IP Multicasting
CIS 185 Advanced Routing
CCNP 1
Rick Graziani
Fall 2008
Multicast Topics
Multicast
Traffic
Sender
Multicast
Traffic
Receiver
IGMP
IGMP
Join/Leave
Receiver
CGMP
CGMP
Non-Receiver
Multicast
Traffic
CGMP
Receiver
PIM
Multicast
Routing
Protocols
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IP Multicast
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.
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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
Rick applications
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delivering data, voice, or video to multiple receivers.
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
Rick packets
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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.
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Uses UDP
0
15
16
Application
Header + data
31
16-bit Source Port Number
16-bit Destination Port Number
16-bit UDP Length
16-bit UDP Checksum
Data (if any)
•
•
•
•
IP multicast traffic uses UDP as the transport layer.
UDP adds no reliability, flow control, or error-recovery
functions to IP.
UDP, simplicity, 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.
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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.
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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),
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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.
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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.
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Multicast IP Address Structure
• Applications allocate multicast addresses dynamically or statically.
• Dynamically:
•
– Provides applications with a group address on demand.
– Have a specific lifetime
– Applications must request this type of address only for as long as
the address is needed.
Statically:
– 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.
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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.
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
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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 multicastcapable 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.
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Multicast MAC Address Structure
•
•
The destination IP address of IP multicast packets maps
to a multicast MAC address.
The multicast MAC address is derived from the IP
multicast address.
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Multicast MAC Address Structure
• Multicasts MAC address
• First 3 bytes (24 bits) are always 01-00-5E.
• Binary: 00000001.00000000.01011110.0xxxxxxx.xxxxxxxx.xxxxxxxx
•
– Indicates a multicast application.
– 25th bit set to 0.
– Usually, is the 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
IP:1110xxxx.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-toMAC address mapping process (uniqueness of IP address).
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Multicast MAC Address Structure
224 - 239
Every multicast IP address
starts with 1110, so this part
doesn’t need to be in the MAC
address
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BUT, these 5 bits don’t make it
to the MAC address.
23 bits of the actual IP
multicast address get
translated into the MAC
address.
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Multicast MAC Address
Structure
• This method for mapping multicast
IP addresses to MAC addresses
results in a 32:1 mapping, whereas
each multicast MAC address
represents a possible 32 distinct
IP multicast addresses.
02
MAC:
00000001.00000000.01011110.00000001.00000001.00000010
IP:
11100000.00000001.00000001.00000010
224.1.1.2
11100000.10000001.00000001.00000010
224.129.1.2
11100001.00000001.00000001.00000010
225.1.1.2
TO
11101111.10000001.00000001.00000010
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239.129.1.2
21
Multicast MAC Address Structure
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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
25th bit “0”
First 3 bytes
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 224.192.255.30 to a multicast MAC address.
224
|
192
|
255
|
30
1110 0000 1100 0000 1111 1111 0001 1110
01-00-5E
25th bit “0”
First 3 bytes
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
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Multicast MAC Address Structure
01:00:5E:01:01:02 [IP 224.129.1.2]
IP Mapped Ethernet
Multicast Frames
01:00:5E:01:01:02 [IP 225.1.1.2]
Joined 224.129.1.2 so NIC is listening for 01-00-5E-01-01-02.
This multicast MAC can belong to several multicast IP
addresses: 224.129.1.2 and 225.1.1.2.
• 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 processes both sets of IP multicast frames.
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.
Remember: Each multicast MAC address represents a possible 32
distinct IP multicast addresses
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Multicast MAC Address Structure
01:00:5E:01:01:02 [IP 224.129.1.2]
IP Mapped Ethernet
Multicast Frames
01:00:5E:01:01:02 [IP 225.1.1.2]
Joined 224.129.1.2 so NIC is listening for 01-00-5E-01-01-02
I’ll let UDP port numbers determine which one I want to process.
• 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:
01:00:5E:01:01:02
[IP 224.129.1.2]
Multicast Group:
01:00:5E:01:01:02
[IP 225.1.1.2]
Multicast Group:
01:00:5E:01:01:02
[IP 224.129.1.2]
• 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 (by default):
– Traffic is routed through the network along the path from the single source
to the single destination host.
– Router does not consider the source address,
– Considers only the destination address.
In multicast forwarding:
– Single source is sending traffic to group of hosts
• Represented by a single multicast group address.
– When a multicast router receives a multicast packet:
• Determines which direction is the upstream direction (toward the
source)
• 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:
1. 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.
2. 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.
3. 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
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:
– Router forwards the packet.
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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
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Source Trees
• The simplest form of a multicast distribution tree is a source tree with:
•
•
– Its root at the source
– 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.
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Source Trees
• Using the (S,G) notation, the SPT for the example shown is
(192.168.1.1, 224.1.1.1).
– Implies that a separate SPT exists for each individual source
sending to each group.
– 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).
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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.
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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).
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Comparing:
Source 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.
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Comparing:
Shared Trees
• Shared trees have the
•
advantage of requiring the
minimum amount of state
information in each router. Better Path
This advantage lowers the
overall memory requirements
and complexity for a network
that only allows shared trees.
• 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 tree–only environment.
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IP Multicast Routing
IP Multicast Routing
PIM Dense
Mode
PIM Sparse
Mode
• 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)
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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.
