HP Network Direr Software Products manual Protocol-based VLANs, Spanning Tree

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.