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Table Of Contents
Configuring Basic IP Multicast
Prerequisites for Configuring Basic IP Multicast
Information About Configuring Basic IP Multicast
The Role of Auto-RP in a PIM Network
Benefits of Auto-RP in a PIM Network
BSR Election and Functionality
How SSM Differs from Internet Standard Multicast
Benefits of Source Specific Multicast
Bidirectional Group Tree Building
How to Configure Basic IP Multicast
Configuring Sparse Mode with Auto-RP
Configuring Sparse Mode with Anycast RP
Configuring Sparse Mode with a Bootstrap Router
Configuring Sparse Mode with a Single Static RP
Configuring Source Specific Multicast
Configuration Examples for Basic IP Multicast
Example: Sparse Mode with Auto-RP
Example: Sparse Mode with Anycast RP
Example: Sparse Mode with Bootstrap Router
Example: BSR and RFC 2362 Interoperable Candidate RP
Example: Sparse Mode with a Single Static RP
Feature Information for Configuring Basic IP Multicast
Configuring Basic IP Multicast
First Published: May 2, 2005Last Updated: September 10, 2010IP multicast is a bandwidth-conserving technology that reduces traffic by delivering a single stream of information simultaneously to potentially thousands of corporate businesses and homes. Applications that take advantage of multicast include video conferencing, corporate communications, distance learning, and distribution of software, stock quotes, and news. This module describes the tasks used to configure basic IP multicast.
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for Configuring Basic IP Multicast" section.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Contents
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Prerequisites for Configuring Basic IP Multicast
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Prerequisites for Configuring Basic IP Multicast
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Information About Configuring Basic IP Multicast
Auto-RP Overview
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The Role of Auto-RP in a PIM Network
Auto-RP automates the distribution of group-to-rendezvous point (RP) mappings in a PIM network. 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 by way of dense mode flooding.
Thus, all routers automatically discover which RP to use for the groups they support. The Internet Assigned Numbers Authority (IANA) has assigned two group addresses, 224.0.1.39 and 224.0.1.40, for Auto-RP.
The mapping agent receives announcements of intention to become the RP from Candidate-RPs. The mapping agent then announces the winner of the RP election. This announcement is made independently of the decisions by the other mapping agents.
IP Multicast Boundary
As shown in Figure 1, address scoping defines domain boundaries so that domains with RPs that have the same IP address do not leak into each other. Scoping is performed on the subnet boundaries within large domains and on the boundaries between the domain and the Internet.
Figure 1 Address Scoping at Boundaries
You can set up an administratively scoped boundary on an interface for multicast group addresses using the ip multicast boundary command with the access-list argument. A standard access list defines the range of addresses affected. When a boundary is set up, no multicast data packets are allowed to flow across the boundary from either direction. The boundary allows the same multicast group address to be reused in different administrative domains.
The Internet Assigned Numbers Authority (IANA) has designated the multicast address range 239.0.0.0 to 239.255.255.255 as the administratively scoped addresses. This range of addresses can be reused in domains administered by different organizations. They would be considered local, not globally unique.
You can configure the filter-autorp keyword to examine and filter Auto-RP discovery and announcement messages at the administratively scoped boundary. Any Auto-RP group range announcements from the Auto-RP packets that are denied by the boundary access control list (ACL) are removed. An Auto-RP group range announcement is permitted and passed by the boundary only if all addresses in the Auto-RP group range are permitted by the boundary ACL. If any address is not permitted, the entire group range is filtered and removed from the Auto-RP message before the Auto-RP message is forwarded.
Benefits of Auto-RP in a PIM Network
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Anycast RP Overview
Anycast RP is a useful application of MSDP. Originally developed for interdomain multicast applications, MSDP used for Anycast RP is an intradomain feature that provides redundancy and load-sharing capabilities. Enterprise customers typically use Anycast RP for configuring a Protocol Independent Multicast sparse mode (PIM-SM) network to meet fault tolerance requirements within a single multicast domain.
In anycast RP, two or more RPs are configured with the same IP address on loopback interfaces. The anycast RP loopback address should be configured with a 32-bit mask, making it a host address. All the downstream routers should be configured so that the anycast RP loopback address is the IP address of their local RP. IP routing will automatically select the topologically closest RP for each source and receiver. Assuming that the sources are evenly spaced around the network, an equal number of sources will register with each RP. That is, the process of registering the sources will be shared equally by all the RPs in the network.
Because a source may register with one RP and receivers may join to a different RP, a method is needed for the RPs to exchange information about active sources. This information exchange is done with MSDP.
In anycast RP, all the RPs are configured to be MSDP peers of each other. When a source registers with one RP, an SA message will be sent to the other RPs informing them that there is an active source for a particular multicast group. The result is that each RP will know about the active sources in the area of the other RPs. If any of the RPs were to fail, IP routing would converge, and one of the RPs would become the active RP in more than one area. New sources would register with the backup RP. Receivers would join the new RP and connectivity would be maintained.
