WAN Subsystem Configuration and Management Manual

Overview of the WAN Subsystem

WAN Subsystem Configuration and Management Manual—522463-011

2-14

Fault Tolerance and Fault Containment

Host-system connection fault tolerance is provided primarily by having two Ethernet

connections to the host for each WAN port and allowing the SWAN concentrator to use

either connection. Only one connection is needed for full functionality.

Fault-containment is provided by the independence of the three CLIPs, each of which

controls two WAN lines. If one CLIP fails, the other four WAN lines remain functional.

Additional fault-containment features include parity on the microprocessor DRAM and

the Ethernet anti-babble circuit that silences a babbling CLIP so that it cannot cause

the other two CLIPs to lose their Ethernet connections. The only component shared

across both of the Ethernet ports and the three CLIPs are the power supply and the

fan.

Path Switching

The preferred path might not always be available because of software or hardware

failures or because of the nature of the software and hardware configuration. However,

since the WAN subsystem and TCP/IP subsystems are designed to be fault-tolerant,

and since fault-containment is built into the SWAN concentrator, IOPs rarely encounter

a “NO PATH AVAILABLE” error. If this error is encountered, the SWAN concentrator

CLIP is unusable.

Table 2-8 describes the algorithm the ConMgr process uses to decide which path an

IOP will use to access the SWAN concentrator. When the conditions described in the

first column are met, the ConMgr process determines which path to use and how the

path will be accessed, as described in the second column.

Note. This algorithm is applicable for the TCP/IP subsystem only. The algorithm for Parallel

Library TCP/IP is shown in Table 2-9 on page 2-16.

Table 2-8. ConMgr Process PATH Switching Algorithm for TCP/IP (page 1 of 2)

When these conditions are met... The ConMgr process will...

A. If the ADAPTER preferred path

TCP/IP process

•

is started, and

•

has not failed, and

•

is primaried in the same proces-

sor as the primary IOP

Use the ADAPTER preferred path TCP/IP

process.

Select the IP address for the CLIP by using

the PATH A IP address.