Cisco IOS Software Releases 12.3 T

Table Of Contents

PIM Dense Mode Fallback Prevention in a Network Following RP Information Loss

Contents

Information About PIM Dense Mode Fallback Prevention in a Network Following RP Information Loss

PIM Sparse Mode and PIM Dense Mode

Auto-RP Overview

PIM Dense Mode Fallback

Cause and Effect of Dense Mode Fallback

Effects of Preventing Dense Mode Fallback

Benefits of Preventing PIM Dense Mode Fallback in a Network Following RP Information Loss

How to Prevent PIM Dense Mode Fallback in a Network Following RP Information Loss

Preventing Dense Mode Fallback

Configuration Example for PIM Dense Mode Fallback Prevention in a Network Following RP Information Loss

Preventing PIM Dense Mode Fallback: Example

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Command Reference

ip pim dm-fallback

Glossary

PIM Dense Mode Fallback Prevention in a Network Following RP Information Loss

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The PIM Dense Mode Fallback Prevention in a Network Following RP Information Loss feature enables you to prevent Protocol Independent Multicast (PIM) dense mode (DM) fallback when all rendezvous points (RPs) fail. Preventing the use of PIM-DM is very important to multicast networks whose reliability is critical. The feature provides a mechanism to keep the multicast groups in sparse mode. The feature also allows you to block multicast traffic for groups not specifically configured.

Release	Modification
12.3(4)T	This feature was introduced.
12.0(28)S	This feature was integrated into Cisco IOS Release 12.0(28)S.
12.2(25)S	This feature was integrated into Cisco IOS Release 12.2(25)S.

Feature History for the PIM Dense Mode Fallback Prevention in a Network Following RP Information Loss Feature

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Contents

•Information About PIM Dense Mode Fallback Prevention in a Network Following RP Information Loss

•How to Prevent PIM Dense Mode Fallback in a Network Following RP Information Loss

•Configuration Example for PIM Dense Mode Fallback Prevention in a Network Following RP Information Loss

•Additional References

•Command Reference

•Glossary

Information About PIM Dense Mode Fallback Prevention in a Network Following RP Information Loss

To configure the PIM Dense Mode Fallback Prevention in a Network Following RP Information Loss feature, you should understand the following concepts:

•PIM Sparse Mode and PIM Dense Mode

•Auto-RP Overview

•PIM Dense Mode Fallback

•Benefits of Preventing PIM Dense Mode Fallback in a Network Following RP Information Loss

PIM Sparse Mode and PIM Dense Mode

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 can operate in dense mode or sparse mode. The router can 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.

Auto-RP Overview

If you configure PIM to operate in sparse mode, you must also choose one or more routers to be rendezvous points (RPs). RPs are used by senders to a multicast group to announce their existence and by receivers of multicast packets to learn about new senders.

Auto-RP is a feature that automates the distribution of group-to-RP mappings in a PIM network. Auto-RP has the following benefits:

•Configuring the use of multiple RPs within a network to serve different group ranges is easy.

•Auto-RP allows load splitting among different RPs and arrangement of RPs according to the location of group participants.

•Auto-RP avoids inconsistent, manual RP configurations that can cause connectivity problems.

Multiple RPs can be used to serve different group ranges or serve as backups to each other. For Auto-RP to 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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Note If you configure PIM in sparse mode or sparse-dense mode and do not configure Auto-RP, you must statically configure an RP.

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Note If router interfaces are configured in sparse mode, Auto-RP can still be used if all routers are configured with a static RP address for the Auto-RP groups.

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PIM Dense Mode Fallback

If you use IP multicast in mission-critical networks, you should avoid the use of PIM-DM (dense mode).

Dense mode fallback describes the event of the PIM mode changing (falling back) from sparse mode (which requires an RP) to dense mode (which does not use an RP). Dense mode fallback occurs when RP information is lost.

By default, if all interfaces in a multicast VPN routing or forwarding instance are configured with the ip pim sparse-mode command, there is no dense mode fallback because dense mode groups cannot be formed over interfaces configured for sparse mode.

Cause and Effect of Dense Mode Fallback

PIM determines whether a multicast group operates in PIM-DM or PIM sparse-dense mode based solely on the existence of RP information in the group-to-RP mapping cache. If Auto-RP is configured or a bootstrap router (BSR) is used to distribute RP information, there is a risk that RP information can be lost if all RPs, Auto-RP, or the BSR for a group fails due to network congestion. This failure can lead to the network either partially or fully falling back into PIM-DM.

If a network falls back into PIM-DM, and if interfaces are configured for Auto-RP, dense mode flooding will occur. Routers that lose RP information will switch all existing states into dense mode and any new states that must be created for the failed group will be created in dense mode.

Effects of Preventing Dense Mode Fallback

The PIM Dense Mode Fallback Prevention in a Network Following RP Information Loss feature provides a method for doing the following:

•Preventing dense mode fallback.

