- Transparent bridging:
- Bridges can program the forwarding tables by "learning" the Ethernet addresses on an Ethernet segment.
- Bridge
start up:
- When the bridge
starts up (reboot),
the
forwarding tables
are empty:
- When the bridge
starts up (reboot),
the
forwarding tables
are empty:
- Bridge operation:
- When the bridge
receives an
Ethernet frame
on the port P:
/* ================================== Learn step ================================== */ if ( Source Addr ∉ Forwarding Table(P) ) Enter Source Addr in Forwarding Table(P); // Learn step !!!
/* ==================================
Forwarding step
================================== */
if ( Destination address ∈ Forwarding Table(P) )
{
Discard frame
}
else
{
Forward the frame on the other port
}
- When the bridge
receives an
Ethernet frame
on the port P:
- Fact:
- During the
initial learning phase:
- Transparent bridging may forward some frames unnecessarily
- During the
initial learning phase:
- Example:
- Node A trnasmits
a frame to
node C on the
same Ethernet segment:
- Because
C is
not found in the
Forwarding table,
the
bridge will
forward
the frame (needlessly):
- Node A trnasmits
a frame to
node C on the
same Ethernet segment:
- The inefficiency will
eventually cease:
- Suppose
node C
replies
to node A's message:
The bridge will learn that node C is the the left segment !!
- From now on:
- the bridge will not forward the frame destined for nodes A and C to the other Ethernet segment !!!
Graphically:
Because now the bridge knows where the nodes A and C are located !!!
- Suppose
node C
replies
to node A's message: