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Cisco IOS IP Configuration Guide, Release 12.2
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About Cisco IOS Software Documentation
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Using Cisco IOS Software
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IP Overview
- Part 2: IP Routing Protocols
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Part 3: IP Multicast
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Configuring IP Multicast Routing
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Configuring Source Specific Multicast
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Configuring Bidirectional PIM
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Configuring Multicast Source Discovery Protocol
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Configuring PGM Host and Router Assist
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Configuring Unidirectional Link Routing
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Using IP Multicast Tools
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Configuring Router-Port Group Management Protocol
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Configuring DVMRP Interoperability
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Index
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Table Of Contents
Configuring IP Multicast Routing
The Cisco IP Multicast Routing Implementation
Basic IP Multicast Routing Configuration Task List
Advanced IP Multicast Routing Configuration Task List
Configuring PIM Dense Mode State Refresh
Configuring a Rendezvous Point
Setting Up Auto-RP in a New Internetwork
Adding Auto-RP to an Existing Sparse Mode Cloud
Announcing the RP and the Group Range It Serves
Assigning the RP Mapping Agent
Verifying the Group-to-RP Mapping
Preventing Join Messages to False RPs
Filtering Incoming RP Announcement Messages
IGMP Features Configuration Task List
Configuring a Router to Be a Member of a Group
Controlling Access to IP Multicast Groups
Modifying the IGMP Host-Query Message and Query Timeout Intervals
Routers That Run IGMP Version 1
Routers That Run IGMP Version 2
Changing the IGMP Query Timeout
Changing the Maximum Query Response Time
Configuring the Router as a Statically Connected Member
Configuring IGMP Leave Latency
Disabling Fast Switching of IP Multicast
SAP Listener Support Configuration Task List
Limiting How Long a SAP Cache Entry Exists
Enabling the Functional Address for IP Multicast over Token Ring LANs
PIM Version 2 Configuration Task List
Configuring PIM Version 2 Only
Configuring PIM Sparse-Dense Mode
Defining a PIM Sparse Mode Domain Border Interface
Making the Transition to PIM Version 2
Deciding When to Configure a BSR
Monitoring the RP Mapping Information
Advanced PIM Features Configuration Task List
Understanding PIM Shared Tree and Source Tree (Shortest-Path Tree)
Understanding Reverse Path Forwarding
Delaying the Use of PIM Shortest-Path Tree
Assigning an RP to Multicast Groups
Modifying the PIM Router Query Message Interval
Understanding the PIM Registering Process
Limiting the Rate of PIM Register Messages
Configuring the IP Source Address of Register Messages
Enabling PIM Nonbroadcast Multiaccess Mode
Configuring an IP Multicast Static Route
Controlling the Transmission Rate to a Multicast Group
Configuring RTP Header Compression
Enabling RTP Header Compression on a Serial Interface
Enabling RTP Header Compression with Frame Relay Encapsulation
Changing the Number of Header Compression Connections
Enabling Express RTP Header Compression
Configuring IP Multicast over ATM Point-to-Multipoint Virtual Circuits
Enabling IP Multicast over ATM Point-to-Multipoint VCs
Configuring an IP Multicast Boundary
Configuring an Intermediate IP Multicast Helper
Configuring Stub IP Multicast Routing
Load Splitting IP Multicast Traffic Across Equal-Cost Paths Configuration Task List
Enabling Native Load Splitting
Enabling Load Splitting Across Tunnels
Configuring the Router at the Opposite End of the Tunnel
Configuring Both Routers to RPF
Monitoring and Maintaining IP Multicast Routing Configuration Task List
Clearing Caches, Tables, and Databases
Displaying System and Network Statistics
IP Multicast Configuration Examples
PIM Dense Mode State Refresh Example
Functional Address for IP Multicast over Token Ring LAN Example
Border Router Configuration Example
RFC 2362 Interoperable Candidate RP Example
RTP Header Compression Examples
Express RTP Header Compression with PPP Encapsulation Example
Express RTP Header Compression with Frame Relay Encapsulation Example
IP Multicast over ATM Point-to-Multipoint VC Example
Administratively Scoped Boundary Example
Load Splitting IP Multicast Traffic Across Equal-Cost Paths Example
IP Multicast Heartbeat Example
Configuring IP Multicast Routing
This chapter describes how to configure IP multicast routing. For a complete description of the IP multicast routing commands in this chapter, refer to the "IP Multicast Routing Commands" chapter of the Cisco IOS IP Command Reference, Volume 3 of 3: Multicast. To locate documentation of other commands in this chapter, use the command reference master index, or search online.
