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DES-7200 Configuration Guide Chapter 4 MLD
Configuration
4-3
4.1.1.2 MLD Version 2
MLDV1 has the following defects in application:
Lack of effective means to control multicast sources.
Being unaware of the location of multicast source, it is comparatively difficult to
establish the multicast route.
It is very difficult to discover an only multicast address. Multiple multicast groups
may use the same multicast address.
On the basis of MLDv1, MLDv2 provides the additional multicast source filtering
mode (INCLUDE/EXCLUDE). In MLDv1, the host determines to join a certain
group only according to the group address, and receives multicast streams sent
from any source to this group address. In MLDv2, the host informs the host of the
multicast group it desires to join in, and also notifies this host of addresses of
multicast sources that it is willing/unwilling to receive. The host can indicate which
sources to receive multicast streams via an INCLUDE list or an EXCLUDE list.
When the host joins a multicast group:
If the host only needs to receive the data streams sent from source {s1,s2,s3…},
then its Report message can contain the information of INCLUDE{s1,s2,s3…}.
If the host doesn't want to receive the data streams sent from source {s1,s2,s3…},
then its Report message can contain the information of EXCLUDE{s1,s2,s3…}.
Through source address filtering, MLDv2 can save network bandwidth and
prevent unnecessary and invalid multicast data streams from occupying the
network bandwidth. This is especially useful when multiple multicast sources are
sharing one multicast address. Although MLDv1 can also realize "source address
filtering" in a certain sense, it is done on the receiving end of multicast streams.
As shown in Fig 1, S1 and S2 are multicast sources sending data streams with
the same multicast address of G. The multicast streams of S1 and S2 will be sent
to all hosts receiving G. If host A only wants to receive the data streams of S1, in
order to avoid the disturbance of data streams from S2, we can only use the
corresponding Client software to perform filtering.
Fig 3 MLDv1 forwarding diagram
If the devices on network can support MLDv2: host A can send the MLDv2
message of join G include {S1} if it only wants to receive data streams from S1,
and host B can send the MLDv2 message of join G include {S2} if it only wants to
receive data streams from S2. The forwarding of data streams will be shown as
follows, allowing the saving of partial bandwidth.