The RP is normally needed only to start new sessions with sources and receivers. The RP facilitates the shared tree so that sources and receivers can establish a direct multicast data flow. If a multicast data flow is already established between a source and the receiver, an RP failure will not affect that session. Anycast RP ensures that new sessions with sources and receivers can begin at any time.
BSR Overview
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BSR Election and Functionality
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 performed by Auto-RP, but the BSR is part of the PIM Version 2 specification. The BSR mechanism interoperates with Auto-RP on Cisco routers.
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.
Following the election of the BSR, candidate RPs use unicast to announce to the BSR their willingness to be the RP. The BSR advertises the entire group-to-RP mapping set to the router link local address 224.0.0.13. Unlike the RP mapping agent in Auto-RP, which is used by Auto-RP to select the RP, every router in the BSR network is responsible for selecting the RP.
BSR lacks the ability to scope RP advertisements; however, BSR is used when vendor interoperability or open standard adherence is a requirement.
BSR Border Interface
A border interface in a PIM sparse mode domain requires precautions to prevent exchange of certain traffic with a neighboring domain reachable through that interface, especially if that domain is also running PIM sparse mode. BSR and Auto-RP messages should not be exchanged between different domains, because routers in one domain may elect RPs in the other domain, resulting in protocol malfunction or loss of isolation between the domains. Configure a BSR border interface to prevent BSR messages from being sent or received through an interface.
Static RP Overview
If you are configuring PIM sparse mode, you must configure a PIM RP for a multicast group. An RP can either be configured statically in each device, or learned through a dynamic mechanism. This task explains how to statically configure an RP, as opposed to the router learning the RP through a dynamic mechanism such as Auto-RP.
PIM designated routers (DRs) forward data from directly connected multicast sources to the RP for distribution down the shared tree. Data is forwarded to the RP in one of two ways. It is encapsulated in register packets and unicast directly to the RP, or, if the RP has itself joined the source tree, it is multicast forwarded per the RPF forwarding algorithm. Last hop routers directly connected to receivers may, at their discretion, join themselves to the source tree and prune themselves from the shared tree.
A single RP can be configured for multiple groups that are defined by an access list. If no RP is configured for a group, the router treats the group as dense using the PIM dense mode techniques. (You can prevent this occurrence by configuring the no ip pim dm-fallback command.)
If dynamic and static group-to-RP mappings are used together and there is an RP address conflict, the RP address configured for a static group-to-RP mapping (with the ip pim rp-address override command) will take precedence.
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SSM Overview
Source Specific Multicast (SSM). SSM is an extension of IP multicast where datagram traffic is forwarded to receivers from only those multicast sources that the receivers have explicitly joined. For multicast groups configured for SSM, only source-specific multicast distribution trees (not shared trees) are created.
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SSM Components
SSM is a datagram delivery model that best supports one-to-many applications, also known as broadcast applications. SSM is a core networking technology for the Cisco implementation of IP multicast solutions targeted for audio and video broadcast application environments and is described in RFC 3569. The following two Cisco IOS components together support the implementation of SSM:
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Protocol Independent Multicast (PIM) SSM, or PIM-SSM, is the routing protocol that supports the implementation of SSM and is derived from PIM sparse mode (PIM-SM). IGMP is the Internet Engineering Task Force (IETF) standards track protocol used for hosts to signal multicast group membership to routers. IGMP Version 3 supports source filtering, which is required for SSM. IGMP For SSM to run with IGMPv3, SSM must be supported in the Cisco IOS router, the host where the application is running, and the application itself.
How SSM Differs from Internet Standard Multicast
The standard IP multicast infrastructure in the Internet and many enterprise intranets is based on the PIM-SM protocol and Multicast Source Discovery Protocol (MSDP). These protocols have proved to be reliable, extensive, and efficient. However, they are bound to the complexity and functionality limitations of the Internet Standard Multicast (ISM) service model. For example, with ISM, the network must maintain knowledge about which hosts in the network are actively sending multicast traffic. With SSM, this information is provided by receivers through the source addresses relayed to the last-hop routers by IGMPv3. SSM is an incremental response to the issues associated with ISM and is intended to coexist in the network with the protocols developed for ISM. In general, SSM provides IP multicast service for applications that utilize SSM.
ISM service is described in RFC 1112. This service consists of the delivery of IP datagrams from any source to a group of receivers called the multicast host group. The datagram traffic for the multicast host group consists of datagrams with an arbitrary IP unicast source address S and the multicast group address G as the IP destination address. Systems will receive this traffic by becoming members of the host group. Membership in a host group simply requires signaling the host group through IGMP Version 1, 2, or 3.