Traditional IP communication allows a host to send packets to a single host (unicast transmission) or to all hosts (broadcast transmission). IP multicast provides a third scheme, allowing a host to send packets to a subset of all hosts (group transmission). These hosts are known as group members.
Packets delivered to group members are identified by a single multicast group address. Multicast packets are delivered to a group using best-effort reliability, just like IP unicast packets.
The multicast environment consists of senders and receivers. Any host, regardless of whether it is a member of a group, can send to a group. However, only the members of a group receive the message.
A multicast address is chosen for the receivers in a multicast group. Senders use that address as the destination address of a datagram to reach all members of the group.
Membership in a multicast group is dynamic; hosts can join and leave at any time. There is no restriction on the location or number of members in a multicast group. A host can be a member of more than one multicast group at a time.
How active a multicast group is and what members it has can vary from group to group and from time to time. A multicast group can be active for a long time, or it may be very short-lived. Membership in a group can change constantly. A group that has members may have no activity.
Routers executing a multicast routing protocol, such as Protocol Independent Multicast (PIM), maintain forwarding tables to forward multicast datagrams. Routers use the Internet Group Management Protocol (IGMP) to learn whether members of a group are present on their directly attached subnets. Hosts join multicast groups by sending IGMP report messages.
Many multimedia applications involve multiple participants. IP multicast is naturally suitable for this communication paradigm.
To identify the hardware platform or software image information associated with a feature, use the Feature Navigator on Cisco.com to search for information about the feature or refer to the software release notes for a specific release. For more information, see the "Identifying Supported Platforms" section in the "Using Cisco IOS Software" chapter.
The Cisco IP Multicast Routing Implementation
The Cisco IOS software supports the following protocols to implement IP multicast routing:
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IGMP is used between hosts on a LAN and the routers on that LAN to track the multicast groups of which hosts are members.
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Figure 66 shows where these protocols operate within the IP multicast environment. The protocols are further described in the sections following the figure.
Figure 66 IP Multicast Routing Protocols
IGMP
To start implementing IP multicast routing in your campus network, you must first define who receives the multicast. IGMP provides a means to automatically control and limit the flow of multicast traffic throughout your network with the use of special multicast queriers and hosts.
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A set of queriers and hosts that receive multicast data streams from the same source is called a multicast group. Queries and hosts use IGMP messages to join and leave multicast groups.
IP multicast traffic uses group addresses, which are Class D IP addresses. The high-order four bits of a Class D address are 1110. Therefore, host group addresses can be in the range 224.0.0.0 to 239.255.255.255.
Multicast addresses in the range 224.0.0.0 to 224.0.0.255 are reserved for use by routing protocols and other network control traffic. The address 224.0.0.0 is guaranteed not to be assigned to any group.
IGMP packets are transmitted using IP multicast group addresses as follows:
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IGMP Versions
IGMP messages are used primarily by multicast hosts to signal their interest in joining a specific multicast group and to begin receiving group traffic.
The original IGMP Version 1 Host Membership model defined in RFC 1112 is extended to significantly reduce leave latency and provide control over source multicast traffic by use of Internet Group Management Protocol, Version 2.
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Provides for the basic Query-Response mechanism that allows the multicast router to determine which multicast groups are active and other processes that enable hosts to join and leave a multicast group. RFC 1112 defines Host Extensions for IP Multicasting.
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Extends IGMP allowing such features as the IGMP leave process, group-specific queries, and an explicit maximum query response time. IGMP Version 2 also adds the capability for routers to elect the IGMP querier without dependence on the multicast protocol to perform this task. RFC 2236 defines Internet Group Management Protocol, Version 2.
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Provides for "source filtering" which enables a multicast receiver host to signal to a router which groups it wants to receive multicast traffic from, and from which sources this traffic is expected.
PIM
The PIM protocol maintains the current IP multicast service mode of receiver-initiated membership. It is not dependent on a specific unicast routing protocol.
PIM is defined in RFC 2362, Protocol-Independent Multicast-Sparse Mode (PIM-SM): Protocol Specification. PIM is defined in the following Internet Engineering Task Force (IETF) Internet drafts:
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PIM can operate in dense mode or sparse mode. It is possible for the router to handle both sparse groups and dense groups at the same time.
In dense mode, a router assumes that all other routers want to forward multicast packets for a group. If a router receives a multicast packet and has no directly connected members or PIM neighbors present, a prune message is sent back to the source. Subsequent multicast packets are not flooded to this router on this pruned branch. PIM builds source-based multicast distribution trees.
