Igmpv3 Host Signaling; Benefits Of Ssm - Cisco Catalyst 3850 series Configuration Manual
Ip multicast routing configuration guide
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Configuring SSM
• For groups within the SSM range, no MSDP Source-Active (SA) messages within the SSM range will
IGMPv3 Host Signaling
IGMPv3 is the third version of the IETF standards track protocol in which hosts signal membership to last-hop
routers of multicast groups. IGMPv3 introduces the ability for hosts to signal group membership that allows
filtering capabilities with respect to sources. A host can signal either that it wants to receive traffic from all
sources sending to a group except for some specific sources (a mode called EXCLUDE) or that it wants to
receive traffic only from some specific sources sending to the group (a mode called INCLUDE).
IGMPv3 can operate with both ISM and SSM. In ISM, both EXCLUDE and INCLUDE mode reports are
accepted by the last-hop router. In SSM, only INCLUDE mode reports are accepted by the last-hop router.
Benefits of SSM
IP Multicast Address Management Not Required
In the ISM service, applications must acquire a unique IP multicast group address because traffic distribution
is based only on the IP multicast group address used. If two applications with different sources and receivers
use the same IP multicast group address, then receivers of both applications will receive traffic from the
senders of both applications. Even though the receivers, if programmed appropriately, can filter out the
unwanted traffic, this situation would cause generally unacceptable levels of unwanted traffic.
Allocating a unique IP multicast group address for an application is still a problem. Most short-lived applications
use mechanisms like Session Description Protocol (SDP) and Session Announcement Protocol (SAP) to get
a random address, a solution that does not work well with a rising number of applications in the Internet. The
best current solution for long-lived applications is described in RFC 2770, but this solution suffers from the
restriction that each autonomous system is limited to only 255 usable IP multicast addresses.
In SSM, traffic from each source is forwarded between routers in the network independent of traffic from
other sources. Thus different sources can reuse multicast group addresses in the SSM range.
Denial of Service Attacks from Unwanted Sources Inhibited
In SSM, multicast traffic from each individual source will be transported across the network only if it was
requested (through IGMPv3, IGMP v3lite, or URD memberships) from a receiver. In contrast, ISM forwards
traffic from any active source sending to a multicast group to all receivers requesting that multicast group. In
Internet broadcast applications, this ISM behavior is highly undesirable because it allows unwanted sources
to easily disturb the actual Internet broadcast source by simply sending traffic to the same multicast group.
This situation depletes bandwidth at the receiver side with unwanted traffic and thus disrupts the undisturbed
reception of the Internet broadcast. In SSM, this type of denial of service (DoS) attack cannot be made by
simply sending traffic to a multicast group.
Easy to Install and Manage
SSM is easy to install and provision in a network because it does not require the network to maintain which
active sources are sending to multicast groups. This requirement exists in ISM (with IGMPv1, IGMPv2, or
IGMPv3).
OL-32598-01
be accepted, generated, or forwarded.
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
IGMPv3 Host Signaling
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