In SSM, delivery of datagrams is based on (S, G) channels. Traffic for one (S, G) channel consists of datagrams with an IP unicast source address S and the multicast group address G as the IP destination address. Systems will receive this traffic by becoming members of the (S, G) channel. In both SSM and ISM, no signaling is required to become a source. However, in SSM, receivers must subscribe or unsubscribe to (S, G) channels to receive or not receive traffic from specific sources. In other words, receivers can receive traffic only from (S, G) channels to which they are subscribed, whereas in ISM, receivers need not know the IP addresses of sources from which they receive their traffic. The proposed standard approach for channel subscription signaling utilizes IGMP INCLUDE mode membership reports, which are supported only in IGMP Version 3.
SSM can coexist with the ISM service by applying the SSM delivery model to a configured subset of the IP multicast group address range. The Internet Assigned Numbers Authority (IANA) has reserved the address range from 232.0.0.0 through 232.255.255.255 for SSM applications and protocols. Cisco IOS software allows SSM configuration for an arbitrary subset of the IP multicast address range from 224.0.0.0 through 239.255.255.255. When an SSM range is defined, an existing IP multicast receiver application will not receive any traffic when it tries to use addresses in the SSM range unless the application is modified to use explicit (S, G) channel subscription or is SSM-enabled through a URL Rendezvous Directory (URD).
SSM Operations
An established network in which IP multicast service is based on PIM-SM can support SSM services. SSM can also be deployed alone in a network without the full range of protocols that are required for interdomain PIM-SM. That is, SSM does not require an RP, so there is no need for an RP mechanism such as Auto-RP, MSDP, or bootstrap router (BSR).
If SSM is deployed in a network that is already configured for PIM-SM, then only the last-hop routers must be upgraded to a Cisco IOS software image that supports SSM. Routers that are not directly connected to receivers do not have to upgrade to a Cisco IOS software image that supports SSM. In general, these non-last-hop routers must only run PIM-SM in the SSM range. They may need additional access control configuration to suppress MSDP signaling, registering, or PIM-SM shared-tree operations from occurring within the SSM range.
The SSM mode of operation is enabled by configuring the SSM range using the ip pim ssm global configuration command. This configuration has the following effects:
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IGMPv3 Host Signaling
IGMPv3 is the third version of the IETF standards track protocol in which hosts signal membership to last-hop routers of multicast groups. IGMPv3 introduces the ability for hosts to signal group membership that allows filtering capabilities with respect to sources. A host can signal either that it wants to receive traffic from all sources sending to a group except for some specific sources (a mode called EXCLUDE) or that it wants to receive traffic only from some specific sources sending to the group (a mode called INCLUDE).
IGMPv3 can operate with both ISM and SSM. In ISM, both EXCLUDE and INCLUDE mode reports are accepted by the last-hop router. In SSM, only INCLUDE mode reports are accepted by the last-hop router.
Benefits of Source Specific Multicast
IP Multicast Address Management
In the ISM service, applications must acquire a unique IP multicast group address because traffic distribution is based only on the IP multicast group address used. If two applications with different sources and receivers use the same IP multicast group address, receivers of both applications will receive traffic from the senders of both applications. Even though the receivers, if programmed appropriately, can filter out the unwanted traffic, this situation would cause generally unacceptable levels of unwanted traffic.
Allocating a unique IP multicast group address for an application is problematic. Most short-lived applications use mechanisms like Session Description Protocol (SDP) and Session Announcement Protocol (SAP) to get a random address, a solution that does not work well with a rising number of applications in the Internet. The best current solution for long-lived applications is described in RFC 2770, but this solution suffers from the restriction that each autonomous system is limited to only 255 usable IP multicast addresses.
In SSM, traffic from each source is forwarded among routers in the network independently of traffic from other sources. Thus different sources can reuse multicast group addresses in the SSM range.
Inhibition of Denial of Service Attacks
In SSM, multicast traffic from each individual source will be transported across the network only if it was requested (through IGMPv3 or URD memberships) from a receiver. In contrast, ISM forwards traffic from any active source sending to a multicast group to all receivers requesting that multicast group. In Internet broadcast applications, this ISM behavior is highly undesirable because it allows unwanted sources to easily disturb the actual Internet broadcast source by simply sending traffic to the same multicast group. This situation depletes bandwidth at the receiver side with unwanted traffic and thus disrupts the undisturbed reception of the Internet broadcast. In SSM, this type of denial-of-service (DoS) attack cannot be made by simply sending traffic to a multicast group.
Installation and Management
SSM is easy to install and provision in a network because it does not require the network to maintain information about which active sources are sending to multicast groups. This requirement exists in ISM (with IGMPv1, IGMPv2, or IGMPv3).
The current standard solutions for ISM service are PIM-SM and MSDP. Rendezvous point (RP) management in PIM-SM (including the necessity for Auto-RP or BSR) and MSDP is required only for the network to learn about active sources. This management is not necessary in SSM. SSM is therefore easier than ISM to install and manage and easier to operationally scale in deployment. Another factor that contributes to the ease of installation of SSM is the fact that it can leverage preexisting PIM-SM networks.