In sparse mode, a router assumes that other routers do not want to forward multicast packets for a group, unless there is an explicit request for the traffic. When hosts join a multicast group, the directly connected routers send PIM join messages toward the rendezvous point (RP). The RP keeps track of multicast groups. Hosts that send multicast packets are registered with the RP by the first hop router of that host. The RP then sends join messages toward the source. At this point, packets are forwarded on a shared distribution tree. If the multicast traffic from a specific source is sufficient, the first hop router of the host may send join messages toward the source to build a source-based distribution tree.
CGMP
CGMP is a protocol used on routers connected to Catalyst switches to perform tasks similar to those performed by IGMP. CGMP is necessary for those Catalyst switches that cannot distinguish between IP multicast data packets and IGMP report messages, both of which are addressed to the same group address at the MAC level.
Basic IP Multicast Routing Configuration Task List
Basic and advanced IP multicast routing configuration tasks are described in the following sections. The basic tasks in the first two sections are required; the tasks in the remaining sections are optional.
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Advanced IP Multicast Routing Configuration Task List
The advanced IP multicast routing tasks described in the following sections are optional:
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See the "IP Multicast Configuration Examples" later in this chapter for examples of multicast routing configurations.
To see information on IP multicast multilayer switching, refer to the Cisco IOS Switching Services Configuration Guide and Cisco IOS Switching Services Command Reference.
Enabling IP Multicast Routing
Enabling IP multicast routing allows the Cisco IOS software to forward multicast packets. To enable IP multicast routing on the router, use the following command in global configuration mode:
Enabling PIM on an Interface
Enabling PIM on an interface also enables IGMP operation on that interface. An interface can be configured to be in dense mode, sparse mode, or sparse-dense mode. The mode determines how the router populates its multicast routing table and how the router forwards multicast packets it receives from its directly connected LANs. You must enable PIM in one of these modes for an interface to perform IP multicast routing.
In populating the multicast routing table, dense mode interfaces are always added to the table. Sparse mode interfaces are added to the table only when periodic join messages are received from downstream routers, or when a directly connected member is on the interface. When forwarding from a LAN, sparse mode operation occurs if an RP is known for the group. If so, the packets are encapsulated and sent toward the RP. When no RP is known, the packet is flooded in a dense mode fashion. If the multicast traffic from a specific source is sufficient, the first hop router of the receiver may send join messages toward the source to build a source-based distribution tree.
There is no default mode setting. By default, multicast routing is disabled on an interface.
Enabling Dense Mode
To configure PIM on an interface to be in dense mode, use the following command in interface configuration mode:
See the "PIM Dense Mode Example" section later in this chapter for an example of how to configure a PIM interface in dense mode.
Enabling Sparse Mode
To configure PIM on an interface to be in sparse mode, use the following command in interface configuration mode:
See the "PIM Sparse Mode Example" section later in this chapter for an example of how to configure a PIM interface in sparse mode.
Enabling Sparse-Dense Mode
If you configure either the ip pim sparse-mode or ip pim dense-mode interface configuration command, then sparseness or denseness is applied to the interface as a whole. However, some environments might require PIM to run in a single region in sparse mode for some groups and in dense mode for other groups.
An alternative to enabling only dense mode or only sparse mode is to enable sparse-dense mode. In this case, the interface is treated as dense mode if the group is in dense mode; the interface is treated in sparse mode if the group is in sparse mode. You must have an RP if the interface is in sparse-dense mode, and you want to treat the group as a sparse group.
If you configure sparse-dense mode, the idea of sparseness or denseness is applied to the group on the router, and the network manager should apply the same concept throughout the network.
Another benefit of sparse-dense mode is that Auto-RP information can be distributed in a dense mode manner; yet, multicast groups for user groups can be used in a sparse mode manner. Thus, there is no need to configure a default RP at the leaf routers.
When an interface is treated in dense mode, it is populated in the outgoing interface list of a multicast routing table when either of the following conditions is true:
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When an interface is treated in sparse mode, it is populated in the outgoing interface list of a multicast routing table when either of the following conditions is true:
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To enable PIM to operate in the same mode as the group, use the following command in interface configuration mode:
Router(config-if)# ip pim sparse-dense-mode
Enables PIM to operate in sparse or dense mode, depending on the group.
Configuring PIM Dense Mode State Refresh
If you have PIM dense mode (PIM-DM) enabled on a router interface, the PIM Dense Mode State Refresh feature is enabled by default.
PIM-DM builds source-based multicast distribution trees that operate on a "flood and prune" principle. Multicast packets from a source are flooded to all areas of a PIM-DM network. PIM routers that receive multicast packets and have no directly connected multicast group members or PIM neighbors send a prune message back up the source-based distribution tree toward the source of the packets. As a result, subsequent multicast packets are not flooded to pruned branches of the distribution tree. However, the pruned state in PIM-DM times out approximately every 3 minutes and the entire PIM-DM network is reflooded with multicast packets and prune messages. This reflooding of unwanted traffic throughout the PIM-DM network consumes network bandwidth.
