IP Multicasting

1
IP Multicasting
http//munz-udo.de
2
Multicast Topics
IGMP Internet Group Management
Protocol CGMP Cisco Group Management
Protocol PIM Protocol Independent
Multicast JAVVIN
3
Multimedia traffic using Unicast
Best case known unicasts
Worst case unknown unicasts

• Application sends one copy of each packet to
  every client's unicast address.
• As a result, unicast transmission has significant
  scaling restrictions.
• If the group is large, the same information must
  be carried multiple times, even on shared links.

4
Multimedia traffic using Broadcast

• Application sends only one copy of each packet
  using a broadcast.
• Inefficient, if only for a small subset of the
  network.
• Every host device must process the broadcast
  multimedia data frame.
• Transmissions may reach data rates as high as 13
  Mbps or more.
• All devices must still process the broadcast
  traffic, even if not using it.
• May use most, if not all, of the allocated
  bandwidth for each device.
• Cisco highly discourages broadcast implementation
  for applications delivering data, voice, or video
  to multiple receivers.

5
Multimedia traffic using Multicast

• The most efficient solution in between
  broadcast and unicast.
• Server sends one copy of each packet to a special
  address that represents multiple clients.
• Server sends out a single data stream to multiple
  clients.
• Client device decides whether or not to listen to
  the multicast address.
• Saves bandwidth and controls network traffic by
  forcing the network to replicate packets only
  when necessary.
• Reduces network bandwidth consumption and host
  processing.
• Cisco switches can process IP multicast packets
  and deliver those packets only to requesting
  receivers at both Layer 2 and Layer 3.

6
Less stress on server

• In a unicast scenario, the server sequences
  through transmission of multiple copies of the
  data, so variability in delivery time is large,
  especially for large transmissions or large
  distribution lists.

7
Uses UDP
Application Header data

• IP multicast traffic uses UDP as the transport
  layer.
• Unlike TCP, UDP adds no reliability, flow
  control, or error-recovery functions to IP.
• Because of the simplicity of UDP, data-packet
  headers contain fewer bytes and consume less
  network overhead than TCP.
• Reliability in multicast is therefore managed at
  the receiving client and by QoS in the network.

8
IP Multicast Routing

• IP multicast uses a virtual group address called
  the multicast IP address.
• IP unicast, a packet is routed from a source
  address to a destination address, hop by hop.
• IP multicast, the packet's destination address is
  not assigned to a single destination.
• Instead, receivers join a group.
• All members of the group receive the packet.
• A host must be a member of the group to receive
  the packet.
• Multicast sources do not need to be members of
  that group.

9
IP Multicast Routing

• Packets sent by group member 3 are received by
  group members 1 and 2, but not by the nonmember
  of the group.
• The nonmember host sends packets to the multicast
  group that are received by all three group
  members.
• Group members 1 and 2 do not send any multicast
  packets.
• The multicast router sends the packets to
  respective multiple interfaces to reach all the
  hosts.
• To avoid duplication, several multicast routing
  protocols use reverse path forwarding (RPF),
  discussed later.

10
Multicast IP Address Structure
224.0.0.0 to 239.255.255.255

• IP multicast uses the Class D addresses, which
  range from 224.0.0.0 to 239.255.255.255.
• These addresses consist of binary 1110 as the
  most significant bits (MSBs) in the first octet,
  followed by a 28-bit group address.
• Unlike with Class A, B, and C IP addresses, the
  last 28 bits of a Class D address are
  unstructured.

11
Multicast IP Address Structure
Destin. IP Multicast
Src. IP Unicast

• The Internet Assigned Numbers Authority (IANA)
  controls the assignment of IP multicast
  addresses.
• The Class D address range is used only for the
  group address or destination address of IP
  multicast traffic.
• The source address for multicast datagrams is
  always the unicast source address.

12
Multicast IP Address Structure

• Applications allocate multicast addresses
  dynamically or statically.
• Dynamic multicast addressing provides
  applications with a group address on demand.
• Because dynamic multicast addresses have a
  specific lifetime, applications must request this
  type of address only for as long as the address
  is needed.
• Statically allocated multicast addresses are
  reserved for specific protocols that require
  well-known addresses, such as OSPF Hello packets
  (224.0.0.5, AllSPFRouters).
• IANA assigns these well-known addresses, which
  are called permanent host groups and are similar
  in concept to the well-known TCP and UDP port
  numbers.

13
Reserved Link Local Addresses

• 224.0.0.0 to 224.0.0.255 (link local destination
  addresses) to be used by network protocols on a
  local network segment.
• Routers do not forward packets in this address
  range.
• Typically sent with a Time-to-Live (TTL) value of
  1.
• Network protocols use these addresses for
  automatic router discovery and to communicate
  important routing information.
• For example, OSPF uses the IP addresses 224.0.0.5
  and 224.0.0.6 to exchange link-state information.
  
• 224.0.0.5, All SPF Routers
• 224.0.0.6 ALL DR Routers

14
Reserved Link Local Addresses

• Address 224.0.0.1 identifies the all-hosts group.
  
