Echelon LonWorks Router - Manual (Page 20)

A forwarding table is used for each domain on each side of the router. Each
forwarding table contains a forwarding flag for each of the 255 subnets and 255
groups in a domain. As shown in Figure 4 and Figure 5, these flags determine
whether or not a message should be forwarded or dropped based on the
destination subnet or group address of the message.
A network management tool initializes the forwarding tables using the network
management messages described in Chapter 7, Network Management Messages,.
By configuring the routing tables based on network topology, a network
management tool can optimize network performance and make the most efficient
use of available bandwidth. Configured routers should be used for looping
topologies; see Loop Topology.
For a L
ON W ORKS router, there are two sets of forwarding tables, one in non-
volatile memory (typically EEPROM) and one in RAM. The non-volatile table is
copied to the RAM table when the router is initially powered-up, after a reset,
and when the router receives the Set Router Mode command with the Initialize
Routing Table option. The RAM table is used for all forwarding decisions.
Several of the operations in shown in Figure 4 and Figure 5 help prevent
message loops for service-pin messages. Service-pin messages require special
handling because they are broadcast to all nodes on the zero-length domain, and
have a source subnet ID of zero. When a router receives a service-pin message
with a source subnet ID of zero, the router modifies the source subnet field of the
message to be the router’s subnet on the receiving side. If the receiving side is
installed in two domains, two service-pin messages are forwarded, one for each
domain. Thus, the router can drop the service-pin message if a loop causes the
message to be received again on the same side.
Learning Router
A Learning Router, like a configured router, determines which packets to forward
based on internal routing tables. A learning router forwards only those messages
which are received on either of the router’s domains and which meet the
forwarding rules shown in Figure 4 and Figure 5. A learning router always
forwards all group-addressed messages. Learning routers maintain their routing
tables in non-volatile memory, and thus retain them after a reset. These tables
control forwarding of subnet and group-addressed messages, and are updated
automatically by the router firmware, rather than their being configured by a
network management tool. The group forwarding tables are configured to always
forward (flood) all messages with group destination addresses.
When a router receives a packet with a destination address using a subnet ID, it
uses the subnet ID to determine whether to forward the packet. Learning
routers learn network topology by examining the source subnet of all messages
received by the router. Whenever a learning router receives a packet from one of
its channels, it uses the source subnet ID to learn the network topology. It sets
the corresponding routing table entries to indicate that the subnet in question is
to be found in the direction from which the packet was received. Because subnets
cannot span two channels connected to an intelligent router, the router can learn
which side a subnet is on whenever that subnet ID appears in the source address.
The subnet forwarding tables are initially configured to forward all messages
with subnet destination addresses. Each time a new subnet ID is observed in the
source address field of a message, its corresponding flag is cleared (that is,
forwarding is disabled) in the subnet forwarding table. The forwarding flag for
10 Introduction to LONWORKS Routers