The PIM Dense Mode State Refresh feature keeps the pruned state in PIM-DM from timing out by periodically forwarding a control message down the source-based distribution tree. The control message refreshes the prune state on the outgoing interfaces of each router in the distribution tree.
This feature also enables PIM routers in a PIM-DM multicast network to recognize topology changes (sources joining or leaving a multicast group) before the default 3-minute state refresh timeout period expires.
By default, all PIM routers that are running a Cisco IOS software release that supports the PIM Dense Mode State Refresh feature automatically process and forward state refresh control messages. To disable the processing and forwarding of state refresh control messages on a PIM router, use the ip pim state-refresh disable global configuration command.
To configure the origination of the control messages on a PIM router, use the following commands beginning in global configuration mode:
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See the "PIM Dense Mode State Refresh Example" section later in this chapter for an example of how to configure the PIM Dense Mode State Refresh feature.
Configuring a Rendezvous Point
If you configure PIM to operate in sparse mode, you must also choose one or more routers to be rendezvous points (RPs). You need not configure the routers to be RPs; they learn how to become RPs themselves. RPs are used by senders to a multicast group to announce their existence and by receivers of multicast packets to learn about new senders. The Cisco IOS software can be configured so that packets for a single multicast group can use one or more RPs.
The RP address is used by first hop routers to send PIM register messages on behalf of a host sending a packet to the group. The RP address is also used by last hop routers to send PIM join and prune messages to the RP to inform it about group membership. You must configure the RP address on all routers (including the RP router).
A PIM router can be an RP for more than one group. Only one RP address can be used at a time within a PIM domain. The conditions specified by the access list determine for which groups the router is an RP.
To configure the address of the RP, use the following command on a leaf router in global configuration mode:
Router(config)# ip pim rp-address rp-address [access-list] [override]
Configures the address of a PIM RP.
Configuring Auto-RP
Auto-RP is a feature that automates the distribution of group-to-RP mappings in a PIM network. This feature has the following benefits:
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Multiple RPs can be used to serve different group ranges or serve as backups of each other. To make Auto-RP work, a router must be designated as an RP-mapping agent, which receives the RP-announcement messages from the RPs and arbitrates conflicts. The RP-mapping agent then sends the consistent group-to-RP mappings to all other routers. Thus, all routers automatically discover which RP to use for the groups they support.
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Setting Up Auto-RP in a New Internetwork
If you are setting up Auto-RP in a new internetwork, you do not need a default RP because you configure all the interfaces for sparse-dense mode. Follow the process described in the section "Adding Auto-RP to an Existing Sparse Mode Cloud," except that you should omit the first step of choosing a default RP.
Adding Auto-RP to an Existing Sparse Mode Cloud
The following sections contain suggestions for the initial deployment of Auto-RP into an existing sparse mode cloud, to minimize disruption of the existing multicast infrastructure.
Choosing a Default RP
Sparse mode environments need a default RP; sparse-dense mode environments do not. If you have sparse-dense mode configured everywhere, you need not choose a default RP.
Adding Auto-RP to a sparse mode cloud requires a default RP. In an existing PIM sparse mode region, at least one RP is defined across the network that has good connectivity and availability. That is, the ip pim rp-address command is already configured on all routers in this network.
Use that RP for the global groups (for example, 224.x.x.x and other global groups). There is no need to reconfigure the group address range that RP serves. RPs discovered dynamically through Auto-RP take precedence over statically configured RPs. Assume it is desirable to use a second RP for the local groups.
Announcing the RP and the Group Range It Serves
Find another router to serve as the RP for the local groups. The RP-mapping agent can double as an RP itself. Assign the whole range of 239.x.x.x to that RP, or assign a subrange of that (for example, 239.2.x.x).
To designate that a router is the RP, use the following command in global configuration mode:
Router(config)# ip pim send-rp-announce type number scope ttl-value [group-list access-list] [interval seconds]
Configures a router to be the RP.
To change the group ranges this RP optimally will serve in the future, change the announcement setting on the RP. If the change is valid, all other routers automatically will adopt the new group-to-RP mapping.
The following example advertises the IP address of Ethernet interface 0 as the RP for the administratively scoped groups:
ip pim send-rp-announce ethernet0 scope 16 group-list 1access-list 1 permit 239.0.0.0 0.255.255.255Assigning the RP Mapping Agent
The RP mapping agent is the router that sends the authoritative discovery packets telling other routers which group-to-RP mapping to use. Such a role is necessary in the event of conflicts (such as overlapping group-to-RP ranges).