• Every multicast-capable host must join this group
  when initializing its IP stack.
• If you send an ICMP echo request to this address,
  all multicast-capable hosts on the network answer
  the request with an ICMP echo reply.
• Address 224.0.0.2 identifies the all-routers
  group.
• Multicast routers join this group on all
  multicast-enabled interfaces.

15
mping A simple demonstration

• Use mping on two or more hosts to show how
  multicast hosts send and receive multicast
  traffic.
• Mping is a little different, in that both hosts
  are senders and receivers.
• http//research.microsoft.com/barc/mbone/mping.asp
  x
• Hosts
• mping ltIP addressgt port TTL time in msec
  between pings
• Example, both hosts mping 224.10.10.10
• When it sends a packet "Sent packet"
• When it receives a packet
• RCV XX bytes from XXX.XXX.XXX.XXX

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Multicast MAC Address Structure

• The destination IP address of IP multicast
  packets maps to a multicast MAC address.
• However, the multicast MAC address is derived
  from the IP multicast address.

17
Multicast MAC Address Structure

• Multicasts must also have a layer 2 group address
  (it cant be FF-FF-FF-FF-FF-FF).
• The first 3 bytes (24 bits) of the multicast MAC
  address are always 01-00-5E.
• Binary 00000001.00000000.01011110.0xxxxxxx.xxxxxx
  xx.xxxxxxxx with the 25th bit set to 0.
• This is a reserved value that indicates a
  multicast application.
• Usually, the first half of the MAC address is the
  vendor code, aka Organizational Unique
  Identifier.
• So, if multicast L2 addresses always begin with
  01-00-5E, where does the other half (24 bits) of
  the address come from?
• 0 23 bits (copied from the IP address)

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Multicast MAC Address Structure

• The second half of the MAC address (24 bits)
  derives from
• 0 23 bits (copied from the IP address)
• The host copies the last 23 bits of the multicast
  IP address into the last 23 bits of the MAC
  address.
• Why the conversion?
• Host If I join multicast group 224.10.8.5, I
  will listen for the MAC address
  01-00-5E-0A-08-05.

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Multicast MAC Address Structure
Loose 5 bits of IP Address
IP1110xxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx
Multicast MAC 00000001.00000000.01011110.0xxxxxxx
.xxxxxxxx.xxxxxxxx

• Because all the IP multicast addresses have the
  first 4 bits set to 1110, the remaining 28 least
  significant bits (LSBs) of multicast IP addresses
  must map into the 23 LSBs of the MAC address.
• As a result, the MAC address loses 5 bits of
  uniqueness in the IP-to-MAC address mapping
  process (uniqueness of IP address).

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Multicast MAC Address Structure
224 - 239
21
Multicast MAC Address Structure

• This method for mapping multicast IP addresses to
  MAC addresses results in a 321 mapping, whereas
  each multicast MAC address represents a possible
  32 distinct IP multicast addresses.

02

• MAC
• 00000001.00000000.01011110.00000001.00000001.00000
  010
• IP
• 11100000.00000001.00000001.00000010
• 11100000.10000001.00000001.00000010
• 11100001.00000001.00000001.00000010
• TO
• 11101111.10000001.00000001.00000010

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Multicast MAC Address Structure
23
Multicast MAC Address Structure

• Convert 224.0.9.45 to a multicast MAC address.

224 0 9
45 1110 0000 0000 0000 0000 1001 0010 1101
01-00-5E First 3 bytes 25th bit 0
0000 0000 0000 1001 0010 1101
0000 0000 0000 1001 0010
1101 01-00-5E- 0 0 - 0 9 -
2 D(13)
01-00-5E-00-09-2D
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Multicast MAC Address Structure

• Convert 224.192.255.44 to a multicast MAC address.

224 192 255
46 1110 0000 1100 0000 1111 1111 0001 1110
01-00-5E First 3 bytes 25th bit 0
0100 0000 1111 1111 0001 1110
0100 0000 1111 1111 0001 1110
01-00-5E- 4 0 - F(15) F - 1
E(14)
01-00-5E-40-FF-1E
25
Multicast MAC Address Structure
01005E010102 IP 224.129.1.2
IP Mapped Ethernet Multicast Frames
01005E010102 IP 225.1.1.2
Joined 224.129.1.2 so NIC is listening for
01-00-5E-01-01-02

• A host that joins one multicast group programs
  its network interface card to listen to the
  IP-mapped MAC address.
• If the same MAC address maps to a second MAC
  multicast address already in use, the host CPU
  processes both sets of IP multicast frames.

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Multicast MAC Address Structure
01005E010102 IP 224.129.1.2
IP Mapped Ethernet Multicast Frames
01005E010102 IP 225.1.1.2
Joined 224.129.1.2 so NIC is listening for
01-00-5E-01-01-02

• For example, multicast IP addresses 224.129.1.2
  and 225.1.1.2 both map to the same multicast MAC
  address 01005E010101.
• As a result, a host that registered to group
  224.129.1.2 also receives the traffic from
  225.1.1.2 because the same MAC multicast address
  is used by both IP multicast flows.
• It is recommended to avoid overlapping when
  implementing multicast applications in the
  multilayer switched network by tuning the
  destination IP multicast address at the
  application level.
• In this case where multiple groups map to the
  same MAC address, usually higher-layer protocols
  let hosts interpret which packets are for the
  application, using UDP port numbers (normally
  different per application).

