Home > Articles > Networking > Routing & Switching

This chapter is from the book

1.3 Multicast Basics

A multicast address is also called a multicast group address. A group member is a host that expresses interest in receiving packets sent to a specific group address. A group member is also sometimes called a receiver or listener. A multicast source is a host that sends packets with the destination IP address set to a multicast group. A multicast source does not have to be a member of the group; sourcing and listening are mutually exclusive.

Because there can be multiple receivers, the path that multicast packets take may have several branches. A multicast data path is known as a distribution tree. Data flow through the multicast distribution trees is sometimes referenced in terms of upstream and downstream. Downstream is in the direction toward the receivers. Upstream is in the direction toward the source. A downstream interface is also known as an outgoing or outbound interface; likewise, an upstream interface is also known as an incoming or inbound interface.

Routers keep track of the incoming and outgoing interfaces for each group, which is known as multicast forwarding state. The incoming interface for a group is sometimes referred to as the IIF. The outgoing interface list for a group is sometimes referred to as the OIL or olist. The OIL can contain 0 to N interfaces, where N is the total number of logical interfaces on the router.

Multicast forwarding state in a router is typically kept in terms of "(S,G)" and "(*,G)" state, which usually are pronounced "ess comma gee" and "star comma gee," respectively. In (S,G), the "S" refers to the unicast IP address of the source. The IP header of the multicast data packet contains S as the packet's source address. The "G" represents the specific multicast group IP address of concern. The IP header of the multicast data packet contains G as the packet's destination address. So for a host whose IP address is acting as a source for the multicast group, (S,G) state would read (,

In (*,G) notation, the asterisk (*) is a wild card used to denote the state that applies to any source sending to group G. A multicast group can have more than one source. If two hosts are both acting as sources for the group, (*, could be used to represent the state a router could contain to forward traffic from both sources to the group. The significance of (S,G) and (*,G) state will become more apparent when we discuss shortest path and shared trees in Chapters 2 and 3.

1.3.1 Reverse Path Forwarding

Multicast routing involves a significant paradigm change from standard unicast routing. In general, routers make unicast routing decisions based on the destination address of the packet. When a unicast packet arrives, the router looks up the destination address of the packet in its routing table. The routing table tells the router out from which interface to forward packets for each destination network. Unicast packets are then routed from source to destination.

In multicast, routers set up forwarding state in the opposite direction of unicast, from receiver to the root of the distribution tree. Routers perform a reverse path forwarding (RPF) check to determine the interface that is topologically closest to the root of the tree (see Figure 1-4). RPF is a central concept in multicast routing. In an RPF check, the router looks in a routing table to determine its RPF interface, which is the interface topologically closest to the root. The RPF interface is the incoming interface for the group.

Figure 1-4Figure 1-4 Reverse path forwarding (RPF)

In a shortest path tree (SPT), the root of the distribution tree is the source. If a router learns that an interested listener for a group is on one of its directly connected interfaces, it tries to join the tree for that group. In Figure 1-5, this router

Figure 1-5Figure 1-5 Shortest path tree (SPT)

The router sends a Join message out the RPF interface to inform the next router upstream it wants to receive packets for this group from this source. This message is an (S,G) Join message. The router receiving the (S,G) Join message adds the interface on which it was received to the OIL for the group and performs an RPF check on the source. This upstream router sends an (S,G) Join message out its RPF interface for the source informing its upstream router that it wants to join the group.

Each upstream router repeats this process of propagating Joins out the RPF interface until this new branch of the tree either a) reaches the router directly connected to the source or b) reaches a router that already has multicast forwarding state for this source-group pair. In this way, a new branch of the tree is created from receiver to source. Once this branch is created and each of the routers as forwarding state for the source-group pair, multicast packets can flow down the tree from source to receiver.

In a shared tree, the root of the distribution tree is a router somewhere in the core of a network. In PIM-SM, this core router is called a rendezvous point (RP). If a router learns that an interested listener for a group is on one of its directly connected interfaces, it tries to join the tree for that group. In Figure 1-6, this router

Figure 1-6Figure 1-6 Rendezvous point tree (RPT)

does not know the address of the source of this group. However, it does know that another router in the network is aware of the source. The router that somehow knows the sources for all multicast groups is the RP (we will find out just how it knows this in Chapters 2 and 3).

The router with the directly connected listener, or the last-hop router, performs an RPF check for the IP address of the RP. This RPF check yields the RPF interface for the RP, and a (*,G) Join is sent out from this interface toward the RP. Notice that this is a (*,G) Join instead of an (S,G) Join because the last-hop router does not know the source. It needs to know only a) that the RP should know the source and b) how to get to the RP. By sending the (*,G) Join toward the RP, the last-hop router is telling the upstream router it is interested in receiving multicast packets for the group via the shared tree, or rendezvous point tree (RPT) as it called in PIM-SM.

The router receiving the (*,G) Join message adds the interface on which it was received to the OIL for the group and performs an RPF check on the RP. This upstream router sends a (*,G) Join message out its RPF interface for the RP, informing its upstream router that it wants to join the group. Each upstream router repeats this process of propagating Joins out of the RPF interface until this new branch of the tree either a) reaches the RP or b) reaches a router that already has multicast forwarding state for the group along the RPT. In this way, a new branch of the tree is created from receiver to RP.

To forward multicast packets down the RPT, the RP itself must be receiving the multicast packets. To receive this traffic, the RP can execute an RPF for the source and send an (S,G) Join toward the source. By joining the SPT, the RP is able to transmit packets down the RPT. Multicast packets now flow from the source to the RP via the SPT and then from RP to the receiver down the RPT.

Further details of SPT, RPT, and PIM-SM operation are examined in greater depth in subsequent chapters. For now, it is most important to understand the concept of reverse path forwarding.

1.3.2 Populating the RPF Table

The routing table used for RPF checks can be the same routing table used to forward unicast data packets, or it can be a separate routing table dedicated to multicast RPF. In either case, this RPF table contains only unicast routes. It does not contain multicast group addresses because RPF checks are performed only on unicast addresses (either the source or the RP).

If the same routing table used to forward unicast data packets is used for RPF, it is populated by the traditional unicast routing protocols (RIP, OSPF, IS-IS, BGP, and so on). If a dedicated multicast RPF table is used, it must be populated by some other means. Some multicast routing protocols (for example, DVMRP) include mechanisms for populating a dedicated RPF table. Others (for example, PIM-SM and PIM-DM) rely on other protocols to set up this table.

Some traditional routing protocols (such as BGP and IS-IS) now have extensions that can be used to differentiate between different sets of routing information (for example, Multiprotocol Extensions to Border Gateway Protocol—MBGP—and Multitopology Routing in IS-IS—M-ISIS). Routes can be tagged as multicast RPF routes and thus distinguished from unicast routes. The advantage of having a dedicated RPF table is that a network administrator can set up separate paths and policies for unicast and multicast traffic. Chapters 7 and 8 examine in detail MBGP and M-ISIS, respectively.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020