Find a router whose connectivity is not likely to be interrupted and assign it the role of RP-mapping agent. All routers within time-to-live (TTL) number of hops from the source router receive the Auto-RP discovery messages. To assign the role of RP mapping agent in that router, use the following command in global configuration mode:
Router(config)# ip pim send-rp-discovery scope ttl-value
Assigns the RP mapping agent.
Verifying the Group-to-RP Mapping
To learn if the group-to-RP mapping has arrived, use the following command in EXEC mode on the designated routers:
Router# show ip pim rp [mapping | metric] [rp-address]
Displays active RPs that are cached with associated multicast routing entries. Information learned by configuration or Auto-RP.
Starting to Use IP Multicast
Use your IP multicast application software to start joining and sending to a group.
Preventing Join Messages to False RPs
Note the ip pim accept-rp global configuration commands previously configured throughout the network. If the ip pim accept-rp command is not configured on any router, this problem can be addressed later. In those routers already configured with the ip pim accept-rp command, you must specify the command again to accept the newly advertised RP.
To accept all RPs advertised with Auto-RP and reject all other RPs by default, use the ip pim accept-rp auto-rp command.
If all interfaces are in sparse mode, a default RP is configured to support the two well-known groups 224.0.1.39 and 224.0.1.40. Auto-RP relies on these two well-known groups to collect and distribute RP-mapping information. When this is the case and the ip pim accept-rp auto-rp command is configured, another ip pim accept-rp command accepting the default RP must be configured, as follows:
ip pim accept-rp <default RP address> 1access-list 1 permit 224.0.1.39access-list 1 permit 224.0.1.40Filtering Incoming RP Announcement Messages
To filter incoming RP announcement messages, use the following command in global configuration mode:
Router(config)# ip pim rp-announce-filter rp-list access-list group-list access-list
Filters incoming RP announcement messages.
IGMP Features Configuration Task List
To configure IGMP features, perform the tasks described in the following sections. The tasks in the first section are required; the tasks in the remaining sections are optional.
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For information about configuring IGMP unidirectional link routing (UDLR), see the chapter "Configuring Unidirectional Link Routing" in this document.
Configuring a Router to Be a Member of a Group
Cisco routers can be configured to be members of a multicast group. This strategy is useful for determining multicast reachability in a network. If a device is configured to be a group member and supports the protocol that is being sent to the group, it can respond (to the ping EXEC command, for example). The device responds to ICMP echo request packets addressed to a group of which it is a member. Another example is the multicast traceroute tools provided in the Cisco IOS software.
To have the router join a multicast group and enable IGMP, use the following command in interface configuration mode:
Controlling Access to IP Multicast Groups
Multicast routers send IGMP host query messages to determine which multicast groups have members in the attached local networks of the router. The routers then forward to these group members all packets addressed to the multicast group. You can place a filter on each interface that restricts the multicast groups that hosts on the subnet serviced by the interface can join.
To filter multicast groups allowed on an interface, use the following command in interface configuration mode:
Router(config-if)# ip igmp access-group access-list
Controls the multicast groups that hosts on the subnet serviced by an interface can join.
Changing the IGMP Version
By default, the router uses IGMP Version 2 (IGMPv2), which allows such features as the IGMP query timeout and the maximum query response time.
All routers on the subnet must support the same version. The router does not automatically detect Version 1 routers and switch to Version 1 as did earlier releases of the Cisco IOS software. However, a mix of IGMP Version 1 and Version 2 hosts on the subnet is acceptable. IGMP Version 2 routers will always work correctly in the presence of IGMP Version 1 hosts.
To control which version of IGMP the router uses, use the following command in interface configuration mode:
Router(config-if)# ip igmp version {3 | 2 | 1}
Selects the IGMP version that the router uses.
Modifying the IGMP Host-Query Message and Query Timeout Intervals
Multicast routers send IGMP host-query messages to discover which multicast groups are present on attached networks. These messages are sent to the all-systems group address of 224.0.0.1 with a time-to-live (TTL) of 1.
Multicast routers send host-query messages periodically to refresh their knowledge of memberships present on their networks. If, after some number of queries, the Cisco IOS software discovers that no local hosts are members of a multicast group, the software stops forwarding onto the local network multicast packets from remote origins for that group and sends a prune message upstream toward the source.
Routers That Run IGMP Version 1
If there are multiple routers on a LAN, a designated router (DR) must be elected to avoid duplicating multicast traffic for connected hosts. PIM routers follow an election process to select a DR. The PIM router with the highest IP address becomes the DR.