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Multicast MAC Address Structure
Multicast Traffic 01005E010102 IP
224.129.1.2
Multicast Group 01005E010102 IP
224.129.1.2
Multicast Group 01005E010102 IP 225.1.1.2

• Furthermore, because switches forward frames
  based on the multicast MAC address if configured
  for Layer 2 multicast snooping, they forward
  frames to all the members corresponding to other
  IP multicast addresses of the same MAC address
  mapping, even if the frames belong to a different
  IP multicast group.

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Reverse Path Forwarding

• Multicast-capable routers and multilayer switches
  create distribution trees that control the path
  that IP multicast traffic takes through the
  network.
• Reverse path forwarding is the mechanism that
  performs an incoming interface check to determine
  whether to forward or drop an incoming multicast
  frame.
• RPF is a key concept in multicast forwarding.
• This RPF check helps to guarantee that the
  distribution tree for multicast is loop-free.
• In addition, RPF enables routers to correctly
  forward multicast traffic down the distribution
  tree.

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Reverse Path Forwarding

• In unicast routing, traffic is routed through the
  network along the path from the single source to
  the single destination host.
• A router that is forwarding unicast packets does
  not consider the source address, by default the
  router considers only the destination address and
  how to forward the traffic toward the
  destination.
• In multicast forwarding, the source is sending
  traffic to an arbitrary group of hosts that is
  represented by a single multicast group address.
• When a multicast router receives a multicast
  packet, it determines which direction is the
  upstream direction (toward the source) and which
  one is the downstream direction (or direction
  toward the receivers).
• A router forwards a multicast packet only if the
  packet is received on the correct upstream
  interface determined by the RPF process.

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Reverse Path Forwarding

• For traffic flowing down a source tree, the RPF
  check procedure works as follows
• The router looks up the source address in the
  unicast routing table to determine whether it
  arrived on the interface that is on the reverse
  path back to the source.
• If the packet has arrived on the interface
  leading back to the source, the RPF check is
  successful and the router replicates and forwards
  the packet to the outgoing interfaces.
• If the RPF check in the previous step fails, the
  router drops the packet and records the drop as
  an RPF failed drop.

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RPF check fails
Reverse-Path Forwarding
151.10.3.21
224.1.1.1

• The router in the figure receives a multicast
  packet from source 151.10.3.21 on interface S0.
• A check of the unicast route table shows that
  this router uses interface S1 as the egress
  (exit) interface for forwarding unicast data to
  151.10.3.21.
• Because the packet instead arrived on interface
  S0, the packet fails the RPF check, and the
  router drops the packet.

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RPF check succeeds
151.10.3.21
224.1.1.1

• With this example, the multicast packet arrives
  on interface S1.
• The router checks the unicast routing table and
  finds that interface S1 is the correct ingress
  (incoming) interface.
• The RPF check passes, and the router forwards the
  packet.

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Non-RPF Traffic
Do Not Forward
Source IP Address is not on these interfaces, but
interface connected to Campus Network Router.

• In multilayer switched networks where multiple
  routers connect to the same LAN segment, only one
  PIM-designated router forwards the multicast
  traffic from the source to the receivers on the
  outgoing interfaces.
• Router A, the PIM-designated router (PIM DR),
  forwards data to VLAN 1 and VLAN 2.
• Router B receives the forwarded multicast traffic
  on VLAN 1 and VLAN 2, and it drops this traffic
  because the multicast traffic fails the RPF
  check. (Source IP is via the other interface.)
• Traffic that fails the RPF check is called
  non-RPF traffic.

34
Multicast Forwarding Tree
Source Tree
Shared Tree

• Multicast-capable routers create multicast
  distribution trees that control the path that IP
  multicast traffic takes through the network to
  deliver traffic to all receivers.
• The following are the two types of distribution
  trees
• Source trees
• Shared trees

35
Source Trees

• The simplest form of a multicast distribution
  tree is a source tree with its root at the source
  and its branches forming a tree through the
  network to the receivers.
• Because this tree uses the shortest path through
  the network, it is also referred to as a shortest
  path tree (SPT).
• SPT for group 224.1.1.1 rooted at the source,
  host A, and connecting two receivers, hosts B and
  C.

36
Source Trees

• Using the (S,G) notation, the SPT for the example
  shown is (192.168.1.1, 224.1.1.1).
• The Source-Group (S,G) notation implies that a
  separate SPT exists for each individual source
  sending to each group.
• For example, if host B is also sending traffic to
  group 224.1.1.1 and hosts A and C are receivers,
  a separate (S,G) SPT would exist with a notation
  of (192.168.2.2, 224.1.1.1).

37
Shared Trees

• Unlike source trees, which have their root at the
  source, shared trees use a single common root
  placed at some chosen point in the network.
• This shared root is called a rendezvous point
  (RP).

• The shared unidirectional tree for the group
  224.1.1.1 with the shared root located at router
  D the RP.
• Source traffic is sent toward the RP on a source
  tree.
• The traffic is then forwarded down the shared
  tree from the RP to reach all the receivers
  unless the receiver is located between the source
  and the RP, in which case the multicast traffic
  is serviced directly.

38
Shared Trees

• Because all sources in the multicast group use a
  common shared tree, a wildcard notation written
  as (, G), pronounced star comma G, represents
  the tree.
• In this case, means all sources, and G
  represents the multicast group.
• Therefore, the shared tree shown in the figure is
  written as
• (, 224.1.1.1).

39
ComparingSource Trees

• Source trees have the advantage of creating the
  optimal path between the source and the
  receivers.

• This advantage guarantees the minimum amount of
  network latency for forwarding multicast traffic.
• However, this optimization requires additional
  overhead because the routers maintain path
  information for each source.
• In a network that has thousands of sources and
  thousands of groups, this overhead quickly
  becomes a resource issue on routers or multilayer
  switches.

40
ComparingShared Trees

• Shared trees have the advantage of requiring the
  minimum amount of state information in each
  router.
• This advantage lowers the overall memory
  requirements and complexity for a network that
  only allows shared trees.

Better Path

• The disadvantage of shared trees is that, under
  certain circumstances, the paths between the
  source and receivers might not be the optimal
  paths, which may introduce additional latency in
  packet delivery.
• May overuse some links and leave others unused
• For example, the shortest path between host A
  (source 1) and host B (a receiver) is between
  router A and router C.
• Network designers need to carefully consider the
  placement of the RP when implementing a shared
  treeonly environment.

41
IP Multicast Routing
42
IP Multicast Routing http//de.wikipedia.org/wiki
/Protocol_Independent_Multicast
PIM Sparse Mode (spärlich)
PIM Dense Mode(dicht)

• Similar to IP unicast, IP multicast uses its own
  routing, management, and Layer 2 protocols.
• The following are two important multicast
  protocols
• Protocol Independent Multicast (PIM)
• PIM Dense Mode
• PIM Sparse Mode (Sparse-dense mode is most common
  in large enterprise networks.)
• Internet Group Management Protocol (IGMP)

43
PIM
PIM Dense Mode
PIM Sparse Mode

• A multicast routing protocol is responsible for
  the construction of multicast delivery trees and
  enabling multicast packet forwarding.
• Different IP multicast routing protocols use
  different techniques to construct multicast trees
  and to forward packets.
• The PIM routing protocol leverages whichever
  unicast routing protocols are used to populate
  the unicast routing table to make multicast
  forwarding decisions.

44
PIM
PIM Dense Mode
PIM Sparse Mode

• Routers use the PIM neighbor discovery mechanism
  to establish PIM neighbors using hello messages
  to the ALL-PIM-Routers (224.0.0.13) multicast
  address for building and maintaining PIM
  multicast distribution trees.
• In addition, routers use PIM hello messages to
  elect the designated router (DR) for a multicast
  LAN network.
• Two distinct versions PIM version 1 and PIM
  version 2.

45
PIM Dense Mode
Source Tree

• PIM dense mode (PIM-DM) multicast routing
  protocols relies on
• Periodic flooding of the network with multicast
  traffic to set up and maintain the distribution
  tree.
• Neighbor information to form the distribution
  tree.
• Source distribution tree to forward multicast
  traffic
• Built by respective routers as soon as any
  multicast source begins transmitting.

46
PIM Dense Mode
Source Tree

• PIM-DM assumes that the multicast group members
  are densely distributed throughout the network
  and that bandwidth is plentiful, meaning that
  almost all hosts on the network belong to the
  group.
• When a router configured for PIM-DM receives a
  multicast packet
• The router performs the RPF check to validate the
  correct interface for the source.
• Forwards on the packet all the interfaces
  configured for multicasting until pruning and
  truncating occurs.

47
PIM Dense Mode
Source Tree

• All downstream routers receive the multicast
  packet until the multicast traffic times out.
• PIM-DM sends a pruning message upstream when
• Traffic arrives on a non-RPF, point-to-point
  interface.
• A leaf router without any receivers sends a prune
  message, where the router, which does not have
  any members or receivers, sends the prune message
  to the upstream router.
• A non-leaf router receives a prune message from
  all of its neighbors.

48
PIM Dense Mode

• In summary, PIM-DM works best when numerous
  members belong to each multimedia group.
• PIM floods the multimedia packet to all routers
  in the network and then prunes routers that do
  not service members of that particular multicast
  group.
• PIM-DM is most useful in the following cases
• Senders and receivers are in close proximity to
  one another.
• There are few senders and many receivers.
• The volume of multicast traffic is high.
• The stream of multicast traffic is constant.
• Nevertheless, PIM-DM is not the method of choice
  for enterprise and service provider customers
  because of its scalability and flooding
  properties.

