3Com 3C15500 - Network Director User Manual page 511

Key Concepts 507
not have a source IP address (initial DHCP requests) or may come from a
different subnet (multicast). In these cases, since it can't be determined
from the packet content what interface to use, the switch chooses the
primary interface defined on the VLAN, 158.101.68.254/255.255.255.0.
Protocol-based VLANs
Some older 3Com switches, namely Switch 4007 and CoreBuilder 3500,
support protocol-based VLANs. With these VLANs, VLAN membership is
based not only on port, but also on protocol type, e.g. IP, IPX, Appletalk.
A single port can be a member of multiple VLANs without the need for
packet tagging. For example a port could be a member of an IPX and an
Appletalk VLAN. When an untagged packet is received on this port, the
protocol information in the packet is examined to determine which VLAN
the packet is to be forwarded on.
3Com Network Director provides limited support for protocol-based
VLANs. 3Com Network Director allows you to view the existing
protocol-based VLANs defined on any Switch 4007's or Corebuilder
3500's on your network. 3Com Network Director does not support the
creation or modification of protocol-based VLANs using the Create VLAN
and Edit Port VLAN Membership wizards.
Spanning Tree
The IEEE Spanning Tree Protocol (STP) standard 802.1D, 1998 Edition
does not take into account VLANs when it calculates STP information —
the calculations are only performed on the basis of physical connections.
For this reason, some network configurations can result in VLANs being
subdivided into a number of isolated sections by the STP system.
Therefore, you must ensure that any LAN configuration on your network
takes into account the expected STP topology and alternative topologies
that may result from link failures.
For example, in the network below VLANs 1 and 2 are connected using
the 802.1Q-tagged link between Switch B and Switch C. By default, this
link has a path cost of 100 and is automatically blocked because the
other inter-switch connections have a path cost of 36 (18+18). This
means that both VLANs are now subdivided — VLAN 1 on Switch units A
and B cannot communicate with VLAN 1 on Switch C, and VLAN 2 on
Switch units A and C cannot communicate with VLAN 2 on Switch B.