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IP Multicasting

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Title: 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

16
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)

18
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.

19
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).

20
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

22
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
24
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.

26
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).

27
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.

28
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.

29
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.

30
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.

31
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.

32
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.

33
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
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