49
PIM Sparse Mode
Shared Tree

• PIM sparse mode (PIM-SM), is based on the
  assumptions that the multicast group members are
  sparsely distributed throughout the network and
  that bandwidth is limited.
• PIM-SM does not imply that the group has few
  members, just that they are widely dispersed.
• In this case, flooding would unnecessarily waste
  network bandwidth and could cause serious
  performance problems.
• Therefore, PIM-SM multicast routing protocols
  rely on more selective techniques to set up and
  maintain multicast trees.

50
PIM Sparse Mode
Shared Tree

• With PIM-SM, each data stream goes to a
  relatively small number of segments in the campus
  network.
• Instead of flooding the network to determine the
  status of multicast members, PIM-SM defines an
  RP.
• When a sender wants to send data, it first does
  so to the RP.
• When a receiver wants to receive data, it
  registers with the RP.
• When the data stream begins to flow from sender
  to RP to receiver, the routers in the path
  automatically optimize the path to remove any
  unnecessary hops.

51
PIM Sparse Mode
Shared Tree

• PIM-SM assumes that no hosts want the multicast
  traffic unless they specifically ask for it.
• In PIM-SM, the shared tree mode can be switched
  to a source tree after a certain threshold to
  have the best route to the source.
• All Cisco IOS routers and switches, by default,
  have the SPT threshold set to 0, such that the
  last-hop router switches to SPT mode as soon as
  the host starts receiving the multicast, to take
  advantage of the best route for the multicast
  traffic.

52
PIM Sparse Mode
Shared Tree

• PIM-SM is optimized for environments where there
  are many multipoint data streams.

53
PIM Sparse-Dense Mode
PIM Dense Mode
PIM Sparse Mode

• PIM can simultaneously support dense mode
  operation for some multicast groups and sparse
  mode operation for others.
• It turned out that it was more efficient to
  choose sparse or dense mode on a per-group basis
  rather than a per-router interface basis.
• PIM sparse-dense mode allows individual groups to
  use either sparse or dense mode depending on
  whether RP information is available for that
  group.
• If the router learns RP information for a
  particular group, it is treated as sparse mode
  otherwise, that group is treated as dense mode.

54
Automating Distribution of RP

• PIM-SM and PIM sparse-dense modes use various
  methods, discussed in this section, to automate
  the distribution of the RP.
• This mechanism has the following benefits
• It eliminates the need to manually configure RP
  information in every router and switch in the
  network.
• It is easy to use multiple RPs within a network
  to serve different group ranges.
• It allows load-splitting among different RPs and
  allows the arrangement of RPs according to the
  location of group participants.
• It avoids inconsistency manual RP configurations
  may cause connectivity problems, if not
  configured properly.
• PIM uses the following mechanisms to automate the
  distribution of the RP
• Auto-RP
• Bo
• Auto-RP is a Cisco proprietary protocol for
  automatically advertising RP-to-group mappings to
  routers in your PIM network.otstrap router (BSR)

55
Auto-RP
Im the RP Mapping Agent, here are the
group-to-RP mappings. (every 60 secs)
Im going to learn about group-to-RP mappings
because I am a member of the 224.0.1.40 multicast
group, Cisco-RP-discovery.

• Auto-RP automates the distribution of group-to-RP
  mappings.
• defines which multicast groups use which RP.
• All routers in the PIM network learn about the
  active group-to-RP mapping from the RP mapping
  agent by automatically joining the
  Cisco-RP-discovery (224.0.1.40) multicast group.
• The RP mapping agent is the router that sends the
  authoritative discovery packets that notify other
  routers which group-to-RP mapping to use (every
  60 seconds).
• Such a role is necessary in the event of
  conflicts (such as overlapping group-to-RP
  ranges).

56
Im a member of the 224.0.1.39 multicast group,
Cisco-RP-announce. This will tell me who the
candidate RPs are.
Im a candidate RPs. I will send this every 60
secs to 224.0.1.39.

• Mapping agents also use IP multicast to discover
  which routers in the network are possible
  candidate RPs by joining the Cisco-RP-announce
  (224.0.1.39) group to receive candidate RP
  announcements.
• Candidate RPs send RP-announce multicast messages
  for the particular groups every 60 seconds.
• The RP mapping agent uses the information
  contained in the announcement to create entries
  in group-to-RP cache.
• RP mapping agents create only one entry per
  group.
• If more than one RP candidate announces the same
  range, then the RP mapping agent uses the IP
  address of the RP to break the tie.

57
Cisco-RP-announce
Im a member of the 224.0.1.39 multicast group,
Cisco-RP-announce. This will tell me who the
candidate RPs are.
Im a candidate RPs. I will send this every 60
secs to 224.0.1.39.

• Mapping agents discover which routers in the
  network are possible candidate RPs.
• RP mapping agent uses the information contained
  in the announcement to create entries in
  group-to-RP cache.