The DR is responsible for the following tasks:
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By default, the DR sends host-query messages every 60 seconds in order to keep the IGMP overhead on hosts and networks very low.
To modify this interval, use the following command in interface configuration mode:
Router(config-if)# ip igmp query-interval seconds
Configures the frequency at which the designated router sends IGMP host-query messages.
Routers That Run IGMP Version 2
IGMPv2 improved the query messaging capabilities of IGMPv1.
The query and membership report messages in IGMPv2 are identical to the IGMPv1 messages with two exceptions.
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Unlike IGMPv1, in which the DR and the IGMP querier are typically the same router; in IGMPv2, the two functions are decoupled. The DR and the IGMP querier are selected based on different criteria and may be different routers on the same subnet. The DR is the router with the highest IP address on the subnet, whereas the IGMP querier is the router with the lowest IP address.
IP addresses in general query messages are used to elect the IGMP querier and this is the election process:
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By default, the timer is 2 times the query interval controlled by the ip igmp query-interval command.
To change the query timeout and to specify the period of time before a new election is performed, use the following command in interface configuration mode:
Configuring IGMP Version 3
IGMP Version 3 (IGMPv3) adds support in Cisco IOS software for "source filtering," which enables a multicast receiver host to signal to a router which groups it wants to receive multicast traffic from, and from which sources this traffic is expected. This membership information enables Cisco IOS software to forward traffic only from those sources from which receivers requested the traffic.
IGMPv3 supports applications that explicitly signal sources from which they want to receive traffic. With IGMPv3, receivers signal membership to a multicast host group in the following two modes:
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IGMPv3 is the industry-designated standard protocol for hosts to signal channel subscriptions in Source Specific Multicast (SSM). For SSM to rely on IGMPv3, IGMPv3 must be available in last hop routers and host operating system network stacks, and be used by the applications running on those hosts.
In SSM deployment cases where IGMPv3 cannot be used because it is not supported by the receiver host or the receiver applications, two Cisco-developed transition solutions enable the immediate deployment of SSM services: URL Rendezvous Directory (URD) and IGMP Version 3 lite (IGMP v3lite). For more information on URD and IGMP v3lite, see the "Configuring Source Specific Multicast" chapter in this document.
Restrictions
Traffic Filtering with Multicast Groups That Are Not Configured in SSM Mode
IGMPv3 membership reports are not utilized by Cisco IOS software to filter or restrict traffic for multicast groups that are not configured in SSM mode. Effectively, Cisco IOS software interprets all IGMPv3 membership reports for groups configured in dense, sparse, or bidirectional mode to be group membership reports and forwards traffic from all active sources onto the network.
Interoperability with IGMP Snooping
You must be careful when using IGMPv3 with switches that support and are enabled for IGMP snooping, because IGMPv3 messages are different from the messages used in IGMP Version 1 (IGMPv1) and Version 2 (IGMPv2). If a switch does not recognize IGMPv3 messages, then hosts will not correctly receive traffic if IGMPv3 is being used. In this case, either IGMP snooping may be disabled on the switch or the router may be configured for IGMPv2 on the interface (which would remove the ability to use SSM for host applications that cannot resort to URD or IGMP v3lite).
Interoperability with CGMP
Networks using CGMP will have better group leave behavior if they are configured with IGMPv2 than IGMPv3. If CGMP is used with IGMPv2 and the switch is enabled for the CGMP leave functionality, then traffic to a port joined to a multicast group will be removed from the port shortly after the last member on that port has dropped membership to that group. This fast-leave mechanism is part of IGMPv2 and is specifically supported by the CGMP fast-leave enabled switch.
With IGMPv3, there is currently no CGMP switch support of fast leave. If IGMPv3 is used in a network, CGMP will continue to work, but CGMP fast-leave support is ineffective and the following conditions apply:
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This join behavior only applies to multicast groups that actually operate in IGMPv3 mode. If legacy hosts only supporting IGMPv2 are present in the network, then groups will revert to IGMPv2 and fast leave will work for these groups.
If fast leave is needed with CGMP-enabled switches, we recommend that you not enable IGMPv3 but configure IGMPv2 on that interface.
If IGMPv3 is needed to support SSM, then you have two configuration alternatives as follows:
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Changing the IGMP Query Timeout
You can specify the period of time before the router takes over as the querier for the interface, after the previous querier has stopped doing so. By default, the router waits two times the query interval controlled by the ip igmp query-interval interface configuration command. After that time, if the router has received no queries, it becomes the querier. This feature requires IGMP Version 2.