•Blocking multicast traffic for groups not specifically configured. When a group is not specifically configured (that is, there is no RP for that group), multicast traffic does not flow across the network. The first hop router that receives data for such a group creates a (*, G) entry for that group with an RP address of 0.0.0.0 and drops these packets. There are no (S, G) entries created on the routers.

When the feature is configured, sparse mode groups operate with an RP address of 0.0.0.0, which causes the following conditions to apply:

•(S, G) state is maintained unchanged and existing flows are unchanged.

•No PIM join or prune messages for (*, G) or (S, G, RPbit) are sent.

•Received (*, G) or (S, G RPbit) joins or prune messages are ignored.

•No registers are sent and traffic at the first hop is dropped.

•Received registers are answered with register stop.

•Asserts are unchanged.

•The (*, G) outgoing interface (OIF) list is maintained only for the Internet Group Management Protocol (IGMP) state.

•Multicast Source Discovery Protocol (MSDP) source active (SA) messages for RP 0.0.0.0 groups are still accepted and forwarded.

•IGMP messages are processed as for any other PIM-SM groups. IGMPv3 (S, G) memberships are expanded into (*, G) IGMP memberships.

•Any group for which no mode information is available (such as PIM sparse mode, bidirectional PIM, or Source Specific Multicast [SSM]) are created in a default mode in which no traffic is forwarded, and no PIM states are established in the network apart from the network first hop.

Benefits of Preventing PIM Dense Mode Fallback in a Network Following RP Information Loss

The benefits of preventing PIM dense mode fallback are the following:

•You can block multicast traffic for groups not specifically configured. When a group is not specifically configured (that is, there is no RP for that group), multicast traffic does not flow across the network. The first hop router that receives data for such a group creates a (*, G) entry for that group with an RP address of 0.0.0.0 and drops these packets. There are no (S, G) entries created on the routers.

•If interfaces are configured with ip pim sparse-dense-mode (such as when Auto-RP is in use), fallback into PIM dense mode can cause undesirable dense mode flooding of multicast packets and outages of multicast traffic flows. These events can be avoided by preventing dense mode fallback.

How to Prevent PIM Dense Mode Fallback in a Network Following RP Information Loss

To prevent PIM dense mode fallback in a network after the loss of RP information, perform the following task:

•Preventing Dense Mode Fallback (required)

Preventing Dense Mode Fallback

By default, PIM dense mode fallback is enabled. That is, a multicast group in the absence of rendezvous point (RP) information will fall to dense mode, regardless of the interface mode configuration.

However, if all of the interfaces in a VRF are configured with PIM sparse mode, no dense mode fallback is achieved by default. If all of the interfaces are already configured as sparse, even though the group mode falls to dense mode, the traffic does not get flooded (due to the sparse characteristic of the interface). But the established flows might be torn down and the state of the network could become indeterministic. The main advantage of no dense mode fallback in this case would be deterministic behavior.

If you have interfaces that are configured with PIM sparse-dense mode, you can disable dense mode fallback by configuring the no ip pim dm-fallback command in global configuration mode.

SUMMARY STEPS

1. enable

2. configure terminal

3. ip multicast-routing

4. interface type slot/port

5. ip pim sparse-mode

or

ip pim sparse-dense-mode

6. exit

7. ip pim send-rp-announce type number scope ttl-value [group-list access-list] [interval seconds]

8. ip pim rp-address rp-address [access-list] [override]

9. no ip pim dm-fallback

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. •Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	ip multicast-routing Example: Router(config)# ip multicast-routing	Enables IP multicast routing.
Step 4	interface type slot/port Example: Router(config)# interface ethernet 0/0	Specifies an interface and places the router in interface configuration mode.
Step 5	ip pim sparse-mode Example: Router(config-if)# ip pim sparse-mode or ip pim sparse-dense-mode Example: Router(config-if)# ip pim sparse-dense-mode	(Optional) Enables PIM sparse mode on the interface. •Enabling PIM sparse mode on all interfaces ensures that dense mode fallback does not occur. Do not perform Step 9. or (Optional) Enables PIM sparse-dense mode on the interface. •Enabling PIM sparse-dense mode causes dense-mode fallback to occur by default. To prevent it, you must perform Step 9.
Step 6	exit Example: Router(config-if)# exit	Exits interface configuration mode.
Step 7	ip pim send-rp-announce type number scope ttl-value [group-list access-list] [interval seconds] Example: Router(config)# ip pim send-rp-announce ethernet 0 scope 16 group-list 1	Configures a router to be the RP.
Step 8	ip pim rp-address rp-address [access-list] [override] Example: Router(config)# ip pim rp-address 10.8.0.20 1	Assigns an RP to multicast groups.
Step 9	no ip pim dm-fallback Example: Router(config)# no ip pim dm-fallback	Prevents PIM dense mode fallback.