Internet Broadcast Applications
The three benefits listed above make SSM ideal for Internet broadcast-style applications for the following reasons:
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Bidir-PIM Overview
Bidir-PIM shares many of its shortest path tree (SPT) operations with PIM-SM. Bidir-PIM also has unconditional forwarding of source traffic toward the RP upstream on the shared tree, but has no registering process for sources as in PIM-SM. These modifications allow forwarding of traffic in all routers based solely on the (*, G) multicast routing entries. This form of forwarding eliminates any source-specific state and allows scaling capability to an arbitrary number of sources.
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Multicast Group Modes
In PIM, packet traffic for a multicast group is routed according to the rules of the mode configured for that multicast group. The Cisco IOS implementation of PIM supports four modes for a multicast group:
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A router can simultaneously support all four modes or any combination of them for different multicast groups.
Bidirectional Shared Tree
In bidirectional mode, traffic is routed only along a bidirectional shared tree that is rooted at the rendezvous point (RP) for the group. In bidir-PIM, the IP address of the RP acts as the key to having all routers establish a loop-free spanning tree topology rooted in that IP address. This IP address need not be a router, but can be any unassigned IP address on a network that is reachable throughout the PIM domain. This technique is the preferred configuration method for establishing a redundant RP configuration for bidir-PIM.
Membership in a bidirectional group is signaled by way of explicit Join messages. Traffic from sources is unconditionally sent up the shared tree toward the RP and passed down the tree toward the receivers on each branch of the tree.
Figure 3 and Figure 4 show the difference in state created per router for a unidirectional shared tree and source tree versus a bidirectional shared tree.
Figure 2 Unidirectional Shared Tree and Source Tree
Figure 3 Bidirectional Shared Tree
For packets that are forwarded downstream from the RP toward receivers, there are no fundamental differences between bidir-PIM and PIM-SM. Bidir-PIM deviates substantially from PIM-SM for traffic that is passed from sources upstream toward the RP.
PIM-SM cannot forward traffic in the upstream direction of a tree because it accepts traffic from only one Reverse Path Forwarding (RPF) interface. This interface (for the shared tree) points toward the RP, thus allowing only downstream traffic flow. Upstream traffic is first encapsulated into unicast register messages, which are passed from the designated router (DR) of the source toward the RP. Second, the RP joins an SPT that is rooted at the source. Therefore, in PIM-SM, traffic from sources destined for the RP does not flow upstream in the shared tree, but downstream along the SPT of the source until it reaches the RP. From the RP, traffic flows along the shared tree toward all receivers.
In bidir-PIM, the packet-forwarding rules have been improved over PIM-SM, allowing traffic to be passed up the shared tree toward the RP. To avoid multicast packet looping, bidir-PIM introduces a new mechanism called designated forwarder (DF) election, which establishes a loop-free SPT rooted at the RP.
DF Election
On every network segment and point-to-point link, all PIM routers participate in a procedure called designated forwarder (DF) election. The procedure selects one router as the DF for every RP of bidirectional groups. This router is responsible for forwarding multicast packets received on that network.
The DF election is based on unicast routing metrics. The router with the most preferred unicast routing metric to the RP becomes the DF. Use of this method ensures that only one copy of every packet will be sent to the RP, even if there are parallel equal-cost paths to the RP.
A DF is selected for every RP of bidirectional groups. As a result, multiple routers may be elected as DF on any network segment, one for each RP. Any particular router may be elected as DF on more than one interface.
Bidirectional Group Tree Building
The procedure for joining the shared tree of a bidirectional group is almost identical to that used in PIM-SM. One main difference is that, for bidirectional groups, the role of the DR is assumed by the DF for the RP.
On a network that has local receivers, only the router elected as the DF populates the outgoing interface list (olist) upon receiving Internet Group Management Protocol (IGMP) Join messages, and sends (*, G) Join and Leave messages upstream toward the RP. When a downstream router wishes to join the shared tree, the RPF neighbor in the PIM Join and Leave messages is always the DF elected for the interface that lead to the RP.
When a router receives a Join or Leave message, and the router is not the DF for the receiving interface, the message is ignored. Otherwise, the router updates the shared tree in the same way as in sparse mode.
In a network where all routers support bidirectional shared trees, (S, G) Join and Leave messages are ignored. There is also no need to send PIM assert messages because the DF election procedure eliminates parallel downstream paths from any RP. An RP never joins a path back to the source, nor will it send any register stops.
Packet Forwarding
A router creates (*, G) entries only for bidirectional groups. The olist of a (*, G) entry includes all the interfaces for which the router has been elected DF and that have received either an IGMP or PIM Join message. If a router is located on a sender-only branch, it will also create a (*, G) state, but the olist will not include any interfaces.
