Brocade FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide - Manual (Page 372)

VLAN on the same router. A virtual routing interface can be associated with the ports in only a single
port-based VLAN. Virtual router interfaces must be defined at the highest level of the VLAN hierarchy.
If you do not need to further partition the port-based VLAN by defining separate Layer 3 VLANs, you
can define a single virtual routing interface at the port-based VLAN level and enable IP, IPX, and
Appletalk routing on a single virtual routing interface.
Some configurations may require simultaneous switching and routing of the same single protocol
across different sets of ports on the same router. When IP, IPX, or Appletalk routing is enabled on a
Brocade Layer 3 Switch, you can route these protocols on specific interfaces while bridging them on
other interfaces. In this scenario, you can create two separate backbones for the same protocol, one
bridged and one routed.
To bridge IP, IPX, or Appletalk at the same time these protocols are being routed, you need to
configure an IP protocol, IP subnet, IPX protocol, IPX network, or Appletalk protocol VLAN and not
assign a virtual routing interface to the VLAN. Packets for these protocols are bridged or switched at
Layer 2 across ports on the router that are included in the Layer 3 VLAN. If these VLANs are built
within port-based VLANs, they can be tagged across a single set of backbone fibers to create
separate Layer 2 switched and Layer 3 routed backbones for the same protocol on a single physical
backbone.
Routing between VLANs using virtual routing interfaces (Layer 3
Switches only)
Brocade calls the ability to route between VLANs with virtual routing interfaces Integrated Switch
Routing (ISR) . There are some important concepts to understand before designing an ISR backbone.
Virtual router interfaces can be defined on port-based, IP protocol, IP subnet, IPX protocol, IPX
network, AppleTalk protocol, and AppleTalk cable VLANs.
To create any type of VLAN on a Brocade Layer 3 Switch, Layer 2 forwarding must be enabled. When
Layer 2 forwarding is enabled, the Layer 3 Switch becomes a Switch on all ports for all non-routable
protocols.
If the router interfaces for IP, IPX, or AppleTalk are configured on physical ports, then routing occurs
independent of the Spanning Tree Protocol (STP). However, if the router interfaces are defined for any
type VLAN, they are virtual routing interfaces and are subject to the rules of STP.
If your backbone consists of virtual routing interfaces all within the same STP domain, it is a bridged
backbone, not a routed one. This means that the set of backbone interfaces that are blocked by STP
will be blocked for routed protocols as well. The routed protocols will be able to cross these paths only
when the STP state of the link is FORWARDING. This problem is easily avoided by proper network
design.
When designing an ISR network, pay attention to your use of virtual routing interfaces and the
spanning-tree domain. If Layer 2 switching of your routed protocols (IP, IPX, AppleTalk) is not required
across the backbone, then the use of virtual routing interfaces can be limited to edge switch ports
within each router. Full backbone routing can be achieved by configuring routing on each physical
interface that connects to the backbone. Routing is independent of STP when configured on a physical
interface.
If your ISR design requires that you switch IP, IPX, or Appletalk at Layer 2 while simultaneously
routing the same protocols over a single backbone, then create multiple port-based VLANs and use
VLAN tagging on the backbone links to separate your Layer 2 switched and Layer 3 routed networks.
There is a separate STP domain for each port-based VLAN. Routing occurs independently across
port-based VLANs or STP domains. You can define each end of each backbone link as a separate
tagged port-based VLAN. Routing will occur independently across the port-based VLANs. Because
each port-based VLAN STP domain is a single point-to-point backbone connection, you are
Routing between VLANs using virtual routing interfaces (Layer 3 Switches only)
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