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Routers and
A router (or bridge) is a special node that
consists of two connected N
Bridges
connected to a separate channel (see figure
below). Routers and bridges pass packets
back and forth between these channels. There
are four types of routers. A repeater is the
simplest form of router, simply forwarding all
packets between the two channels. A bridge
simply forwards all packets which match its
domains between the two channels. Using a
bridge or repeater, a subnet can exist across
multiple channels. A learning router monitors
the network traffic and learns the network
topology at the domain/subnet level. The
learning router then uses its knowledge to
selectively route packets between channels.
Like a learning router, a configured router
selectively routes packets between channels
by consulting internal routing tables. Unlike a
learning router, the contents of the internal
routing tables are specified using network
management commands.
Initially, each router sets its internal routing tables
to indicate that all subnets could lie on either side
of the router. Suppose that node 6, in the figure
below, generates a message bound for node 2.
Learning router 1 initially picks up the message.
It examines the source subnet field of the
message and notes in its internal routing tables
Channel
Channel
MG.60.N2.02 - VLT is a registered Danfoss trademark
EURON
1
2
3
Subnet 1
9
10
11
Subnet 3
VLT
®
5000 / VLT
that subnet 2 lies below it. The router then
chips, each
compares the source and destination subnet IDs
and, since they are different, the message is
passed on. Meanwhile, learning router 2 also
passes the message on, making an appropriate
notation in its internal routing tables regarding the
location of subnet 2.
Suppose now that node 2 generates an
acknowledgment. This acknowledgment is
picked up by learning router 1, which now notes
the location of subnet 1. Learning router 1
examines its internal routing tables, and, noting
that subnet 2 lies below, passes the message
on. When the message appears on subnet 2, it
is noted by both node 6 (the destination) and
learning router 2. Learning router 2 does not pass
it on but merely notes that subnet 1, like subnet
2, lies somewhere above. Learning router 2 will
not learn of the existence or location of subnet 3
until a message is originated from there. Subnets
cannot cross routers. While bridges and
repeaters allow subnets to span multiple
channels, the two sides of a router must belong
to separate subnets. Since routers are selective
about the packets they forward to each channel,
the total capacity of a system can be increased
in terms of nodes and connections.
Learning
R
Router 1
4
Learning
R
Router 2
12
Learning Routers
Source: Echelon Corp.
®
6000 HVAC / VLT
®
8000 AQUA
Channel
5
6
7
Subnet 2
8
9
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