Foundry Networks Switch and Router Installation And Configuration Manual page 176

Foundry Switch and Router Installation and Configuration Guide
Link Redundancy and Load Balancing
The configuration in Figure 6.2 on page 6-12 uses a single POS link between the two remote devices. However,
you can provide link redundancy using either of the following methods:
• Enable the Spanning Tree Protocol (STP) on all the port-based VLANs of which the POS ports are members,
then set the path costs to different values for different VLANs, so that STP prefers one POS port for some of
the VLANs but not others.
• Configure the POS ports as a trunk group.
Using STP for Redundancy and Load Balancing
Figure 6.4 shows an example of link redundancy using STP. In this example, STP is enabled in port-based
VLANs 10, 20, 30, and 40. The POS ports are members of each of these VLANs. Each VLAN runs a separate
spanning tree (a separate instance of STP).
Each client and its server on the other
Foundry device are in the same sub-net.
Traffic between each client and its server
is bridged over a PPP link on the SONET
link on the Layer 2 POS ports.
192.168.1.10
E
C1
192.168.2.10
C2 E
192.168.3.10
C3 E
192.168.4.10
C4 E
Foundry switch A
(root STP bridge)
Figure 6.4 POS Layer 2 port redundancy using STP – default port path cost used
If you use the default settings for the STP parameters as shown in Figure 6.4, enabling STP on the POS ports for
all the port-based VLANs provides redundancy but does not provide load sharing. In this example, STP blocks
one of the POS ports for all the port-based VLANs.
Assuming the POS ports are operating at the same speed, the default STP path cost for each of the ports is the
same. Therefore, if you use the default STP path cost for each port, the spanning tree in each of the port-based
VLANs selects the same POS port for forwarding and blocks the other POS port.
To also provide load balancing, change the path cost of each the POS ports in some of the VLANs, but no others.
For example, to configure the POS ports on a switch so that STP uses one port for forwarding VLANs 10 and 20
and the other port for forwarding VLANs 30 and 40, change the port path cost on the first POS port to a lower
value for VLANs 10 and 20. On the other POS port, change the port path cost to the same lower value for the
other VLANs, 30 and 40. Figure 6.5 shows this configuration.
6 - 14
FWD VLAN 10, path cost 80
FWD VLAN 20, path cost 80
FWD VLAN 30, path cost 80
P FWD VLAN 40, path cost 80
O
S
Ethernet over POS
P
O
S
FWD VLAN 10, path cost 80
FWD VLAN 20, path cost 80
FWD VLAN 30, path cost 80
FWD VLAN 40, path cost 80
Spanning Tree is enabled on the client
and server VLANs. For each VLAN, one
POS link is active and the other is a standby.
Each POS port uses the same port path cost.
Thus, STP blocks one of the ports for all the VLANs.
This example assumes that Foundry switch A
is the root bridge for all the VLANs.
To provide load balancing in
addition to redundancy, configure
the STP path cost of the POS ports
to different values on different VLANs.
FWD
FWD
FWD
FWD P
O
S
P
O
S
BLK
BLK
BLK
BLK
Foundry switch B
192.168.1.20
E
S1
192.168.2.20
E S2
192.168.3.20
E S3
192.168.4.20
E S4
December 2000