IP Multicasting: Explaining Multicast

© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicBSCI Module 7 Lesson 1 1

BSCI Module 7 Lesson 1

IP Multicasting: Explaining Multicast


Objectives

Describe the IP multicast group.

Compare and contrast Unicast packets and multicast packets.

List the advantages and disadvantages of multicast traffic.

Discuss two types of multicast applications.

Describe the types of IP multicast addresses.

Describe how receivers can learn about a scheduled multicast session.


Multicast Overview


IP Multicast

Distribute information to large audiences over an IP network


Why Multicast?

Used when sending same data to multiple receivers

Better bandwidth utilization

Less host/router processing

Used when addresses of receivers unknown

Used when simultaneous delivery for a group of receivers is required (simulcast)


Unicast vs. Multicast


Multicast AdvantagesEnhanced efficiency: Controls network traffic and reduces server and CPU loads

Optimized performance: Eliminates traffic redundancy

Distributed applications: Makes multipoint applications possible


Types of Multicast Applications

One-to-many

A single host sending to two or more (n) receivers

Many-to-many

Any number of hosts sending to the same multicast group; hosts are also members of the group (sender = receiver)

Many-to-one

Any number of receivers sending data back to a source (via unicast or multicast)


Corporate Broadcasts

Distance Learning

Training

Videoconferencing

Whiteboard/Collaboration

Multicast File TransferData and File Replication

Real-Time Data Delivery—FinancialVideo-on-Demand

Live TV and Radio Broadcast to the Desktop

IP Multicast Applications


Self Check

1. List some advantages of multicast transmission over unicast transmission.

2. How does the best effort delivery nature of UDP impact multicast transmissions?

3. What are the 3 basic types of multicast applications?

4. Give examples of one-to-many.

5. What model is used when a host can be a sender as well as a receiver simultaneously?


Multicast Addressing


IP Multicast Address Structure

IP group addresses:

Class D address (high-order three bits are set)

Range from 224.0.0.0 through 239.255.255.255


Multicast Addressing

IPv4 Header

Options Padding

Time to Live Protocol Header Checksum

Identification Flags Fragment Offset

Version IHL Type of Service Total Length

Source Address

Destination AddressDestination

Source

Source Address can never be

Source Address can never be

Class D Multicast Group Address

Class D Multicast Group Address

224.0.0.0 - 239.255.255.255 (Class D) Multicast Group Address RangeDestination

1.0.0.0 - 223.255.255.255 (Class A, B, C)Source


IP Multicast Address Groups

Local scope addresses224.0.0.0 to 224.0.0.255

Global scope addresses224.0.1.0 to 238.255.255.255

Administratively scoped addresses (private)239.0.0.0 to 239.255.255.255


Local Scope Addresses

Well-known addresses assigned by IANA

Reserved use: 224.0.0.0 through 224.0.0.255224.0.0.1 (all multicast clients on subnet)

224.0.0.2 (all routers on subnet)

224.0.0.4 (all DVMRP routers)

224.0.0.13 (all PIMv2 routers)

224.0.0.5, 224.0.0.6, 224.0.0.9, and 224.0.0.10 used by unicast routing protocols


IANA Ethernet MAC Address Range

01-00-5e-00-00-00

through

01-00-5e-7f-ff-ff

Available range of MAC addresses for IP multicast

Because the 7th hexadecimal character is 0 – 7, the 25th bitwill always be 0.

00000001:00000000:01011110:00000000:00000000:00000000


00000001:00000000:01011110:00000000:00000000:00000000

IANA Ethernet MAC Address Range

through

Within this range, these MAC addresses have the first 25 bits in common.

The remaining 23 bits are available for mapping to the lower 23 bits of the IP multicast group address.

