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Case Study
A few months ago I converted all our interfaces over to
IGMP Version 3. Then I started getting complaints from
our lab/classroom support group that Norton Ghost was
failing for them. It would hang after about 3 minutes. So
far the fix, without understanding why it works, has been
to revert the interfaces to IGMP version 2. The switches
downstream from these interfaces are running CGMP and
CGMP LEAVE (which is actually a form of IGMP
snooping/spoofing for IGMP Leaves sent to 224.0.0.2). I
suspect that the fact that these switches are actually
looking at IGMP packets may have something to do with
the problem that reverting to v2 fixed...
— Alan Crosswell
Engineering Workshops
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Case Study
This author traveled to Los Alamos, New Mexico to help
debug a multicast problem that had everyone stumped.
Everyone was assuming the only known router on the
subnet was also acting as the multicast gateway.
Unfortunately, this wasn’t the case. A nominally Layer
2 switch on the subnet was accidentally configured with
PIM active, and won the PIM Designated Router
election. Of course, this Layer 2 switch had no upstream
to anywhere.
— Bill Nickless
Engineering Workshops
One Approach to
Multicast on the LAN
• Avoid snooping, as it causes more problems than it
solves.
• Keep subnets small. A smaller subnet is less likely
to have people joining several different multicast
groups, traffic for each of which is sent to the entire
subnet.
• If at all possible, use routers, not switches or
bridges.
• If you have to use switches, try to at least buy them
all from the same vendor, so you won’t have
inconsistent behavior as well as unexpected
behavior.
Engineering Workshops
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23
Another Approach to
Multicast on the LAN
• The previous approach reflects gigaPoP/WAN bias.
• On a campus, it just isn't possible to use routers
everywhere.
• Switches and snooping may be evils, but they are
necessary evils. Learn to cope with them.
http://www.cisco.com/warp/public/473/22.html
is a good place to start.
Engineering Workshops
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Lab 1: Multicast on the LAN
Engineering Workshops
25
SSM
Engineering Workshops
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PIM-SM
• SM stands for “Sparse Mode.”
– RFC 2362 and draft-ietf-pim-sm-v2-new-06.txt
– There is also a Dense Mode, but we don’t
recommend using it.
– Cisco has a proprietary “Sparse-Dense” mode
which is used for RP discovery.
• PIM-SM allows for both RPTs and SPTs.
• There are two ways to use PIM-SM…
Engineering Workshops
27
ASM and SSM
• ASM: Any-Source Multicast. Traditional multicast – data
and joins are forwarded to an RP.
– All routers in a PIM domain must have RP mapping.
– When load exceeds threshold, forwarding switches to an
SPT. The default threshold is one packet; in this case,
the sole purpose of the RPT is to learn which sources
are active. (With IGMPv2, the receiver can only specify
the group, not specific sources.)
– State increases (not everywhere) as number of sources
and number of groups increase.
– SPT state is refreshed when data is forwarded and with
Join/Prune control messages.
• SSM: Source-Specific Multicast. PIM-SM without RPs –
instead, the source is learned out-of-band, and the SPT is
built directly to it.
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SSM
• Source-Specific Multicast (SSM) is a subset of ASM,
so
– SSM concepts apply directly to ASM, but
– SSM is a lot simpler than ASM.
For these reasons, we cover SSM first in this
workshop.
• 232 / 8 is assigned to SSM as an address space. Other
address ranges can also be set up for SSM — this is
primarily a function of the receiving network.
• Source activity and IP addresses are assumed
known.
• IGMPv3 allows for “Include” lists of (S,G) pairs.
Engineering Workshops
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SSM
• SSM - draft-ietf-ssm-arch-01.txt
– 232/8 – IANA assigned
– No RPTs
– Guarantees ONE source on any delivery tree
• Content security – no unwanted sources
– Reduced protocol dependence – more later...
– Solves address allocation issues for inter-domain one-to-many
• tree address is 64 bits – S,G
– Host must learn source address out-of-band (e.g, from a web page)
– Host-to-router join request specifies source as well as group
• requires IGMPv3 for include-source list
– SSM behavior in 232/8 by default
• Configurable to expand range
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SSM in Action
• Each (S,G) pair listed in the IGMPv3
include list generates a (S,G) Join directly
towards the source.
• That’s it. It’s very simple. All you need to
implement is :
– Edge routers need IGMPv3
– Interior routers need filters to prevent RP
(*,G) Joins & other RP state for the SSM
address block
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SSM Group Addresses
• 232 / 8 is assigned to SSM as an address space.
– You don’t have to ask, you can just pick one and
use it.
• How can this be ?
– Note that all joins are unique as long as the
combination of S and G are unique. Not only can
one source support multiple groups, but if there
are two sources using the same group address,
everything works just fine.
Engineering Workshops
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Lab 2: SSM
Engineering Workshops