58
Cisco-RP-discovery
Im the RP Mapping Agent, here are the
group-to-RP mappings. (every 60 secs)
Im going to learn about group-to-RP mappings
because I am a member of the 224.0.1.40 multicast
group, Cisco-RP-discovery.
Auto-RP

• All routers in the PIM network learn about the
  active group-to-RP mapping from the RP mapping
  agent.
• Note It is recommended that a RP mapping agent
  be configured on the router with the best
  connectivity and stability.

59
Configuring Multicast
60
Configuring PIM DM/SM

• First, enable multicast routing (disabled by
  default)
• Router(config)ip multicast-routing
• Next, enable PIM on an interface.
• It is best to enable PIM on every interface in
  every router in the network, using the following
  interface command
• Router(config-if)ip pim dense-mode sparse
  mode sparse-dense-mode

61
Configuring PIM DM/SM

• The recommended method for enabling multicast on
  an interface is the use of the ip pim
  sparse-dense-mode command.
• This command allows the router to use either
  dense or sparse mode, depending on the existence
  of RP information for each multicast group.
• This makes it much easier to switch the entire
  network from dense mode to sparse mode (or vice
  versa) as needed.

62
Configuring PIM DM/SM

• PIM SM only
• Because PIM SM uses a shared tree, you must also
  specify the rendezvous point address.
• The RP doesnt need to know it is the RP.
• A PIM router can be an RP for more than one
  group.
• To designate the RP on a leaf router
• Router(config)ip pim rp-address ltaddressgt

63
Bootstrap Router

• A bootstrap router (BSR) is a router or Layer 3
  device that is responsible for distributing RP.
• Another way to distribute group-to-RP mapping
  information.
• BSR works only with PIM version 2.
• Uses hop-to-hop flooding of special BSR messages
  instead of multicast to distribute the
  group-to-RP mapping.

64
BSR Election

• BSR uses an election mechanism to select the BSR
  router from a set of candidate routers and
  multilayer switches in the domain.
• The BSR election uses the BSR priority of the
  device contained in the BSR messages that flow
  hop-by-hop through the network.

65
Whos the BSR?
TTL1
TTL1
TTL1

• BSR sends BSR messages with a TTL of 1 with its
  IP address to enable candidate BSRs to learn
  automatically about the elected BSR.
• Neighboring PIM version 2 routers or multilayer
  switches receive the BSR message and multicast
  the message out all other interfaces (except the
  one on which it was received) with a TTL of 1 to
  distribute the BSR messages hop-by-hop.

66
Candidate RPs
TTL1
BSR sends this info to all PIM routers
Candidate RPs send advertisement to BSR with
groups they are responsible for.
TTL1
TTL1
All routers can now map mulitcast groups to a
specific RP.

• Candidate RPs send candidate RP advertisements
  showing the group range for which each is
  responsible to the BSR, which stores this
  information in its local candidate RP cache.
• The BSR includes this information in its
  bootstrap messages and disseminates it to all PIM
  routers using 224.0.1.13 with a TTL of 1 in the
  domain hop-by-hop.
• Based on this information, all routers can map
  multicast groups to specific RPs.
• As long as a router is receiving the bootstrap
  message, it has a current RP map.
• Routers and multilayer switches select the same
  RP for a given group because they all use a
  common RP hashing algorithm.

67
Comparison and Compatibility of PIM Version 1 and
Version 2

• PIM version 2 is a standards-based multicast
  protocol in the Internet Engineering Task Force
  (IETF).
• Cisco highly recommends using PIM version 2 in
  the entire multilayer switched network.
• Cisco's PIM version 2 implementation allows
  interoperability and transition between version 1
  and version 2, although there are a few caveats.

68
IGMP - Internet Group Management Protocol

• Hosts use IGMP to dynamically register themselves
  in a multicast group on a particular LAN.
• Hosts identify group memberships by sending IGMP
  messages to their local multicast router.
• Routers and multilayer switches, configured for
  IGMP, listen to IGMP messages and periodically
  send out queries to discover which groups are
  active or inactive on a particular subnet or
  VLAN.
• The following list indicates the current versions
  of IGMP
• IGMP version 1 (IGMPv1) RFC 1112
• IGMP version 2 (IGMPv2) RFC 2236
• IGMP version 3 (IGMPv3) RFC 3376
• IGMP version 3 lite (IGMPv3 lite)

69
IGMP

• IGMP v1 version v1
• No way to expressly leave a multicast group.
• Its up to the router to timeout the group
  membership
• IGMP v2 version v2
• Includes leave processing mechanism
• IGMP v3 version v3
• Supports "source filtering," which enables a
  multicast receiver host to signal to a router
  which groups it wants to receive multicast
  traffic from, and from which source(s) this
  traffic is expected.
• IOS release 12.1(5) and later.
• Current IOS release (12.2) still uses IGMPv2 as
  the default

70
IGMPv1

• One multicast router per LAN must periodically
  transmit host membership query messages to
  determine which host groups have members on the
  router's directly attached LAN networks.