If a packet is received from the RPF interface toward the RP, the packet is forwarded downstream according to the olist of the (*, G) entry. Otherwise, only the router that is the DF for the receiving interface forwards the packet upstream toward the RP; all other routers must discard the packet.
Benefits of Bidirectional PIM
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How to Configure Basic IP Multicast
The tasks described in this section configure the basic IP multicast modes. No single task in this section is required; however, at least one of the tasks must be performed to configure IP multicast in a network. More than one of the tasks may be needed. This section contains the following tasks:
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Configuring Sparse Mode with Auto-RP
This section contains information about and instructions on how to configure auto-rendezvous point (Auto-RP). Auto-RP can also be optionally used with anycast RP, which is described in the "Configuring Sparse Mode with Anycast RP"section.
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What to Do Next
Proceed to the "Verifying IP Multicast Operation" module.
Configuring Sparse Mode with Anycast RP
This section describes how to configure sparse mode with anycast RP for RP redundancy.
Anycast RPs are configured statically, and interfaces are configured to operate in Protocol Independent Multicast-Sparse Mode (PIM-SM). In an anycast RP configuration, two or more RPs are configured with the same IP address on loopback interfaces. The Anycast RP loopback address should be configured with a 32-bit mask, making it a host address. An Anycast RP configuration is easy to configure and troubleshoot because the same host address is used as the RP address regardless of which router it is configured on.
Anycast RP allows two or more rendezvous points (RPs) to share the load for source registration and have the ability to act as hot backup routers for each other. Multicast Source Discovery Protocol (MSDP) is the key protocol that makes anycast RP possible.
Figure 4 MSDP Sharing Source Information Between RPs in Each Domain
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What to Do Next
Proceed to the "Verifying IP Multicast Operation" module.
Configuring Sparse Mode with a Bootstrap Router
This section describes how to configure a bootstrap router (BSR), which provides a fault-tolerant, automated RP discovery and distribution mechanism so that routers learn the group-to-RP mappings dynamically.
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Step 1
enable
Example:Router> enable
Enables privileged EXEC mode.
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Step 2
configure terminal
Example:Router# configure terminal
Enters global configuration mode.
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ip multicast-routing [distributed]
Example:Router(config)# ip multicast-routing
Enables IP multicast routing.
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interface type number
Example:Router(config)# interface ethernet 1
Selects an interface that is connected to hosts on which PIM can be enabled.
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ip pim sparse-mode
Example:Router(config-if)# ip pim sparse-mode
Enables sparse mode.
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end
Example:Router(config-if)# end
Returns to global configuration mode.
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Repeat Steps 1 through 6 on every multicast-enabled interface on every router.
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ip pim bsr-candidate interface-type interface-number [hash-mask-length [priority]]
Example:Router(config)# ip pim bsr-candidate ethernet 1 0 192
Configures the router to announce its candidacy as a bootstrap router (BSR).
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ip pim rp-candidate interface-type interface-number [group-list access-list] [interval seconds] [priority value]
Example:Router(config)# ip pim rp-candidate ethernet 2 group-list 4 priority 192
Configures the router to advertise itself as a PIM Version 2 candidate RP to the BSR.
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Step 10
Repeat Step 8 and Step 9 on all RP and BSR routers.
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interface type number
Example:Router(config)# interface ethernet 1
Selects an interface that is connected to hosts on which PIM can be enabled.
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ip pim bsr-border
Example:Router(config-if)# ip pim bsr-border
Prevents the bootstrap router (BSR) messages from being sent or received through an interface.
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end
Example:Router(config-if)# end
Ends the current configuration session and returns to privileged EXEC mode.
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Repeat Steps 11 through 13 on all the routers that have boundary interfaces where the messages should not be sent or received.
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show ip pim rp [mapping] [rp-address]
Example:Router# show ip pim rp
(Optional) Displays active rendezvous points (RPs) that are cached with associated multicast routing entries.
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show ip pim rp-hash [group-address] [group-name]
Example:Router# show ip pim rp-hash 239.1.1.1
(Optional) Displays which rendezvous point (RP) is being selected for a specified group.
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show ip pim bsr-router
Example:Router# show ip pim bsr-router
(Optional) Displays the bootstrap router (BSR) information.
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show ip igmp groups [group-name | group-address | interface-type interface-number] [detail]
Example:Router# show ip igmp groups
(Optional) Displays the multicast groups having receivers that are directly connected to the router and that were learned through IGMP.
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show ip mroute
Example:Router# show ip mroute cbone-audio
(Optional) Displays the contents of the IP mroute table.
What to Do Next
Proceed to the "Verifying IP Multicast Operation" module.
Configuring Sparse Mode with a Single Static RP
A rendezvous point (RP) is required in networks running Protocol Independent Multicast sparse mode (PIM-SM). In PIM-SM, traffic will be forwarded only to network segments with active receivers that have explicitly requested multicast data.