Available range of MAC addresses for IP multicast

00000001:00000000:01011110:01111111:11111111:11111111


Multicast IP Address to MAC Address


224.1.1.1224.129.1.1225.1.1.1225.129.1.1 . . .238.1.1.1238.129.1.1239.1.1.1239.129.1.1

0x0100.5E01.0101

1 - Multicast MAC Address(FDDI and Ethernet)

Multicast AddressingSince the only 23 bits of the multicast IP address are usedand the first four bits are always 1110, bits 5 – 8 (five bits)are always ignored!

This means that there are 32 IP addresses that match any onemulticast MAC address!!!

Since the only 23 bits of the multicast IP address are usedand the first four bits are always 1110, bits 5 – 8 (five bits)are always ignored!

This means that there are 32 IP addresses that match any onemulticast MAC address!!!

11100000.00000001.00000001.00000001 = 224.1.1.1


32 IP Address to 1 Multicast MAC Address


Summary

IP multicast is a much more efficient means of delivering content where a single sender needs to deliver the content to multiple receivers. This task may be achieved through the use of multicast groups.

IP multicasts are designated by the use of a specific Class D IP address range. This is achieved through global scope addresses, which are assigned dynamically, and administratively scoped, which are assigned locally and are reserved for use inside private domains.


Resources

Wikipedia IP Multicast articlehttp://en.wikipedia.org/wiki/IP_Multicast

Webopedia Mbone articlehttp://www.webopedia.com/TERM/M/Mbone.html



IP Multicasting: IGMP and Layer 2 Issues


Objectives

Explain the operations of IGMPv2 and how IGMPv2 utilizes Join Group and Leave Group messages.

Explain the operations of IGMPv3 and how IGMPv2 and IGMPv3 interoperate.

Describe the methods used to deal with multicast in a Layer 2 switching environment.


IGMP Overview


Internet Group Management Protocol (IGMP)

How hosts tell routers about group membership

Routers solicit group membership from directly connected hosts

RFC 1112 specifies IGMPv1

Supported on Windows 95


Supported on latest service pack for Windows and most UNIX systems


Supported in Window XP and various UNIX systems


IGMPv2RFC 2236

Group-specific queryRouter sends query membership message to a single group rather than all hosts (reduces traffic).

Leave group messageHost sends leave message if it leaves the group and is the last member (reduces leave latency in comparison to v1).

Query-interval response timeThe Query router sets the maximum Query-Response time (controls burstiness and fine-tunes leave latencies).

Querier election processIGMPv2 routers can elect the Query Router without relying on the multicast routing protocol.


IGMPv3

IGMPv3 adds the ability to filter multicasts based on the multicast source.

IGMPv3 allows hosts to indicate that they want to receive traffic only from particular sources within a multicast group.

IGMPv3 is the industry-designated standard protocol for hosts to signal channel subscriptions in Source Specific Multicast (SSM).

http://www.cisco.com/en/US/products/sw/iosswrel/ps1834/products_feature_guide09186a0080080515.html


IGMPv3 (cont.)

IGMPv3 runs in two modes: INCLUDE and EXCLUDE

INCLUDE mode—In this mode, the receiver announces membership to a host group and provides a list of IP addresses (the INCLUDE list) from which it wants to receive traffic.

EXCLUDE mode—In this mode, the receiver announces membership to a host group and provides a list of IP addresses (the EXCLUDE list) from which it does not want to receive traffic.

This indicates that the host wants to receive traffic only from other sources whose IP addresses are not listed in the EXCLUDE list.


IGMPv2—Joining a Group

224.1.1.1

Join Group


IGMPv2—Leaving a Group

IGMPv2 has explicit Leave Group messages, which reduces overall leave latency.


IGMPv2—Leaving a Group (Cont.)

Hosts H2 and H3 are members of group 224.1.1.1.

1. H2 sends a leave message.



2. Router sends group-specific query.

Karen Alderson

In the original slide the screen of computer H3 was a different color to bring it to the attention of the learner. Please change the screen color of computer H3 from the light teal color to a white.



