Foundry Networks Switch and Router Installation And Configuration Manual page 175

Figure 6.3 shows the network in Figure 6.2 from the perspective of client C1 and server SI. The devices appear to
one another to be on the same Ethernet LAN segment. Conceptually, the devices are connected by a Layer 2
switch or bridge. The clients are not aware that the Layer 2 connection between them is a POS WAN link between
two geographically distant devices.
192.168.1.10
C1
Figure 6.3 IP Host perspective of the switching link
Notice that the devices in Figure 6.2 are configured with a separate port-based VLAN for each sub-net. The
separate port-based VLANs provide a private Layer 2 broadcast domain for each sub-net. The separate port-
based VLANs ensure that devices on one sub-net do not receive broadcast traffic from devices on the other sub-
nets. Partitioning the ports into separate port-based VLANs is especially useful for co-location implementations,
when multiple clients lease ports on the same devices. The port-based VLANs ensure that one client's ports do
not receive broadcast traffic from another client's ports.
In fact, to enable Layer 2 switching on a POS port, you must add the port as a tagged port to each of the port-
based VLANs that contains the sub-nets you want to bridge. The ports are tagged so that they can properly
multiplex traffic from the different VLANs for sending the traffic over the PPP link, and demultiplex the traffic at the
other end of the link. For example, when client C2 sends traffic to server S2, the traffic is tagged with the VLAN ID
(20). The POS ports on both devices also are members of VLAN 20 and can therefore forward traffic between C2
and S2 at Layer 2.
In addition, since the traffic is tagged, broadcast traffic on the 192.168.2.x sub-net (in VLAN 20) goes only to other
devices on the same sub-net, because VLAN 20 is its own Layer 2 broadcast domain. Only the ports in VLAN 20
receive broadcasts from a host connected to a port in VLAN 20. For example, the POS ports forward a broadcast
packet from S2 to client C2 but not to clients C1, C3, or C4.
December 2000
192.168.1.10 192.168.1.20
C1 S1
192.168.1.20
S1
Layer 2 switch
Using Packet Over SONET (POS) Modules
Layer 2 remote bridging is
transparent to Layer 3 IP traffic
that uses the PPP link over the
POS ports.
The client and server appear to
each other to be locally attached.
E
Conceptually, the client and server are
connected by a Layer 2 switch.
E
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