This section describes how to configure sparse mode with a single static RP.
Prerequisites
All access lists that are needed when sparse mode is configured with a single static RP should be configured prior to beginning the configuration task.
Restrictions
The same RP address cannot be used for both bidirectional and sparse mode PIM groups.
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What to Do Next
Proceed to the "Verifying IP Multicast Operation" module.
Configuring Source Specific Multicast
This section describes how to configure Source Specific Multicast (SSM).
Prerequisites
If you want to use an access list to define the SSM range, configure the access list before you reference the access list in the ip pim ssm command.
Restrictions
Address Management Restrictions
Address management is necessary to some degree when SSM is used with Layer 2 switching mechanisms. Cisco Group Management Protocol (CGMP), IGMP snooping, and Router-Port Group Management Protocol (RGMP) currently support only group-specific filtering, not (S, G) channel-specific filtering. If different receivers in a switched network request different (S, G) channels sharing the same group, they will not benefit from these existing mechanisms. Instead, both receivers will receive all (S, G) channel traffic (and filter out the unwanted traffic on input). Because SSM can reuse the group addresses in the SSM range for many independent applications, this situation can lead to unexpected traffic filtering in a switched network. It is therefore important to follow the recommendations set forth in the IETF drafts for SSM in regard to using random IP addresses in the SSM range to minimize the chance for reuse of a single address by different applications. For example, an application service providing a set of television channels should, even with SSM, use a different group for each television (S, G) channel. This setup guarantees that multiple receivers to different channels within the same application service will never experience traffic aliasing in networks that include Layer 2 switches.
IGMP Snooping and CGMP Limitations
IGMPv3 uses new membership report messages that may not be recognized correctly by older IGMP snooping switches, in which case hosts will not receive traffic properly. IGMP uses a new link-local address for the destination of these messages. This new link-local address is 224.0.0.22.
State Maintenance Limitations
In PIM-SSM, the last-hop router will continue to send (S, G) Join messages periodically if appropriate (S, G) subscriptions are on the interfaces. Therefore, as long as receivers send (S, G) subscriptions, the shortest path tree (SPT) state from the receivers to the source will be maintained, even if the source is not sending traffic for longer periods of time (or never sends).
This case is opposite to that of PIM-SM, in which the (S, G) state is maintained only if the source is sending traffic and receivers are joining the group. If a source stops sending traffic for more than 3 minutes in PIM-SM, the (S, G) state is deleted and will be reestablished only after packets from the source arrive again through the RPT. Because no mechanism in PIM-SSM notifies a receiver that a source is active, the network must maintain the (S, G) state in PIM-SSM as long as receivers are requesting receipt of that channel.
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What to Do Next
Proceed to the "Verifying IP Multicast Operation" module.
Configuring Bidirectional PIM
This section describes how to configure bidirectional PIM (bidir-PIM).
Prerequisites
All access lists needed when configuring bidirectional PIM must be configured prior to beginning the configuration task.
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Configuration Examples for Basic IP Multicast
This section contains the following examples:
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Example: Sparse Mode with Auto-RP
The following example configures sparse mode with Auto-RP:
ip multicast-routingip pim autorp listenerip pim send-rp-announce Loopback0 scope 16 group-list 1ip pim send-rp-discovery Loopback1 scope 16access-list 1 permit 239.254.2.0 0.0.0.255access-list 1 permit 239.254.3.0 0.0.0.255...access-list 10 permit 224.0.1.39access-list 10 permit 224.0.1.40access-list 10 permit 239.254.2.0 0.0.0.255access-list 10 permit 239.254.3.0 0.0.0.255Example: Sparse Mode with Anycast RP
The main purpose of an Anycast RP implementation is that the downstream multicast routers will have just one address for an RP. The example given in Figure 5 shows how loopback interface 0 of the RPs (RP1 and RP2) is configured with the 10.0.0.1 IP address. If this 10.0.0.1 address is configured on all RPs as the address for loopback interface 0 and then configured as the RP address, IP routing will converge on the closest RP. This address must be a host route; note the 255.255.255.255 subnet mask.
The downstream routers must be informed about the 10.0.0.1 RP address. In Figure 5, the routers are configured statically with the ip pim rp-address 10.0.0.1 global configuration command. This configuration could also be accomplished using the Auto-RP or bootstrap router (BSR) features.
The RPs in Figure 5 must also share source information using MSDP. In this example, loopback interface 1 of the RPs (RP1 and RP2) is configured for MSDP peering. The MSDP peering address must be different from the anycast RP address.