3. A remaining member host sends report, so group remains active.






IGMPv3—Joining a Group

Joining member sends IGMPv3 report to 224.0.0.22 immediately upon joining.


IGMPv3—Joining Specific Source(s)

IGMPv3 Report contains desired sources in the Include list. Only “Included” sources are joined.


IGMPv3—Maintaining State

Router sends periodic queries: All IGMPv3 members respond.

Reports contain multiple group state records.


Self Check

1. What is the primary purpose of IGMP?

2. When 2 IGMP routers are located on the same Ethernet segment, which router will be the designated querier?

3. What does the ICMPv2 Query router doe when it receives a Leave Message?


IGMP Layer 2 Issues


Determining IGMP Version Running

Determining which IGMP version is running on an interface.

rtr-a>show ip igmp interface e0Ethernet0 is up, line protocol is up Internet address is 1.1.1.1, subnet mask is 255.255.255.0 IGMP is enabled on interface Current IGMP version is 2 CGMP is disabled on interface IGMP query interval is 60 seconds IGMP querier timeout is 120 seconds IGMP max query response time is 10 seconds Inbound IGMP access group is not set Multicast routing is enabled on interface Multicast TTL threshold is 0 Multicast designated router (DR) is 1.1.1.1 (this system) IGMP querying router is 1.1.1.1 (this system) Multicast groups joined: 224.0.1.40 224.2.127.254


Layer 2 Multicast Frame Switching

Problem: Layer 2 flooding of multicast frames

Typical Layer 2 switches treat multicast traffic as unknown or broadcast and must flood the frame to every port (in VLAN).

Static entries may sometimes be set to specify which ports receive which groups of multicast traffic.

Dynamic configuration of these entries may reduce administration.


Layer 2 Multicast Switching Solutions

Cisco Group Management Protocol (CGMP): Simple, proprietary; routers and switches

IGMP snooping: Complex, standardized, proprietary implementations; switches only


Layer 2 Multicast Frame Switching CGMPSolution 1: CGMP

Runs on switches and routers.

CGMP packets sent by routers to switches at the CGMP multicast MAC address of 0100.0cdd.dddd.

CGMP packet contains:Type field: join or leave

MAC address of the IGMP client

Multicast MAC address of the group

Switch uses CGMP packet information to add or remove an entry for a particular multicast MAC address.


IGMP SnoopingSolution 2: IGMP snooping

Switches become IGMP-aware.

IGMP packets are intercepted by the CPU or by special hardware ASICs.

Switch examines contents of IGMP messages to learn which ports want what traffic.

Effect on switch without Layer 3-aware Hardware/ASICs

Must process all Layer 2 multicast packets

Administration load increased with multicast traffic load

Effect on switch with Layer 3-aware Hardware/ASICs

Maintain high-throughput performance but cost of switch increases


Self Check

1. What command is used to determine the version of IGMP active on an interface?

2. How does a typical layer 2 switch treat multicast traffic?

3. What is CGMP?

4. What type of switch is recommending for use with IGMP snooping?


Summary IGMPv2 is a protocol used by multicast clients to join a

multicast group.

IGMPv3 allows a receiver to specify a source.

If controls such as CGMP and IGMP snooping are not added at the Ethernet switching level, all multicast frames are flooded.

CGMP is a Cisco proprietary protocol used to implement multicast efficiently.

IGMP snooping is a standard protocol that has a function similar to CGMP.


Resources

Wikipedia IGMP articlehttp://en.wikipedia.org/wiki/IGMP

Multicast in a Campus Network: CGMP and IGMP Snooping

http://www.cisco.com/warp/public/473/22.html

IP Multicast Glossary of Termshttp://www.cisco.com/en/US/tech/tk828/tech_brief0900aecd801bca26.html



IP Multicasting: Multicast Routing Protocols


Objectives

Describe a multicast network in terms of the IP multicast routing protocols and processes used over various segments.

