HP 600 Series, Services And Applications

Page: 67 / 316

First, consider the network in figure 7. Assume station addresses are
statically assigned. Assume system A’s station address is 080009000000H, and
system B’s station address is 010000000000. Each address is an individual
address on its associated LAN. When packets are transmitted, the first part
of the MAC header transmitted is the
destination station address. When A
sends a frame to B, the first bit transmitted is a “1”, since the order of
transmission for Ethernet is LSB (least-significant bit) of the most-significant
byte first. This bit is also the multicast bit. Thus, the packet that was
intended to be sent as a unicast packet is unintentionally sent as a multicast
packet. Other systems on the Ethernet will interpret the packet transmitted
by system A as a multicast packet. The accidental multicast packet may be
misinterpreted by systems as a valid multicast packet, with unpredictable
results. Worse, all bridges connected to the Ethernet will forward the packet.
In this case, normal unicast packets are treated the same as broadcast
packets, with potentially disastrous effects on network utilization.
Figure 7. Unsupported Ethernet/Token Ring Bridged Network
The second problem arises when station addresses are determined
dynamically. ARP (the Address Resolution Protocol) is used by IP end nodes
to determine the station address of a destination system for which the IP
address (network-layer address) is known.
SRT Bridge
ROUTER
System B
System A
R
o
u
ti
n
g
S
e
r
v
ic
e
s
N
o
te
s
2
Bridging Service
Token Ring Solutions
2-15