20 NETernity* RM921N / RM921ND Hardware Reference Manual
Publication No. 597-0000000013-000 Rev. F
Layer 3 and above, use information from the IP header. This can be an IP address,
optionally qualified by port number. When a packet arrives, where the destination is
not in the switch fabric database, it is “flooded” to applicable ports (i.e., VLAN setup
may restrict the ports to which the packet will be forwarded). Once a specific port for
the destination is known, the packet is forwarded only to that specific port with all
processing done by the Switch Fabric.
3.3.3
Virtual LAN (VLAN)
VLAN is a mechanism useful for both security and improved performance. It allows
a network to be partitioned into several virtual networks so that packets from one
are not forwarded to another. Restricting forwarding reduces traffic (improves
performance) and can be a component of a security control. VLAN support is
available at both Layer 2 (port based) and Layer 3. Packets received at the switch
either contain a VLAN ID (per 802.1q) or are assigned based on a per port default.
3.3.4
Routing Domains
The switch has the ability to create up to 32 Routing Domains. These are based upon
the previous definition of a VLAN, and therefore, contains the same port
membership as the VLAN. The Routing Domain adds the concept of a subnet to the
VLAN. This creates an exception to the isolation of a VLAN.
The switch can forward packets between routing domains, performing the duties of
a standard IP router. When the destination of a packet is to a Routing Domain
directly connected to the switch (e.g., across VLANs), the fabric can forward the
packet at wire speed without any additional information.
In larger configurations, the destination may be one (or more) hops away. Thus, the
switch must forward the packet to an IP address that is directly connected, which is
known to be responsible for the next step of the route. To locate the appropriate next
hop, a route table is required. The purpose of the table is to associate the final
destination IP subnet with a specific IP address on one of the connected routing
domains. For this purpose, the route table of the host Linux system is used. When a
forwarding decision is required, the switch control application queries the Linux
route table to find the next hop. Using the Linux table provides the advantage of a
well‐defi
ned interface to read and change. Also, it allows the use of standard routing
daemons for automatic update of routes.
These daemons will receive routing protocol messages (e.g., RIP, OPSF, etc.) and use
them to automatically update the routing table. This provides for dynamic
configuration and maintenance of the routes. In addition (or as an alternative),
routes can be added as part of the switch initialization process. These routes will be
permanent and always present for the switch. The two techniques (dynamic update
with standard routing protocols or static routes) can be used together or
independently.