Figure 5 AnyCast RP Configuration
Many routing protocols choose the highest IP address on loopback interfaces for the router ID. A problem may arise if the router selects the anycast RP address for the router ID. It is recommended that you avoid this problem by manually setting the router ID on the RPs to the same address as the MSDP peering address (for example, the loopback 1 address in Figure 5). In Open Shortest Path First (OSPF), the router ID is configured using the router-id router configuration command. In Border Gateway Protocol (BGP), the router ID is configured using the bgp router-id router configuration command. In many BGP topologies, the MSDP peering address and the BGP peering address must be the same in order to pass the RPF check. The BGP peering address can be set using the neighbor update-source router configuration command.
The anycast RP example above uses IP addresses taken from RFC 1918. These IP addresses are normally blocked at interdomain borders and therefore are not accessible to other ISPs. You must use valid IP addresses if you want the RPs to be reachable from other domains.
The following example shows how to perform an Anycast RP configuration.
On RP 1
ip pim rp-address 10.0.0.1interface loopback 0ip address 10.0.0.1 255.255.255.255!interface loopback 1ip address 10.1.1.1. 255.255.255.255!ip msdp peer 10.1.1.2 connect-source loopback 1ip msdp originator-id loopback 1On RP 2
ip pim rp-address 10.0.0.1interface loopback 0ip address 10.0.0.1 255.255.255.255interface loopback 1ip address 10.1.1.2. 255.255.255.255!ip msdp peer 10.1.1.1 connect-source loopback 1ip msdp originator-id loopback 1All Other Routers
ip pim rp-address 10.0.0.1no ip pim dm-fallbackExample: Sparse Mode with Bootstrap Router
The following example is a configuration for a candidate BSR, which also happens to be a candidate RP:
!ip multicast-routing!interface Ethernet0ip address 172.69.62.35 255.255.255.240ip pim sparse-mode!interface Ethernet1ip address 172.21.24.18 255.255.255.248ip pim sparse-mode!interface Ethernet2ip address 172.21.24.12 255.255.255.248ip pim sparse-mode!ip pim bsr-candidate Ethernet2 30 10ip pim rp-candidate Ethernet2 group-list 5access-list 5 permit 239.255.2.0 0.0.0.255Example: BSR and RFC 2362 Interoperable Candidate RP
When Cisco and non-Cisco routers are being operated in a single PIM domain with PIM Version 2 BSR, care must be taken when configuring candidate RPs because the Cisco IOS implementation of the BSR RP selection is not fully compatible with RFC 2362.
RFC 2362 specifies that the BSR RP be selected as follows (RFC 2362, 3.7):
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Cisco routers always select the candidate RP based on the longest match on the announced group address prefix before selecting an RP based on priority, hash function, or IP address.
Inconsistent candidate RP selection between Cisco and non-Cisco RFC 2362-compliant routers in the same domain if multiple candidate RPs with partially overlapping group address ranges are configured can occur. Inconsistent candidate RP selection can prevent connectivity between sources and receivers in the PIM domain. A source may register with one candidate RP and a receiver may connect to a different candidate RP even though it is in the same group.
The following example shows a configuration that can cause inconsistent RP selection between a Cisco and a non-Cisco router in a single PIM domain with PIM Version 2 BSR:
access-list 10 permit 224.0.0.0 7.255.255.255ip pim rp-candidate ethernet1 group-list 10 priority 20access-list 20 permit 224.0.0.0 15.255.255.255ip pim rp-candidate ethernet2 group-list 20 priority 10In this example, a candidate RP on Ethernet interface 1 announces a longer group prefix of 224.0.0.0/5 with a lower priority of 20. The candidate RP on Ethernet interface 2 announces a shorter group prefix of 224.0.0.0/4 with a higher priority of 10. For all groups that match both ranges a Cisco router will always select the candidate RP on Ethernet interface 1 because it has the longer announced group prefix. A non-Cisco fully RFC 2362-compliant router will always select the candidate RP on Ethernet interface 2 because it is configured with a higher priority.
To avoid this interoperability issue, do not configure different candidate RPs to announce partially overlapping group address prefixes. Configure any group prefixes that you want to announce from more than one candidate RP with the same group prefix length.
The following example shows how to configure the previous example so that there is no incompatibility between a Cisco router and a non-Cisco router in a single PIM domain with PIM Version 2 BSR:
access-list 10 permit 224.0.0.0 7.255.255.255ip pim rp-candidate ethernet1 group-list 10 priority 20access-list 20 permit 224.0.0.0 7.255.255.255access-list 20 permit 232.0.0.0 7.255.255.255ip pim rp-candidate ethernet2 group-list 20 priority 10In this configuration the candidate RP on Ethernet interface 2 announces group address 224.0.0.0/5 and 232.0.0.0/5 which equal 224.0.0.0/4, but gives the interface the same group prefix length (5) as the candidate RP on Ethernet 1. As a result, both a Cisco router and an RFC 2362-compliant router will select the RP Ethernet interface 2.