Describe multicast distribution trees including source trees and shared trees.

Describe the (S,G) and (*,G) multicast distribution trees entry formats.

Explain IP multicast routing.

Identify the characteristics of each of the PIM modes.

Describe the operation of PIM-DM, PIM-SM, and PIM sparse-dense modes.


Multicast Distribution Trees


Multicast Protocol Basics

Types of multicast distribution trees:

Source distribution trees; also called shortest path trees (SPTs)

Shared distribution trees; rooted at a meeting point in the network

A core router serves as a rendezvous point (RP)


Shortest Path or Source Distribution Tree

Receiver 1

B

E

A D F

Source 1Notation: (S, G) S = Source G = Group

C

Receiver 2

Source 2



Receiver 1

B

E

A D F

Source 1Notation: (S, G) S = Source G = Group

C

Receiver 2

Source 2

Multicast Distribution TreesShortest Path or Source Distribution Tree



Receiver 1

B

E

A D F

Notation: (*, G) * = All Sources G = Group

C

Receiver 2

(RP) PIM Rendezvous Point

Shared Tree

(RP)

Shared Distribution Tree



Receiver 1

B

E

A F

Source 1 Notation: (*, G) * = All Sources G = Group

C

Receiver 2

Source 2

(RP) PIM Rendezvous Point

Shared Tree

Source Tree

D (RP)

Shared Distribution Tree


Multicast Distribution Tree Identification

(S,G) entries – Source Distribution Trees

For this particular source sending to this particular group

Traffic is forwarded through the shortest path from the source

(*,G) entries – Shared Trees

For any (*) source sending to this group

Traffic is forwarded through a meeting point for this group



CharacteristicsCharacteristics of Distribution Trees

Source or Shortest Path trees

Uses more memory but optimal paths from source to all receivers; minimizes delay

Shared trees

Uses less memory but sub-optimal paths from source to all receivers; may introduce extra delay


Self Check

1. What is an advantage of Shortest Path Trees compared to Shared Trees?

2. What does multicast use to make it’s forwarding decisions?

3. In the STP notation (S,G), what do each of the characters represent?

4. What is the root of the shared tree called?

5. Compare Shared Distribution trees and SPT trees in terms of router memory and packet delivery delay.


Multicast Routing


Protocols for IP Multicast Routing

PIM is used between routers so that they can track which multicast packets to forward to each other and to their directly connected LANs.


Protocol-Independent Multicast (PIM) PIM maintains the current IP multicast service mode of

receiver-initiated membership.

PIM is not dependent on a specific unicast routing protocol.

With PIM, routers maintain forwarding tables to forward multicast datagrams.

PIM can operate in dense mode or sparse mode.Dense mode protocols flood multicast traffic to all parts of the network and prune the flows where there are no receivers using a periodic flood-and-prune mechanism.

Sparse mode protocols use an explicit join mechanism where distribution trees are built on demand by explicit tree join messages sent by routers that have directly connected receivers.


Multicast Tree Creation

PIM Join/Prune Control Messages

Used to create/remove Distribution Trees

Shortest Path trees

PIM control messages are sent toward the Source

Shared trees

PIM control messages are sent toward RP


Multicast Forwarding

Multicast routing operation is the opposite of unicast routing.

Unicast routing is concerned with where the packet is going.

Multicast routing is concerned with where the packet comes from.

Multicast routing uses Reverse Path Forwarding (RPF) to prevent forwarding loops.


Reverse Path Forwarding (RPF)

The RPF Calculation

The multicast source address is checked against the unicast routing table.

This determines the interface and upstream router in the direction of the source to which PIM Joins are sent.

This interface becomes the “Incoming” or RPF interface.

A router forwards a multicast datagram only if received on the RPF interface.



RPF Calculation

Based on Source Address.

Best path to source found in Unicast Route Table.

Determines where to send Joins.

Joins continue towards Source to build multicast tree.

