Configuring Virtual LANs (VLANs)
December 2005 © Foundry Networks, Inc. 11 - 9
It is possible that STP will block one or more ports in a protocol VLAN that uses a virtual routing interface to route
to other VLANs. For IP protocol and IP sub-net VLANs, even though some of the physical ports of the virtual
routing interface are blocked, the virtual routing interface can still route so long as at least one port in the virtual
routing interface’s protocol VLAN is not blocked by STP.
If you enable Single STP (SSTP) on the device, the ports in all VLANs on which STP is enabled become members
of a single spanning tree. The ports in VLANs on which STP is disabled are excluded from the single spanning
tree.
For more information, see “Configuring Spanning Tree Protocol (STP) and IronSpan Features” on page 7-1.
Virtual Routing Interfaces
A virtual routing interface is a logical routing interface that Foundry Layer 3 Switches use to route Layer 3 protocol
traffic between protocol VLANs.
Foundry devices send Layer 3 traffic at Layer 2 within a protocol VLAN. However, Layer 3 traffic from one protocol
VLAN to another must be routed.
If you want the device to be able to send Layer 3 traffic from one protocol VLAN to another, you must configure a
virtual routing interface on each protocol VLAN, then configure routing parameters on the virtual routing interfaces.
For example, to enable a FastIron Layer 3 Switch to route IP traffic from one IP sub-net VLAN to another, you must
configure a virtual routing interface on each IP sub-net VLAN, then configure the appropriate IP routing
parameters on each of the virtual routing interfaces.
Figure 11.6 shows an example of Layer 3 protocol VLANs that use virtual routing interfaces for routing.
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