• IGMP query messages are addressed to the all-host
  group (224.0.0.1) and have an IP TTL equal to 1.
• A TTL of 1 ensures that the corresponding router
  does not forward the query messages to any other
  multicast router.
• When the end station receives an IGMP query
  message, the end station responds with a host
  membership report for each group to which the end
  station belongs.
• IGMP messages are specified in the IP datagram
  with a protocol value of 2.

71
IGMPv1

• Routers use IGMP to query hosts on a subnet as to
  what multicast groups they belong to.
• Hosts dont have to wait for the query to join a
  group they can immediately send a join request
• Routers keep track of the multicast groups that
  are active on a subnet (not the actual hosts that
  are in each group)

72
IGMPv1

• IGMP Queriers (routers) send queries every 60
  seconds.
• If a host does not respond with its membership
  information, the router will timeout the hosts
  group membership
• This process could take up to 3 minutes (not
  good).
• IGMPv1 Queriers are determined by a multicast
  routing protocol, not IGMPv1.
• The specific multicast routing protocol elects a
  designated router for the subnet.
• This router also becomes the IGMPv1 Querier.

73
IGMPv1

• From the routers perspective, it is not a host
  that joins the multicast group, but an interface.
• All the router wants to know is if a segment is
  supposed to receive the multicast traffic.
• It does not keep track of the exact hosts that
  are making the multicast requests. (Unless using
  CGMP)
• The multicast traffic is sent to an entire cable
  segment, not to a single host.

74
IGMPv2

• RFC 2236 (November 1997)
• Primarily to address the issues of leave and join
  latencies.
• IGMP Queriers (routers) send two kinds of
  queries
• General queries (same as IGMPv1 queries)
• Group-specific queries (directed at single group)

75
IGMPv2 - Join
To 224.0.0.2

• The process of joining a multicast group is the
  same in IGMPv2 as in IGMPv1.
• Like IGMPv1, IGMPv2 hosts do not have to wait for
  a query to join.
• When a host wants to join a multicast group, it
  sends a host membership report to the all-router
  group address 224.0.0.2.

76
IGMPv2 - Join
To 224.0.0.2

• When the host and server reside on different
  subnets, the join message must go to a router.
• When the router intercepts the message, it looks
  at its IGMP table.
• If the network number is not in the table the
  router adds the information contained in the IGMP
  message.
• When the router receives a multicast packet, it
  forward the packet to only those interface that
  have hosts with processes belonging to that group.

77
IGMPv2 - Join
To 224.0.0.2

• IGMPv2 defines a procedure for electing the
  multicast querier (router) for each network
  segment.
• Router with the lowest IP address becomes the
  Querier.
• Initially, every router believes itself to be the
  querier for every one of the routers interface
  that are multicast-enabled.
• IGMPv2 has group-specific queries.
• General query multicasts to the all-hosts
  224.0.0.1
• Group-specific query multicasts to the multicast
  group address.

78
IGMPv2 - Join
To 224.0.0.2

• Similar to IGMPv1, IGMPv2 router multicasts
  periodic membership queries to the all-hosts
  (224.0.0.1) group address.
• Only one member (host) per group responds with a
  report to a query.
• IGMP uses interval and timeout timers for this
  process.
• http//www.cisco.com/univercd/cc/td/doc/product/la
  n/c3550/1214ea1/3550scg/swmcast.htm

79
IGMPv2 - Leave

• Leave group messages provides hosts with a
  method of notifying routers and multilayer
  switches on the network that they are leaving a
  group.

80
IGMPv2 - Leave

• Hosts 2 and 3 are members of multicast group
  224.1.1.1.
• Host 2 sends an IGMPv2 leave message to the
  all-multicast-routers group (224.0.0.2) to inform
  all routers and multilayer switches on the subnet
  that it is leaving the group.
• Router 1, the query router, receives the message,
  but because it keeps a list only of the group
  memberships that are active on a subnet and not
  individual hosts that are members, it sends a
  group-specific query to the target group
  (224.1.1.1) to determine whether any hosts remain
  for the group.
• Host 3 is still a member of multicast group
  224.1.1.1 and receives the group-specific query.
• It responds with an IGMPv2 membership report to
  inform Router 1 that a member is still present.
• When Router 1 receives the report, it keeps the
  group active on the subnet.
• If no response is received, the query router
  stops forwarding its traffic to the subnet.

81
IGMPv3

• IGMPv3 is the next step in the evolution of IGMP.
  
• IGMPv3 adds support for source filtering that
  enables a multicast receiver to signal to a
  router the groups from which it wants to receive
  multicast traffic, and also from which sources to
  expect traffic.
• This membership information enables Cisco IOS
  software to forward traffic from only those
  sources from which receivers requested the
  traffic.
• IGMPv3 supports applications that explicitly
  signal sources from which they want to receive
  traffic.

82
Layer 2 Multicast Protocols
Multicast Traffic 1.5-Mbps IP multicastbased
video feed sent from a corporate video server
Sent out all interface on that VLAN

• Similar to Layer 3 hardware switching properties
  of switches, switches also have Layer 2 features
  to control multicast traffic.
• The default behavior for a Layer 2 interface on a
  switch is to forward all multicast traffic to
  every Layer 2 interface that belongs to the
  destination VLAN on the switch.
• This behavior reduces the efficiency of
  multilayer switching at Layer 2, whose purpose is
  to limit traffic to the interfaces that need to
  receive the data.

