298 BigIron RX Series Configuration Guide53-1002253-01Configuring virtual routing interfaces11Integrated Switch Routing (ISR)Brocade Integrated Switch Routing (ISR) feature enables VLANs configured on the device to routeLayer 3 traffic from one protocol-based VLAN to another instead of forwarding the traffic to anexternal router. The VLANs provide Layer 3 broadcast domains for the protocols, but do not inthemselves provide routing services. This is true even if the source and destination protocols are onthe same device.ISR eliminates the need for an external router by allowing you to route between VLANs using virtualrouting interfaces (ves). You configure a separate virtual routing interface on each VLAN that youwant to use to route packets. For example, if you configure two IP protocol VLANs on a device, youcan configure a virtual routing interface on each of the IP protocol VLAN, then configure IP routingparameters for the IP protocol VLAN. Thus, the device forwards IP broadcasts within each VLAN atLayer 2 but routes Layer 3 traffic between the VLANs using the virtual routing interfaces.NOTEThe device uses the lowest MAC address on the device (the MAC address of port 1/1) as the MACaddress for all ports within all virtual routing interfaces you configure on the device.The routing parameters and the syntax for configuring them are the same as when you configure aphysical interface for routing (for example, interface ve 10). The logical interface allows the deviceto internally route traffic between the protocol-based VLANs without using physical interfaces.All the ports within a protocol-based VLAN must be in the same port-based VLAN. Theprotocol-based VLAN cannot have ports in multiple port-based VLANs, unless the ports in theport-based VLAN to which you add the protocol-based VLAN are 802.1q tagged.You can configure multiple protocol-based VLANs within the same port-based VLAN. In addition, aport within a port-based VLAN can belong to multiple protocol-based VLANs of the same type ordifferent types. For example, if you have a port-based VLAN that contains ports 1/1 – 1/10, youcan configure port 1/5 as a member of an AppleTalk protocol VLAN, an IP protocol VLAN, and anIPX protocol VLAN, and so on.If the router interface for IP is configured on physical ports, then routing occurs independent of theSpanning Tree Protocol (STP). However, if the router interfaces are defined for IP VLAN, they arevirtual routing interfaces and are subject to the rules of STP.If your backbone is consisted of virtual routing interfaces all within the same STP domain, it is abridged backbone, not a routed one. This means that the set of backbone interfaces that areblocked by STP will be blocked for routed protocols as well. The routed protocols will be able tocross these paths only when the STP state of the link is FORWARDING. This problem is easilyavoided by proper network design.When designing an ISR network, pay attention to your use of virtual routing interfaces and thespanning-tree domain. If Layer 2 switching of your routed protocols (IP, IPX, AppleTalk) is notrequired across the backbone, then the use of virtual routing interfaces can be limited to edgeswitch ports within each router. Full backbone routing can be achieved by configuring routing oneach physical interface that connects to the backbone. Routing is independent of STP whenconfigured on a physical interface.If your ISR design requires that you switch IP, IPX, or Appletalk at Layer 2 while simultaneouslyrouting the IP protocol over a single backbone, then create multiple port-based VLANs and useVLAN tagging on the backbone links to separate your Layer 2 switched and Layer 3 routednetworks.