Multicast data flows down tree.

10.1.1.1

E1

E2

Unicast Route TableNetwork Interface10.1.0.0/24 E0

Join

Join

E0



10.1.1.1

E1E0

E2

Join

Join

RPF Calculation (cont.)

Repeat for other receivers…



RPF Calculation

What if we have equal-cost paths?We can’t use both.

Tie-BreakerUse highest Next-Hop IP address.

10.1.1.1

E1E0

E2 Unicast Route TableNetwork Intfc Nxt-Hop10.1.0.0/24 E0 1.1.1.110.1.0.0/24 E1 1.1.2.1

1.1.2.11.1.1.1Join


Self Check

1. Why is Protocol Independent Multicast called Independent?

2. Describe dense mode operation.

3. What does multicast routing use to prevent forwarding loops?

4. What is the RPF interface?

5. What if the RFP calculation finds 2 equal-cost paths?


PIM Dense Mode Operation


PIM-DM Flood and Prune

Initial Flooding


PIM-DM Flood and Prune (Cont.)


PIM-DM Flood and Prune (Cont.)

Results After Pruning


Self Check

1. What happens to packets arriving through the non-RPF interface?

2. When would prune messages be sent on the RPF interface?

3. How often do prune messages expire in PIM-DM?

4. What happens when the prune messages expire?


PIM Sparse Mode Operation


PIM Sparse Mode

PIM-SM works with any of the underlying unicast routing protocols.

PIM-SM supports both source and shared trees.

PIM-SM is based on an explicit pull model.

PIM-SM uses an RP.Senders and receivers “meet each other.”

Senders are registered with RP by their first-hop router.

Receivers are joined to the shared tree (rooted at the RP) by their local DR.


PIM-SM Shared Tree Join

Receiver

RP

(*, G) Join

Shared Tree

(*, G) State created onlyalong the Shared Tree.


PIM-SM Sender Registration

Receiver

RP

(S, G) Join

Source

Shared Tree

(S, G) Register (unicast)

Source Tree

(S, G) State created onlyalong the Source Tree.Traffic Flow



Receiver

RPSource

Shared Tree

Source Tree RP sends a Register-Stop back to the first-hop router to stop the Register process.

(S, G) Register-Stop (unicast)

Traffic Flow

(S, G) Register (unicast)

(S, G) traffic begins arriving at the RP through the Source tree.



Receiver

RPSource

Shared Tree

Source Tree

Traffic FlowSource traffic flows nativelyalong SPT to RP.

From RP, traffic flows downthe Shared Tree to Receivers.


PIM-SM SPT Switchover

Receiver

RP

(S, G) Join

Source

Source Tree

Shared Tree

Last-hop router joins the Source Tree.

Additional (S, G) State is created along new part of the Source Tree.

Traffic Flow



Receiver

RPSource

Source Tree

Shared Tree

(S, G)RP-bit Prune

Traffic begins flowing down the new branch of the Source Tree.

Additional (S, G) State is created along along the Shared Tree to prune off (S, G) traffic.

Traffic Flow



Receiver

RPSource

Source Tree

Shared Tree

(S, G) Traffic flow is now pruned off of the Shared Tree and is flowing to the Receiver through the Source Tree.

Traffic Flow



Receiver

RPSource

Source Tree

Shared Tree

(S, G) traffic flow is no longer needed by the RP so it Prunes the flow of (S, G) traffic.

Traffic Flow

(S, G) Prune



Receiver

RPSource

Source Tree

Shared Tree

(S, G) Traffic flow is now only flowing to the Receiver through a single branch of the Source Tree.

Traffic Flow


“The default behavior of PIM-SM is that routers with directly connected members will join the Shortest Path Tree as soon as they detect a new multicast source.”

