3Com 4007, Implementation Manual, Page: 218 / 684

218 C
HAPTER
11: IP M
ULTICAST
F
ILTERING WITH
IGMP
Effects of MAC
Address Aliasing
Operating as a Layer 2 device, your module filters IP multicast traffic by
referring to hexidecimal MAC addresses that correspond to binary IP
multicast group addresses.
A multicast MAC address is created by selecting only the low order
23-bits in the Class D binary IP address, translating that portion to
hexidecimal format, and attaching it to a standard set of bits that signify
it is a multicast packet (01-00-5E). For example, IP address 224.10.8.5
becomes MAC address 01-00-5E-0A-08-05.
Because only a portion of the binary IP address is translated, several
different binary addresses can map to the same hexidecimal MAC
address. This situation is called
MAC address aliasing.
Because a switching module cannot distinguish such packets, MAC
address aliasing has two main implications:
■
Some packets are forwarded to more ports than actually require it.
For example, if requests for multicast group 226.1.2.3 are registered
on port 1 and requests for group 227.1.2.3 are registered on port 2,
these IP addresses map to the same MAC address and the module
forwards traffic for both groups to both ports.
■
Packets with certain addresses can never be filtered by IGMP.
In most cases, such packets are routing protocol advertisements that
use addresses from the block of permanent reserved addresses that is
administered by the Internet Assigned Numbers Authority (IANA). To
ensure these advertisements make their way through the network, it is
important that these packets do not get filtered by IGMP. However, if
an IP multicast application uses a group address that maps (due to
MAC address aliasing) to one of these permanent addresses, these
packets cannot be filtered by IGMP either.