83
Layer 2 Multicast Protocols
Multicast Table
Multicast Traffic 1.5-Mbps IP multicastbased
video feed sent from a corporate video server
Sent only to those hosts that have joined that
multicast group.

• Layer 2 switches have some degree of multicast
  awareness to avoid flooding multicasts to all
  switch ports.
• The following are the two methods to control
  multicast at Layer 2 on multilayer switches
• IGMP snooping
• Cisco Group Management Protocol (CGMP)

84
IGMP Snooping
I have to examine every multicast packet to see
if there are any join or leave requests. Whew!
This is a lot of work!
Multicast Table
Multicast Traffic 1.5-Mbps IP multicastbased
video feed sent from a corporate video server
Sent only to those hosts that have joined that
multicast group.

• IGMP snooping is an IP multicast constraining
  mechanism that examines Layer 2 and Layer 3 IP
  multicast information to maintain a Layer 2
  multicast table.
• IGMP snooping operates on multilayer switches,
  even switches that do not support Layer 3
  routing.
• IGMP snooping requires the LAN switch to examine,
  or snoop, the IGMP join and leave messages,
  sent between hosts and the first-hop multicast
  router.
• The IGMP protocol transmits messages as IP
  multicast packets as a result, switches cannot
  distinguish IGMP packets from normal IP multicast
  data at Layer 2.

85
IGMP Snooping
I have to examine every multicast packet to see
if there are any join or leave requests. Whew!
This is a lot of work!
Multicast Table
Multicast Traffic 1.5-Mbps IP multicastbased
video feed sent from a corporate video server
Sent only to those hosts that have joined that
multicast group.

• Therefore, a switch running IGMP snooping must
  examine every multicast data packet to determine
  whether it contains any pertinent IGMP control
  information.
• If IGMP snooping is implemented on a low-end
  switch with a slow CPU, this could have a severe
  performance impact when data is transmitted at
  high rates.
• The solution to this problem is to implement IGMP
  snooping with special ASICs that can perform IGMP
  snooping in hardware.
• Without specialized ASICs for IGMP snooping to
  operate with hardware switching, CGMP is the
  preferable choice for low-end switches.

86
CGMP

• CGMP (Cisco Group Management Protocol) is a
  Cisco-developed protocol that allows Catalyst
  switches to learn about the existence of
  multicast clients from Cisco routers and Layer 3
  switches.

• CGMP is based on a client/server model.
• The router is considered a CGMP server, with the
  switch taking on the client role.
• The basis of CGMP is that the IP multicast router
  sees all IGMP packets and, therefore, can inform
  the switch when specific hosts join or leave
  multicast groups.
• The switch then uses this information to
  construct a forwarding table.

87
CGMP
Multicast Packets
IGMP Join Request

• When the router sees an IGMP control packet, the
  router creates a CGMP packet.
• This CGMP packet contains the request type
  (either join or leave), the multicast group
  address, and the actual MAC address of the
  client.
• The packet is sent to a well-known address to
  which all switches listen.
• Each switch then interprets the packet and
  creates the proper entries in a forwarding table.

88
CGMP

• CGMP is a legacy multicast switching protocol.
• All current-generation (and future) Catalyst
  switches support IGMP snooping.
• IGMP snooping has several advantages over CGMP,
  such as the ability to operate without a
  first-hop router, and is less CPU intensive.

89
Configuring IGMP

• IGMP Version 2 mode is the default for all
  systems using Cisco IOS Release 11.3(2)T or
  later. To determine the current version use
• Routershow ip igmp interface type-number
• To change versions (per interface only)
• Router(config-if)ip igmp version 2 1

90
Configuring IGMP joins

• A router is configured to be a member of a
  specific multicast group if you want that router
  to respond to commands addressed to that group,
  such as pings.
• Typically, you will manually configure your
  router to belong to a multicast group for testing
  purposes.
• To have the router join a multicast group, enter
  the following command in interface configuration
  mode.
• Router(config-if)ip igmp join-group
  group-address

91
Configuring CGMP

• CGMP can run on an interface only if PIM is
  configured on the same interface.
• CGMP is disabled by default.
• To enable CGMP on the router, enter the following
  command in the interface configuration mode
• Router(config-if)ip cgmp

92
Configuring CGMP

• Configuring CGMP on the switch allows IP
  multicast packets to be switched to only those
  ports that have IP multicast clients.
• The switch must be connected to an CGMP-enabled
  router.
• CGMP on the switch automatically identifies the
  ports to which the CGMP-capable router is
  attached.
• IOS-based switch (enabled by default)
• Switch(config) cgmp
• Set-based switch
• Switch(enable) set cgmp enable

93

• ? Suggested Reading
• Developing IP Multicast Networks The Definitive
  Guide to Designing and Deploying CISCO IP
  Multicast Networks
• by Beau Williamson Cisco Press ISBN
  1578700779