System information

26 IBM and Cisco LAN Switching: Interoperability and Migration Guide
to move them to fast Ethernet and switched ports, mostly because it is
feasible and not very costly.
3.2 Scenario 2: Multiple subnets - growth environment
In this generalized scenario the installation in question is in a growth
environment and the owning organization is pursuing a strategy to implement
advanced functions such as multimedia and telephony. Additionally,
bandwidth requirements may be expected to grow considerably.
This is an obvious case for implementing VLANs and a more powerful
backbone switch with layer-3 and 4 function to classify data flows and enforce
service policies.
In this environment we want to gradually build a VLAN-based network. We
recommend that the VLAN be based on the IEEE 802.1Q standard. Most
Cisco Catalyst switches support this standard as well as the ISL protocol. For
more information about products that support IEEE 802.1Q refer to the
redbook
Application-Driven Networking: Class of Service in IP, Ethernet and
ATM Networks, SG24-5384-00.
Interoperability and coexistence of IBM and Cisco hubs and switches in this
environment are quite feasible. Not all IBM switches support port-based
802.1Q VLAN, but if several workstations attached in a wiring closet belong to
the same VLAN, they might as well share the same switch or hub.
Of course, this may impede campus-wide VLANs. However, while large, flat
campus-wide VLANs have their advantages, there are also many potential
pitfalls. A potential source of problems in such a network is the time required
and possible instability of the spanning tree protocol (STP) to converge.
Today, backbone switches handle millions of packets per second on layers 3
and 4 at a reasonable cost. It is probably a healthier design to localize VLAN
with respect to resources (such as servers), and geography (such as wiring
closets or buildings) and leave access control and policy to the backbone
switch.
Inter-Switch Link, ISL protocol, is not supported by any IBM boxes. It is a
Cisco proprietary protocol.
Note