Foundry Networks Switch and Router Installation And Configuration Manual page 935

Configuring Virtual LANs (VLANs)
interface within each of the protocol, subnet or network VLANs that are defined to the port-based VLAN. This
configuration would require three virtual interfaces for a single port-based VLAN.
If you do not need to further partition the port-based VLAN by defining separate Layer 3 VLANs, you can define a
single virtual interface at the port-based VLAN level and enable IP, IPX, and Appletalk routing on a single virtual
interface.
Bridging and Routing the Same Protocol Simultaneously on the Same Device
(routers 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 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 Interfaces (routers only)
Foundry calls the ability to route between VLANs with virtual 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 router becomes a Layer 2 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 interfaces and are subject to the rules of STP.
If your backbone is comprised of virtual 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 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 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's STP domain is
a single point-to-point backbone connection, you are guaranteed to never have an STP loop. STP will never block
the virtual router interfaces within the tagged port-based VLAN, and you will have a fully routed backbone.
Dynamic Port Assignment (routers only)
When defining IP, IPX, Appletalk, IP sub-net, and IPX network VLANs on a Foundry router, you do not need to
exclude ports, because ports are not dynamically assigned. Therefore, only those ports implicitly defined as
members of an Appletalk, IP protocol, IP sub-net, IPX protocol, or IPX network VLAN are members of that VLAN.
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