To change the query timeout, use the following command in interface configuration mode:
Changing the Maximum Query Response Time
By default, the maximum query response time advertised in IGMP queries is 10 seconds. If the router is using IGMP Version 2, you can change this value. The maximum query response time allows a router to quickly detect that there are no more directly connected group members on a LAN. Decreasing the value allows the router to prune groups faster.
To change the maximum query response time, use the following command in interface configuration mode:
Router(config-if)# ip igmp query-max-response-time seconds
Sets the maximum query response time advertised in IGMP queries.
Configuring the Router as a Statically Connected Member
Sometimes either there is no group member on a network segment or a host cannot report its group membership using IGMP. However, you may want multicast traffic to go to that network segment. The following are two ways to pull multicast traffic down to a network segment:
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To configure the router itself to be a statically connected member of a group (and allow fast switching), use the following command in interface configuration mode:
Router(config-if)# ip igmp static-group group-address
Configures the router as a statically connected member of a group.
Configuring IGMP Leave Latency
In IGMPv2 and IGMPv3, hosts send IGMP messages to indicate that they do not wish to receive a particular group, source, or channel any more. The length of time between the host wanting to leave and the router stopping forwarding is called the IGMP leave latency. IGMP leave latency is only relevant when the last host on a subnet that was a member to a group, source, or channel intends to leave, because as long as there are still other interested members, the router still needs to forward the traffic.
When a router receives such a membership message that indicates a leave, by default, it needs to verify if there are still other members interested in the traffic. To do so, the IGMP querying router sends out a group-specific or group-source-specific query. This query contains the last member query interval (LMQI), which is the time within which other still interested hosts need to send a membership report or else the router will stop forwarding. Because IGMP messages may get lost between router and hosts, the router by default does not immediately stop forwarding after the LMQI has expired, but instead it repeats this process of sending the group or group-source-specific query and waiting for membership reports for a total of times specified by the last member query count (LMQC). Only thereafter will the router stop forwarding.
By default in Cisco IOS software and in the IGMPv2 and IGMPv3 RFCs, the LMQI is 1 second, and the LMQC is 2. Therefore, the default leave latency for individual leaves in Cisco IOS software is 3 seconds.
IGMPv3 explicit tracking allows to reduce the leave latency to approximately 0 for hosts that support IGMPv3. This feature is not available for hosts that support only IGMPv2 because of the protocol limitation.
In IGMPv2, if there is only one IP multicast receiving host connected to a subnet, the ip igmp immediate-leave group-list command can be configured so the router immediately stop forwarding traffic for the group, resulting in a leave latency of 0.
To change the values of the LMQI, use the following command in interface configuration mode:
To change the values of the LMQC, use the following commands in interface configuration mode:
Router(config-if)# ip igmp last-member-query-count lmqc
Configures the number of times that the router sends IGMP group-specific or group-source-specific (with IGMPv3) query messages.
Configuring the TTL Threshold
The TTL value controls whether packets are forwarded out of an interface. You specify the TTL value in hops. Only multicast packets with a TTL greater than the interface TTL threshold are forwarded on the interface. The default value is 0, which means that all multicast packets are forwarded on the interface. To change the default TTL threshold value, use the following command in interface configuration mode:
Router(config-if)# ip multicast ttl-threshold ttl-value
Configures the TTL threshold of packets being forwarded out an interface.
Disabling Fast Switching of IP Multicast
Fast switching of IP multicast packets is enabled by default on all interfaces (including generic routing encapsulation [GRE] and DVMRP tunnels), with one exception: It is disabled and not supported over X.25 encapsulated interfaces. Note the following properties of fast switching:
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Disable fast switching if you want to log debug messages, because when fast switching is enabled, debug messages are not logged.
To disable fast switching of IP multicast, use the following command in interface configuration mode:
SAP Listener Support Configuration Task List
To configure Session Announcement Protocol (SAP) listener support, perform the tasks described in the following sections. The task in the first section is required; the task in the remaining section is optional.
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Enabling SAP Listener Support
Use session description and announcement protocols and applications to assist the advertisement of multicast multimedia conferences and other multicast sessions and to communicate the relevant session setup information to prospective participants. Sessions are described by the Session Description Protocol (SDP), which is defined in RFC 2327. SDP provides a formatted, textual description of session properties (for example, contact information, session lifetime, and the media) being used in the session (for example, audio, video, and whiteboard) with their specific attributes like TTL scope, group address, and User Datagram Protocol (UDP) port number.
Many multimedia applications rely on SDP for session descriptions. However, they may use different methods to disseminate these session descriptions. For example, IP/TV relies on the Web to disseminate session descriptions to participants. In this example, participants must know of a Web server that provides the session information.
