ISDN
- Bridging Versus Routing
Bridging
offers one of the most straightforward and flexible methods
of interconnecting network segments. Bridges are uncomplicated
devices. No changes are required to existing applications
or communications software when bridges are introduced to
a network.
Bridging
Features
With most bridges, you only have to connect them to your
network and switch them on. A bridge automatically learns
addresses of all active stations on its Local Area Network
(LAN). It examines all the packets on the LAN, reading their
source and destination addresses, and forwards packets destined
for remote networks over the Wide Area Network (WAN). Packets
running between addresses on the local network are not passed
onto the WAN. This means that traffic intended only for
the local segment does not cross the bridge and clutter
up the rest of the network.
Bridging Limitations
However,
there are some packets that a bridge must forward across
the WAN to all parts of the network, such as broadcasts,
multicasts, and packets with unknown destinations. These
packets cause the following problems when they are bridged
across ISDN:
If you have configured more circuits than you have B-channels,
the bridge tries to transmit these packets on all circuits.
However, transmission must be complete on the first two
circuits before more circuits can be opened. As a result,
transmission can be delayed on some circuits, or packets
may be discarded.
Every
broadcast and multicast transmission to an unknown destination
results in an ISDN call. Sometimes it is necessary to pass
this type of transmission across the WAN. Every call costs
money.
Recommendation
for Routing
In most circumstances, it is more appropriate to use routing
to interconnect LANs using ISDN
Filtering
If you use bridging to interconnect LANs using ISDN, you
can avoid some of the limitations by using filtering. Filtering
allows the bridge to discard those packets that do not need
to pass across links. This controls the amount of traffic
sent across ISDN links.
