Foundry Networks Switch and Router Installation And Configuration Manual page 928

Foundry Switch and Router Installation and Configuration Guide
Figure 25.5 shows an example of two devices that have the same Layer 2 port-based VLANs configured across
them. Notice that only one of the VLANs requires tagging.
User-configured port-based VLAN
T = 802.1Q tagged port
Segment 1
Tagging is required for the ports
on Segment 1 because the ports
are in multiple port-based VLANs.
Without tagging, a device receiving
VLAN traffic from the other device
would not be sure which VLAN the
traffic is for.
Figure 25.5 VLANs configured across multiple devices
Spanning Tree Protocol (STP)
The default state of STP depends on the device type:
• STP is disabled by default on Foundry Layer 3 Switches.
• STP is enabled by default on Foundry Layer 2 Switches.
Also by default, each port-based VLAN has a separate instance of STP. Thus, when STP is globally enabled,
each port-based VLAN on the device runs a separate spanning tree.
You can enable or disable STP on the following levels:
• Globally – Affects all ports on the device.
NOTE: When you configure a VLAN, the VLAN inherits the global STP settings. However, once you begin
to define a VLAN, you can no longer configure STP globally. From that point on, you can configure STP only
within individual VLANs.
• Port-based VLAN – Affects all ports within the specified port-based VLAN. When you enable or disable STP
within a port-based VLAN, the setting overrides the global setting. Thus, you can enable STP for the ports
within a port-based VLAN even when STP is globally disabled, or disable the ports within a port-based VLAN
when STP is globally enabled.
STP is a Layer 2 protocol. Thus, you cannot enable or disable STP for individual protocol VLANs or for IP sub-net,
IPX network, or AppleTalk cable VLANs. The STP state of a port-based VLAN containing these other types of
VLANs determines the STP state for all the Layer 2 broadcasts within the port-based VLAN. This is true even
though Layer 3 protocol broadcasts are sent on Layer 2 within the VLAN.
It is possible that STP will block one or more ports in a protocol VLAN that uses a virtual interface to route to other
VLANs. For IP protocol and IP sub-net VLANs, even though some of the physical ports of the virtual interface are
blocked, the virtual interface can still route so long as at least one port in the virtual interface's protocol VLAN is
not blocked by STP.
25 - 8
T
T
Segment 1
T T
Segment 2
Segment 2
Tagging is not required for the ports
on Segment 2 because each port is
in only one port-based VLAN.
T
T
T
December 2000