PIM-SM Frequently Forgotten Fact


Multiple RPs with Auto RP

PIM Sparse-Dense-Mode


Self Check

1. What types of deployments is PIM-SM appropriate for?

2. When using PIM-SM, to what device does the receiver send a Join when wishing to receive multicast traffic?

3. How is a Register message used?

4. Explain the implications of the default value of the SPT-Threshold in Cisco routers.

5. Describe the potential issues with PIM-SM.


Summary IP multicast requires multiple protocols and processes for proper packet

forwarding.

Source and shared trees may be used to define multicast packet flows to group members.

Multicast routing utilizes the distribution trees for proper packet forwarding.

PIM is the routing protocol for multicast.

PIM-DM uses flood and prune.

PIM-SM uses less device and bandwidth resources and is typically chosen to implement multicast.

PIM sparse-dense mode is the recommended methodology for maximum efficiency in IP multicast.


Q and A


Resources

Internet Protocol IP Multicast Technologyhttp://www.cisco.com/en/US/tech/tk828/tech_brief09186a00800a4415.html

IP Multicast Deployment Fundamentalshttp://www.cisco.com/en/US/tech/tk828/tech_brief09186a00800e9952.html

Cisco IOS Multicast Q&Ahttp://www.cisco.com/en/US/tech/tk828/technologies_q_and_a_item09186a00801bb25d.shtml



IP Multicasting: Multicast Configuration and Verification


Objectives

Enable PIM Sparse Mode and Sparse-Dense Mode on a router interface.

Verify the multicast routing table.

Determine if PIM neighbors are correctly configured.

Verify RP information and the IGMP group state.

Configure a router as a statically connected member.

Verify IGMP snooping.


Multicast Configuration


Enabling IP Multicast Routing

ip multicast-routing

router(config)#

Enables multicast routing. Enabling IP multicast routing allows the Cisco IOS

software to forward multicast packets.


Enabling PIM on an Interface

ip pim { sparse-mode | sparse-dense-mode }

router(config-if)#

Enables PIM SM on an interface; the sparse-dense-mode option enables mixed sparse-dense groups.

Enabling PIM on an interface also enables IGMP operation on that interface.

Recommended method is to use sparse-dense-mode option.


Announcing the RP and the Group Range It Serves

ip pim send-rp-announce {interface type} scope {ttl} group-list {acl}

router(config)#

Configures a router to be the RP for the local group as defined in the access list.

The following example advertises the IP address of Ethernet 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 0.255.255.255

router(config)#


Assigning the RP Mapping Agent

ip pim send-rp-discovery {interface type} scope {ttl}

router(config)#

The RP mapping agent is the router that tells other routers which group-to-RP range 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 ttl number of hops from the source router receive the Auto-RP Discovery messages.


Self Check

1. What is the purpose if the ip multicast-routing command?

2. How is IGMP enabled on an interface?

3. What is the recommended method for configuring an interface for PIM-SM operation?

4. What is the potential issue when configuring an interface for sparse mode or dense mode, rather than sparse-dense?

5. What is the RP mapping agent?


Verifying Multicast Configuration


Inspecting Multicast Routing Table

Displays the contents of the IP multicast routing tablesummary: Displays a one-line, abbreviated summary of each entry in the IP multicast routing table.

count: Displays statistics about the group and source, including number of packets, packets per second, average packet size, and bits per second.

active: Displays the rate at which active sources are sending to multicast groups. Active sources are those sending at a rate specified in the kbps argument or higher. The kbps argument defaults to 4 kbps.

show ip mroute [group-address] [summary] [count] [active kbps]

router#


show ip mroute

NA-1#sh ip mrouteIP Multicast Routing TableFlags: D - Dense, S - Sparse, B - Bidir Group, s - SSM Group, C - Connected L - Local, P - Pruned, R - RP-bit set, F - Register flag, T - SPT-bit set, J - Join SPT, M - MSDP created entry, X - Proxy Join Timer Running, A - Advertised via MSDP, U - URD, I - Received Source Specific Host ReportOutgoing interface flags: H - Hardware switchedTimers: Uptime/ExpiresInterface state: Interface, Next-Hop or VCD, State/Mode