Example: Sparse Mode with a Single Static RP
The following example shows how to set the PIM RP address to 192.168.0.1 for all multicast groups (224/4) and defines all groups to operate in sparse mode:
ip pim rp-address 192.168.0.1The following example shows how to set the PIM RP address to 172.16.0.2 for the multicast range 239/8:
access list 10 239.0.0.0 0.255.255.255ip pim rp-address 172.16.0.2 10Example: SSM with IGMPv3
The following example shows how to configure a router (running IGMPv3) for SSM:
ip multicast-routing!interface Ethernet3/1ip address 172.21.200.203 255.255.255.0description backbone interfaceip pim sparse-mode!interface Ethernet3/2ip address 131.108.1.2 255.255.255.0ip pim sparse-modedescription ethernet connected to hostsip igmp version 3!ip pim ssm defaultExample: SSM Filtering
The following example shows how to configure filtering on legacy RP routers running Cisco IOS software releases that do not support SSM routing. This filtering will suppress all unwanted PIM-SM and MSDP traffic in the SSM range. Without this filtering, SSM will still operate, but there may be additional RPT traffic if legacy first-hop and last-hop routers exist in the network.
ip access-list extended no-ssm-rangedeny ip any 232.0.0.0 0.255.255.255permit ip any any! Deny sources registering in SSM rangeip pim accept-register list no-ssm-rangeip access-list extended msdp-nono-listdeny ip any 232.0.0.0 0.255.255.255permit ip any any! Filter generated SA messages in SSM range. This configuration is needed only if there! are sources directly connected to this router. The ip pim accept-register command! filters remote sources. See /en/US/tech/tk828/technologies_tech_note09186a0080093fda.shtml for other SA! messages that typically need to be filtered.ip msdp redistribute list msdp-nono-list! Filter received SA messages in SSM range. "Filtered on receipt" means messages are! neither processed nor forwarded. This filter needs to be configured for each MSDP peer.ip msdp sa-filter in msdp-peer1 list msdp-nono-list...ip msdp sa-filter in msdp-peerN list msdp-nono-listExample: Bidir-PIM
By default, a bidirectional RP advertises all groups as bidirectional. An access list on the RP can be used to specify a list of groups to be advertised as bidirectional. Groups with the deny keyword will operate in dense mode. A different, nonbidirectional RP address is required for groups that operate in sparse mode because a single access list only allows either a permit or deny keyword.
The following example shows how to configure an RP for both sparse mode and bidirectional mode groups. The groups identified as 224/8 and 227/8 are bidirectional groups, and 226/8 is a sparse mode group. The RP must be configured to use different IP addresses for the sparse mode and bidirectional mode operations. Two loopback interfaces are used to allow this configuration. The addresses of these loopback interfaces must be routed throughout the PIM domain in such a way that the other routers in the PIM domain can communicate with the RP.
ip multicast-routing!...!interface loopback 0description One loopback address for this router's Bidir Mode RP functionip address 10.0.1.1 255.255.255.0!interface loopback 1description One loopback address for this router's Sparse Mode RP functionip address 10.0.2.1 255.255.255.0!...!ip pim bidir-enableip pim rp-address 10.0.1.1 45 bidirip pim rp-address 10.0.2.1 46!access-list 45 permit 224.0.0.0 0.255.255.255access-list 45 permit 227.0.0.0 0.255.255.255access-list 46 permit 226.0.0.0 0.255.255.255Additional References
Related Documents
IP multicast commands: complete command syntax, command mode, command history, defaults, usage guidelines, and examples
Standards
MIBs
RFCs
Technical Assistance
Feature Information for Configuring Basic IP Multicast
Table 1 lists the features in this module and provides links to specific configuration information.
For information on a feature in this technology that is not documented here, see the "IP Multicast Features Roadmap."
Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Note
Table 1 Feature Information for Configuring Basic IP Multicast
PIM Dense Mode Fallback Prevention in a Network Following RP Information Loss
12.3(4)T
12.0(28)S
12.2(33)SRA
12.2(33)SXH
15.0(1)SThe PIM Dense Mode Fallback Prevention in a Network Following RP Information Loss feature enables you to prevent PIM-DM fallback when all RPs fail. Preventing the use of dense mode is very important to multicast networks whose reliability is critical. This feature provides a mechanism to keep the multicast groups in sparse mode, thereby preventing dense mode flooding.
The following sections provide information about this feature:
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The following command was introduced by this feature:
ip pim dm-fallback.Source Specific Multicast (SSM)
12.3(4)T
12.2(25)S
12.0(28)S
12.2(33)SXH
12.2(33)SRA
15.0(1)S
Cisco IOS XE 3.1.0SGSSM is an extension of IP multicast where datagram traffic is forwarded to receivers from only those multicast sources that the receivers have explicitly joined. For multicast groups configured for SSM, only source-specific multicast distribution trees (not shared trees) are created.
The following sections provide information about this feature:
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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.
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