MBONE applications (for example, vic, vat, and wb) and other applications rely on multicast session information sent throughout the network. In these cases, a protocol called Session Announcement Protocol (SAP) is used to transport the SDP session announcements. SAP Version 2 uses the well-known session directory multicast group 224.2.127.254 to disseminate SDP session descriptions for global scope sessions and group 239.255.255.255 for administrative scope sessions.
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To enable the Cisco IOS software to listen to Session Directory announcements, use the following command on a multicast-enabled interface in interface configuration mode:
Router(config-if)# ip sap listen
Enables the Cisco IOS software to listen to Session Directory announcements.
Limiting How Long a SAP Cache Entry Exists
By default, entries are deleted 24 hours after they were last received from the network. To limit how long a SAP cache entry stays active in the cache, use the following command in global configuration mode:
Router(config)# ip sap cache-timeout
Limits how long a SAP cache entry stays active in the cache.
Enabling the Functional Address for IP Multicast over Token Ring LANs
By default, IP multicast datagrams on Token Ring LAN segments use the MAC-level broadcast address 0xFFFF.FFFF.FFFF. That default places an unnecessary burden on all devices that do not participate in IP multicast. The IP Multicast over Token Ring LANs feature defines a way to map IP multicast addresses to a single Token Ring MAC address.
This feature defines the Token Ring functional address (0xc000.0004.0000) that should be used over Token Ring. A functional address is a severely restricted form of multicast addressing implemented on Token Ring interfaces. Only 31 functional addresses are available. A bit in the destination MAC address designates it as a functional address.
The implementation used by Cisco complies with RFC 1469, IP Multicast over Token-Ring Local Area Networks.
If you configure this feature, IP multicast transmissions over Token Ring interfaces are more efficient than they formerly were. This feature reduces the load on other machines that do not participate in IP multicast because they do not process these packets.
The following restrictions apply to the Token Ring functional address:
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To enable the mapping of IP multicast addresses to the Token Ring functional address 0xc000.0004.0000, use the following command in interface configuration mode:
Router(config-if)# ip multicast use-functional
Enables the mapping of IP multicast addresses to the Token Ring functional address.
For an example of configuring the functional address, see the section "Functional Address for IP Multicast over Token Ring LAN Example" later in this chapter.
Configuring PIM Version 2
PIM Version 2 includes the following improvements over PIM Version 1:
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PIM Version 1, together with the Auto-RP feature, can perform the same tasks as the PIM Version 2 BSR. However, Auto-RP is a standalone protocol, separate from PIM Version 1, and is Cisco proprietary. PIM Version 2 is a standards track protocol in the IETF. We recommend that you use PIM Version 2.
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Either the BSR or Auto-RP should be chosen for a given range of multicast groups. If there are PIM Version 1 routers in the network, do not use the BSR.
The Cisco PIM Version 2 implementation allows interoperability and transition between Version 1 and Version 2, although there might be some minor problems. You can upgrade to PIM Version 2 incrementally. PIM Versions 1 and 2 can be configured on different routers within one network. Internally, all routers on a shared media network must run the same PIM version. Therefore, if a PIM Version 2 router detects a PIM Version 1 router, the Version 2 router downgrades itself to Version 1 until all Version 1 routers have been shut down or upgraded.
PIM uses the BSR to discover and announce RP-set information for each group prefix to all the routers in a PIM domain. This is the same function accomplished by Auto-RP, but the BSR is part of the PIM Version 2 specification.
To avoid a single point of failure, you can configure several candidate BSRs in a PIM domain. A BSR is elected among the candidate BSRs automatically; they use bootstrap messages to discover which BSR has the highest priority. This router then announces to all PIM routers in the PIM domain that it is the BSR.
Routers that are configured as candidate RPs then unicast to the BSR the group range for which they are responsible. The BSR includes this information in its bootstrap messages and disseminates it to all PIM routers in the domain. Based on this information, all routers will be able to map multicast groups to specific RPs. As long as a router is receiving the bootstrap message, it has a current RP map.
Prerequisites
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PIM Version 2 Configuration Task List
There are two approaches to using PIM Version 2. You can use Version 2 exclusively in your network, or migrate to Version 2 by employing a mixed PIM version environment. When deploying PIM Version 2 in your network, use the following guidelines:
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To configure PIM Version 2, perform the tasks described in the following sections. The tasks in the first section are required; the tasks in the remaining sections are optional.
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Specifying the PIM Version
All systems using Cisco IOS Release 11.3(2)T or later start in PIM Version 2 mode by default. To reenable PIM Version 2 or specify PIM Version 1 for some reason, control the PIM version by using the following command in interface configuration mode:
Configuring PIM Version 2 Only
To configure PIM Version