Configuration Example for PIM Dense Mode Fallback Prevention in a Network Following RP Information Loss

This section contains the following configuration example:

•Preventing PIM Dense Mode Fallback: Example

Preventing PIM Dense Mode Fallback: Example

The following example disables PIM dense mode fallback:

ip multicast-routing

interface ethernet 0/0

ip pim sparse-dense-mode

ip pim send-rp-announce ethernet 0 scope 16 group-list 1 

ip pim rp-address 10.8.0.20 1

no ip pim dm-fallback

Additional References

The following sections provide references related to the PIM Dense Mode Fallback Prevention in a Network Following RP Information Loss feature.

Related Documents

Related Topic	Document Title
Cisco IOS IP multicast commands	Cisco IOS IP Command Reference, Volume 3 of 4: Multicast, Release 12.3 T
Cisco IOS IP multicast configuration	"IP Multicast" part of the Cisco IOS IP Configuration Guide

Standards

Standards	Title
No new or modified standards are supported by this feature, and support for existing standards have not been modified by this feature.	—

MIBs

MIBs	MIBs Link
No new or modified MIBs are supported by this feature, and support for existing MIBs have not been modified by this feature.	To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL: http://www.cisco.com/go/mibs

RFCs

RFCs	Title
No new or modified RFCs are supported by this feature, and support for existing RFCs have not been modified by this feature.	—

Technical Assistance

Description	Link
Technical Assistance Center (TAC) home page, containing 30,000 pages of searchable technical content, including links to products, technologies, solutions, technical tips, and tools. Registered Cisco.com users can log in from this page to access even more content.	http://www.cisco.com/public/support/tac/home.shtml

Command Reference

This section documents the following new command. All other commands used with this feature are documented in the Cisco IOS Release 12.3 T command reference publications.

•ip pim dm-fallback

ip pim dm-fallback

To enable Protocol Independent Multicast (PIM) dense mode fallback, use the ip pim dm-fallback command in global configuration mode. To prevent PIM dense mode fallback and to block all multicast traffic for groups not specifically configured, use the no form of this command.

ip pim dm-fallback

no ip pim dm-fallback

Syntax Description

This command has no arguments or keywords.

Defaults

PIM dense mode fallback is enabled. That is, a multicast group in the absence of rendezvous point (RP) information will fall to dense mode, regardless of the interface mode configuration.

Command Modes

Global configuration

Command History

Release	Modification
12.3(4)T	This command was introduced.
12.0(28)S	This command was implemented in Cisco IOS Release 12.0(28)S.
12.2(25)S	This command was implemented in Cisco IOS Release 12.2(25)S.

Usage Guidelines

Use this command to prevent a router from falling back into PIM dense mode when the RP becomes unavailable. This command also causes the router to block all multicast traffic for groups not specifically configured with an RP.

When IP multicast is used in mission-critical networks, you should avoid the use of PIM-DM. PIM makes the determination as to whether a multicast group operates in PIM-DM or PIM sparse-dense mode based solely on the existence of RP information in the group-to-RP mapping cache. If Auto-RP is configured or a bootstrap router (BSR) is used to distribute RP information, there is a risk that RP information can be lost if all RPs, Auto-RP, or the BSR for a group fails due to network congestion. This failure can lead to the network either partially or fully falling back into PIM-DM.

If a network falls back into PIM-DM, dense mode flooding will occur. Routers that lose RP information will switch all existing states into dense mode and any new states that must be created for the failed group will be created in dense mode.

Examples

The following example disables PIM dense mode fallback:

ip multicast-routing

interface ethernet 0/0

ip pim sparse-dense-mode

ip pim send-rp-announce ethernet 0 scope 16 group-list 1 

ip pim rp-address 10.8.0.20 1

no ip pim dm-fallback

Related Commands

Command	Description
ip pim dense-mode	Enables PIM dense mode on the interface.
ip pim sparse-dense-mode	Enables PIM to operate in sparse or dense mode, depending on the group.

Glossary

PIM—Protocol Independent Multicast. Multicast routing architecture that allows the addition of IP multicast routing on existing IP networks. PIM is unicast routing protocol-independent and can be operated in two modes: dense and sparse.

PIM dense mode—One of the two PIM operational modes. PIM dense mode is data-driven and resembles typical multicast routing protocols. Packets are forwarded on all outgoing interfaces until pruning and truncation occurs. In dense mode, receivers are densely populated, and it is assumed that the downstream networks want to receive and will probably use the datagrams that are forwarded to them. The cost of using dense mode is its default flooding behavior. Sometimes called dense mode PIM or PIM-DM.

PIM sparse mode—One of the two PIM operational modes. PIM sparse mode tries to constrain data distribution to a minimal number of routers in the network. Packets are sent only if they are explicitly requested at the rendezvous point. In sparse mode, receivers are widely distributed, and the assumption is that downstream networks will not necessarily use the datagrams that are sent to them. The cost of using sparse mode is its reliance on the periodic refreshing of explicit join messages and its need for RPs. Sometimes called sparse mode PIM or PIM-SM.

RP—rendezvous point. Router specified in PIM sparse mode implementations to track membership in multicast groups and to forward messages to known multicast group addresses.

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Note Refer to Internetworking Terms and Acronyms for terms not included in this glossary.

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