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PIM
PIM Dense
Mode
PIM Sparse
Mode
• Routers use the PIM neighbor discovery mechanism to establish PIM
•
neighbors using hello messages to:
– Build and maintain PIM multicast distribution trees
• ALL-PIM-Routers (224.0.0.13) multicast address
– Elect the designated router (DR) for a multicast LAN network.
Two distinct versions: PIM version 1 and PIM version 2.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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PIM Sparse Mode
Shared
Tree
• PIM-SM is optimized for environments where there are many
multipoint data streams.
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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.
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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
• It 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
– Bootstrap router (BSR)
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Auto-RP: Cisco-RP-announce
I’m a member of the 224.0.1.39 multicast
group, Cisco-RP-announce. This will tell
me who the candidate RPs are.
I’m 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.
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Auto-RP: Cisco-RP-discovery
I’m the RP Mapping Agent, here are the
group-to-RP mappings. (every 60 secs)
I’m going to learn about
group-to-RP mappings
because I am a member
of the 224.0.1.40
multicast group, CiscoRP-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.
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I’m the RP Mapping Agent, here are the
group-to-RP mappings. (every 60 secs)
I’m going to learn about
group-to-RP mappings
because I am a member
of the 224.0.1.40
multicast group, CiscoRP-discovery.
FYI
Auto-RP
• 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).
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I’m a member of the 224.0.1.39 multicast
group, Cisco-RP-announce. This will tell
me who the candidate RPs are.
I’m a candidate RPs. I
will send this every 60
secs to 224.0.1.39.
FYI
• Mapping agents also use IP multicast to discover which routers in the
•
•
network are possible candidate RPs by joining the Cisco-RPannounce (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.
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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.
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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.
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TTL=1
Who’s the BSR?
TTL=1
TTL=1
• 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.
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Candidate
RPs
BSR sends this info
to all PIM routers
Candidate RPs send
advertisement to
BSR with groups
they are responsible
for.
•
•
•
•
•
TTL=1
TTL=1
TTL=1
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.
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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.
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Configuring Multicast
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Configuring PIM DM/SM
•
First, enable multicast routing (disabled by default):
Router(config)#ip multicast-routing
•
•
Next, enable PIM on each interface.
The recommended method for enabling multicast
on an interface is the use of the ip pim sparsedense-mode command.
Router(config-if)#ip pim {dense-mode |
sparse mode | sparse-dense-mode}
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Configuring RPs
• Manually designate the RP:
Router(config)#ip pim rp-address <address>
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Configuring RPs
Auto RP
• Configure a mapping agent to learn of about all candidate RPs so it
can compike a list of which routers are RP for which grpups and
advertise the list to client routers.
Router(config)#ip pim send-rp-discovery scope <ttl>
• Configure a candidate RP to announce itself as a possible RP for a
multicast range.
Router(config)#ip pim send-rp-announce <interface>
scope <ttl>
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Configuring RPs
BSR
• Define the BSR.
Router(config)#ip pim bsr-candidate <interface>
<hashing-function>
• Configure a candidate RP.
Router(config)#ip pim rp-candidate <interface>
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IGMP - Internet Group Management Protocol
IGMP
IGMP
Join/Leave
•
•
•
•
Receiver
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)
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IGMP
IGMP
IGMP
Join/Leave
Receiver
• IGMP v1 – version v1
•
•
– No way to expressly leave a multicast group.
– It’s 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
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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
Rick of
Graziani
70
2. graziani@cabrillo.edu
IGMPv1
• Routers use IGMP to query hosts on a subnet as to what multicast
•
groups they belong to.
– Hosts don’t 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)
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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
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.
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IGMPv1
• From the router’s 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.
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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)
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IGMPv2 - Join
Membership report 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.
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IGMPv2 - Join
Membership report to
224.0.0.2
•
When the router receives a multicast packet, it forward the
packet to only those interface that have hosts with
processes belonging to that group.
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IGMPv2 - Join
Membership report 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 router’s 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.
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IGMPv2 - Join
General Membership
Query 224.0.0.1
Membership report 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/lan/c3550/1214ea
1/3550scg/swmcast.htm
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IGMPv2 - Leave
Leave msg to
224.0.0.2
Membership
report to
224.0.0.2
Group-specific Query
224.1.1.1
• Leave group messages — provides hosts with a method of
notifying routers and multilayer switches on the network that they
are leaving a group.
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IGMPv2 Leave
Leave msg to
224.0.0.2
Membership
report to
224.0.0.2
Group-specific Query
224.1.1.1
•
•
•
•
•
•
•
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.
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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.
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Layer 2 Multicast Protocols
Multicast Traffic:
1.5-Mbps IP
multicast–based
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.
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Layer 2 Multicast Protocols
Multicast Table
Multicast Traffic:
1.5-Mbps IP
multicast–based
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)
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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
multicast–based
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.
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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
multicast–based
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.
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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:
Router#show ip igmp interface type-number
• To change versions (per interface only):
Router(config-if)#ip igmp version {2 | 1}
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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
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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.
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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.
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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.
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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
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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
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FYI - Non-RPF Traffic
Source IP Address is not on these
interfaces, but interface
connected to Campus Network
Router.
Do Not
Forward
• 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.
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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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IP Multicasting
CIS 185 Advanced Routing
CCNP 1
Rick Graziani
Fall 2008