(*, 224.1.1.1), 00:07:54/00:02:59, RP 10.127.0.7, flags: S Incoming interface: Null, RPF nbr 0.0.0.0 Outgoing interface list: Serial1/3, Forward/Sparse, 00:07:54/00:02:32

(172.16.8.1, 224.1.1.1), 00:01:29/00:02:08, flags: TA Incoming interface: Serial1/4, RPF nbr 10.139.16.130 Outgoing interface list: Serial1/3, Forward/Sparse, 00:00:57/00:02:02


Finding PIM Neighbors

show ip pim interface [type number] [count]

router#

Displays information about interfaces configured for PIM

show ip pim neighbor [type number]

router#

Lists the PIM neighbors discovered by the router

mrinfo [hostname | address]

router#

Queries which neighboring multicast routers are peering with the local router or router specified


show ip pim interface

NA-2#show ip pim interfaceAddress Interface Ver/ Nbr Query DR DR Mode Count Intvl Prior10.139.16.133 Serial0/0 v2/S 1 30 1 0.0.0.010.127.0.170 Serial1/2 v2/S 1 30 1 0.0.0.010.127.0.242 Serial1/3 v2/S 1 30 1 0.0.0.0


show ip pim neighbor

NA-2#show ip pim neighborPIM Neighbor TableNeighbor Interface Uptime/Expires Ver DRAddress Priority10.139.16.134 Serial0/0 00:01:46/00:01:28 v2 None10.127.0.169 Serial1/2 00:01:05/00:01:40 v2 1 (BD)10.127.0.241 Serial1/3 00:01:56/00:01:18 v2 1 (BD)


Checking RP Information

Displays active RPs that are cached with associated multicast routing entries.

Mapping—displays all group-to-RP mappings that the router is aware of

show ip pim rp [group-name | group-address | mapping]

router(config)#

Displays how IP multicast routing does Reverse Path Forwarding (RPF).

Source Address—source address of the host for which RPF information is displayed

Name—name of the host for which RPF information is displayed

show ip rpf {source address | name }

router(config)#


show ip pim rp

P4-2#show ip pim rpGroup: 224.1.2.3, RP: 10.127.0.7, uptime 00:00:20, expires never

P4-2#show ip pim rp mappingPIM Group-to-RP Mappings

Group(s) 224.0.1.39/32 RP 10.127.0.7 (NA-1), v1 Info source: local, via Auto-RP Uptime: 00:00:21, expires: neverGroup(s) 224.0.1.40/32 RP 10.127.0.7 (NA-1), v1 Info source: local, via Auto-RP Uptime: 00:00:21, expires: neverGroup(s): 224.0.0.0/4, Static RP: 10.127.0.7 (NA-1)


show ip rpf

(towards the RP)NA-2#show ip rpf 10.127.0.7RPF information for NA-1 (10.127.0.7) RPF interface: Serial1/3 RPF neighbor: ? (10.127.0.241) RPF route/mask: 10.127.0.7/32 RPF type: unicast (ospf 1) RPF recursion count: 0 Doing distance-preferred lookups across tables

(towards the source)NA-2#show ip rpf 10.139.17.126RPF information for ? (10.139.17.126) RPF interface: Serial0/0 RPF neighbor: ? (10.139.16.134) RPF route/mask: 10.139.17.0/25 RPF type: unicast (ospf 1) RPF recursion count: 0 Doing distance-preferred lookups across tables


Checking the Group State

show ip igmp interface [type number]

router#

Displays multicast-related information about an interface

show ip igmp groups [group-address | type number]

router#

Displays the multicast groups that are directly connected to the router and that were learned via IGMP


Configure a Router as a Group Member

ip igmp join-group group address

Router (config-if)#

Configure a router to join a specific multicast group and enable IGMP on an interface.

