best when numerous members belong to eachmultimedia group. PIM floods the multimedia packet to all routers in thenetwork and then prunes routers that do not service members of thatparticular multicast group.

Consider when planning multicast in the campus network that PIM-DM ismost useful in the following cases:

Senders and receivers are in close proximity to one another.

PIM-SM goes through a dense-mode flooding phase before fullyrelying on the RP for multicast forwarding.

There are few senders and many receivers.

The volume of multicast traffic is high.

The stream of multicast traffic is constant.

Nevertheless, PIM-DM is not the method of choice for most campusnetworks enterprise because of its scalability limitations and floodingproperties.

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The second approach to multicast routing, PIM sparse mode (PIM-SM), is based on the assumptions that the multicast group membersare sparsely distributed throughout the network and that bandwidth islimited.

It is important to note that PIM-SM does not imply that the group hasfew members, just that they are widely dispersed. In this case,flooding would unnecessarily waste network bandwidth and couldcause serious performance problems. Therefore, PIM-SM multicastrouting protocols rely on more selective techniques to set up andmaintain multicast trees.

PIM-SM protocols begin with an empty distribution tree and addbranches only as the result of explicit requests to join the distribution.The figure illustrates a sample of PIM sparse mode.

With PIM-SM, each data stream goes to a relatively small number ofsegments in the campus network. Instead of flooding the network todetermine the status of multicast members, PIM-SM defines an RP.When a sender wants to send data, it first does so to the RP. When areceiver wants to receive data, it registers with the RP, as shown inthe figure. When the data stream begins to flow from sender to RP toreceiver, the routers in the path automatically optimize the path toremove unnecessary hops. PIM-SM assumes that no hosts want themulticast traffic unless they specifically ask for it. In PIM-SM, theshared tree mode can be switched to a source tree after a certainthreshold to have the best route to the source. All Cisco IOS routers


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and switches, by default, have the SPT threshold set to 0, such that the last-hop router switches to SPT mode as soon as the host starts receiving themulticast, to take advantage of the best route for the multicast traffic.

Note: To clarify a common confusion, sparse-mode only uses the RP as ainitial point to forward multicast by default; configuration options exist toforeword all traffic to the RP.

PIM-SM is optimized for environments where there are many multipoint datastreams. When planning for multicast deployments in the campus network,choose PIM-SM with IP under the following scenarios:

There are many multipoint data streams.

At any moment, there are few receivers in a group.

The type of traffic is intermittent or busty.

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PIM can simultaneously support dense mode operation for somemulticast groups and sparse mode operation for others. Cisco hasimplemented an alternative to choosing just dense mode or justsparse mode on a router interface, however. This was necessitatedby a change in the paradigm for forwarding multicast traffic via PIMthat became apparent during its development. It turned out that it wasmore efficient to choose sparse or dense mode on a per-group basisrather than a per-router interface basis. Sparse-dense modefacilitates this ability.

PIM sparse-dense mode enables individual groups to use eithersparse or dense mode depending on whether RP information isavailable for that group. If the router learns RP information for aparticular group, sparse mode is used; otherwise, dense mode isused. Note, PIM spare-dense mode is seldom used in campusnetworks.


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PIM version 2 is a standards-based multicast protocol in the Internet Engineering Task Force (IETF). Cisco highlyrecommends using PIM version 2 in the entire multilayer switched network. The Cisco PIM version 2 implementationenables interoperability and transition between version 1 and version 2, although there are a few caveats. For example,if a PIM version 2 router detects a PIM version 1 router, the version 2 router downgrades itself to version 1 until allversion 1 routers have been shut down or upgraded.

PIM version 2 uses the BSR to discover and announce RP-to-group mapping information for each group prefix to all therouters in a PIM domain. This is the same function accomplished by Auto-RP. However, the BSR feature is part of thePIM version 2 specifications because bootstrap messages are sent on a hop-by-hop basis, and a PIM version 1 routerprevents these messages from reaching all routers in a network. Therefore, if a network has a PIM version 1 router withCisco routers, it is best to use Auto-RP rather than the bootstrap mechanism. Nevertheless, Auto-RP is a standaloneprotocol, separate from PIM version 1, and is Cisco proprietary. The BSR mechanism interoperates with Auto-RP onCisco routers.

A PIM version 2 BSR that is also an Auto-RP mapping agent automatically advertises the RP elected by Auto-RP. Thatis, Auto-RP prevails in its imposition of a single RP on every router in the group.

In summary, PIM version 2 includes the following improvements over PIM version 1:

A single, active RP exists per multicast group, with multiple backup RPs. This single RP compares to multipleactive RPs for the same group in PIM version 1.

A BSR provides a fault-tolerant, automated RP discovery and distribution mechanism. Thus, routers dynamicallylearn the group-to-RP mappings.

Sparse mode and dense mode are properties of a group, as opposed to an interface. Cisco stronglyrecommends sparse-dense mode configurations.

PIM join and prune messages have more flexible encodings for multiple address families. A more flexible hello packet format replaces the query packet to encode current and future capability options.

Register messages to an RP indicate whether they were sent by a border router or a designated router.

PIM no longer uses the IGMP protocol for transport; PIM version 2 uses standalone packets.

When planning for PIM deployments in the campus network, prefer to use PIM version 2 over PIM version 1.

Note: Several newer versions of PIM sparse-mode protocols are gaining popularity in campus networks. Theseversions are outside the scope of this course:

Any Source Multicast (ASM), which uses RPs with either shortest path tree or shared tree.

Source Specific Multicast (SSM), which does not use RP and is designed using only shortest path tree.

Bidirectional PIM (Bi-dir PIM), which is based on shared trees only.

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In addition to the preceding commands, every Cisco switch supportsadditional multicast tuning features such as rate limiting and altering theprotocol timers.

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Ch07 IP Multicast

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