Configuring Source Specific Multicast
Benefits
IPC-464
Cisco IOS IP Configuration Guide
Because the router returns a Content-Type of text and HTML, the best way to include the URD intercept
URL into a web page is to use a frame. By defining the size of the frame, you can also hide the URD
intercept URL on the displayed page.
By default, URD is disabled on all interfaces. When URD is configured through the ip urd interface
configuration command on an interface, it will be active only for IP multicast addresses in the SSM
range.
Benefits
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).
The current standard solutions for ISM service are PIM-SM and MSDP. Rendezvous point (RP)
management in PIM-SM (including the necessity for Auto-RP or BSR) and MSDP is required only for
the network to learn about active sources. This management is not necessary in SSM, which makes SSM
easier than ISM to install and manage, and therefore easier than ISM to operationally scale in