ip igmp static-group group-address

Router (config-if)#

Configures the router as a statically connected member of a group


show ip igmp interface

rtr-a>show ip igmp interface e0Ethernet0 is up, line protocol is up Internet address is 1.1.1.1, subnet mask is 255.255.255.0 IGMP is enabled on interface Current IGMP version is 2 CGMP is disabled on interface IGMP query interval is 60 seconds IGMP querier timeout is 120 seconds IGMP max query response time is 10 seconds Inbound IGMP access group is not set Multicast routing is enabled on interface Multicast TTL threshold is 0 Multicast designated router (DR) is 1.1.1.1 (this system) IGMP querying router is 1.1.1.1 (this system) Multicast groups joined: 224.0.1.40 224.2.127.254


show ip igmp groups

rtr-a>sh ip igmp groupsIGMP Connected Group MembershipGroup Address Interface Uptime Expires Last Reporter224.1.1.1 Ethernet0 6d17h 00:01:47 1.1.1.12224.0.1.40 Ethernet0 6d17h never 1.1.1.17


Verifying IGMP Snooping on a Switch

Displays information about multicast groups.

If igmp keyword is used, only IGMP-learned information is shown.

show multicast group [igmp] [mac_addr] [vlan_id]

switch>

Displays information on dynamically learned and manually configured multicast router ports.

If igmp keyword is used, only IGMP-learned information is shown.

show multicast router [igmp] [mod_num/port_num] [vlan_id]

switch>


Verifying IGMP Snooping—Example


Switch> show igmp statistics 10IGMP enabled

IGMP statistics for vlan 10:

IGMP statistics for vlan 10: Transmit: General Queries: 0 Group Specific Queries: 0 Reports: 0 Leaves: 0

Receive: General Queries: 1 Group Specific Queries: 0 Reports: 2 Leaves: 0 Total Valid pkts: 4 Total Invalid pkts: 0 Other pkts: 1 MAC-Based General Queries: 0 Failures to add GDA to EARL: 0 Topology Notifications: 0

Verifying IGMP Snooping—Example (Cont.)


Switch> show multicast router igmpPort Vlan---------- ---------------- 4/1 10

Total Number of Entries = 1'*' - Configured'+' - RGMP-capable

Switch> show multicast group igmp

VLAN Dest MAC/Route Des [CoS] Destination Ports or VCs / [Protocol Type]---- ---------------------------------------------------------------------10 01-00-5e-00-01-28 4/110 01-00-5e-01-02-03 4/1-2

Total Number of Entries = 2

Verifying IGMP Snooping—Example (Cont.)


Self Check

1. What command can be used to view the multicast routing table?

2. What commands can be used with PIM-SM to check PIM-enabled interfaces?

3. Why should the show ip pim rp mapping command be used instead of the show ip pim rp command?

4. Why might you configure a router to be a multicast group member?


Summary

Configuring a simple multicast network requires a global multicast command, a multicast command for each interface, and the specification of an RP discovery method.

Effective methods for verifying a multicast network include checking the multicast routing table and checking PIM neighbors.

Configuring IGMP snooping on an Ethernet switch avoids the problem of multicast frame flooding.


Q and A


Resources

Cisco IOS IP Multicast Configuration Guidehttp://cisco.com/en/US/partner/products/ps6350/products_configuration_guide_book09186a0080435b9f.html

IP Multicast Deployment Fundamentalshttp://cisco.com/en/US/partner/tech/tk828/tech_brief09186a00800e9952.html

Multicast Quick-Start Configuration Guidehttp://cisco.com/en/US/partner/tech/tk828/technologies_tech_note09186a0080094821.shtml

Basic Multicast Troubleshooting Toolshttp://cisco.com/en/US/partner/tech/tk828/technologies_tech_note09186a0080093f21.shtml


Documents

IP Multicasting: Explaining Multicast