HP 200 Series Services And Applications page 68

Bridging Service
Token Ring Solutions
Figure 8 shows the sequence of packets sent when system A (from figure 7)
tries to determine the station address of system B using ARP, and then sends
a data packet to B. First, system A broadcasts an ARP request, which the
SRT bridge forwards onto the token ring. The packet is received by system
B, which responds by sending an ARP reply with its station address appropri-
ately inserted in the ARP reply packet in the proper order for its media type.
Next, system A uses the station address obtained in the ARP reply to send a
data packet to system B. This address, however, is improperly ordered for
the Ethernet network on which it will now be used, since it was returned (in
the ARP reply) by a system on a token ring. This is again the first problem.
Figure 8. A dynamically determined address is misordered.
Tunneling
Although the SRT bridge does not support bridging data between Ethernet
systems and token ring systems for the reasons described above, tunneling
provides a mechanism to transmit Ethernet data over token ring backbones
and to transmit token ring data over Ethernet backbones. Since source-
routing support has not been extended to DECnet or XNS, this capability is
especially useful for connecting Ethernet-resident DEC VAXes, XNS
systems, and systems using non-routable protocols over token ring back-
bone networks. Figure 9 shows HP routers connecting Ethernet-resident
DEC VAXes and terminal servers over a token ring backbone by using
tunneling.
2-16