Routing Between Vlan S; Virtual Routing Interfaces (Layer 3 Switches Only); Bridging And Routing The Same Protocol Simultaneously On The Same Device (Layer 3 Switches Only); Routing Between Vlan S Using Virtual Routing Interfaces (Layer 3 Switches Only ) - Foundry Networks FESX Manual

Foundry Configuration Guide for the FESX, FSX, and FWSX
• Removing a configured port-based VLAN from a Foundry Networks Layer 2 Switch or Layer 3 Switch
automatically removes any protocol-based VLAN, IP sub-net VLAN, AppleTalk cable VLAN, or IPX network
VLAN, or any Virtual Ethernet router interfaces defined within the Port-based VLAN.
Routing Between VLANs
Foundry Layer 3 Switches can locally route IP, IPX, and Appletalk between VLANs defined within a single router.
All other routable protocols or protocol VLANs (for example, DecNet) must be routed by another external router
capable of routing the protocol.

Virtual Routing Interfaces (Layer 3 Switches Only)

You need to configure virtual routing interfaces if an IP, IPX, or Appletalk protocol VLAN, IP sub-net VLAN,
AppleTalk cable VLAN, or IPX network VLAN needs to route protocols to another port-based 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.
Bridging and Routing the Same Protocol Simultaneously
on the Same Device (Layer 3 Switches Only)
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 Foundry 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 sub-net, 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)
Foundry 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 sub-net, IPX protocol, IPX network,
AppleTalk protocol, and AppleTalk cable VLANs.
To create any type of VLAN on a Foundry 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.
11 - 14 © Foundry Networks, Inc. December 2005