Technical data

Troubleshooting Techniques and Tools
1.2 Isolating Problems
The following example shows typical output from these two commands:
$ TCPIP SHOW ROUTE /PERMANENT /DEFAULT
PERMANENT
Type Destination Gateway
PN 0.0.0.0 rufus.lkg.dec.com
$ TCPIP SHOW ROUTE /DEFAULT
DYNAMIC
Type Destination Gateway
DN 0.0.0.0 10.10.2.66
$
To set a default route, enter a command similar to the following:
$ TCPIP SET ROUTE /DEFAULT /GATE=n.n.n.n
You can also set a default route by running the TCPIP$CONFIG
procedure and selecting option 1 for Core, and then option 3 for Routing.
TCPIP$CONFIG prompts with:
* Do you want to configure dynamic ROUTED or GATED routing [NO]:
Take the default value by pressing the Enter key. TCPIP$CONFIG then
displays the current configuration and asks whether you want to reconfigure
a default route:
The current configuration for the default route is:
PERMANENT
Type Destination Gateway
PN 0.0.0.0 rufus.lkg.dec.com
* Do you want to reconfigure a default route [YES]:
Enter the Default Gateway host name []:
• Next, use ping to see whether you can reach the routing gateway.
1.2.9 Checking the Routes Known to a Gateway
The traceroute command helps you locate problems between the local host and
the remote destination by tracing the route of UDP packets from the local host to
a remote host. Tracing attempts to determine the name and IP address of each
gateway along the route to the remote host.
The traceroute command works by sending UDP packets with small time-to-live
(TTL) values and an invalid port number to the remote system. The TTL values
increase in increments of one for each group of three UDP packets sent. When a
gateway receives a packet, it decrements the TTL. If the TTL is zero, the packet
is not forwarded, and an ICMP ‘‘time exceeded’’ message is returned.
Intermediate gateways are detected when they return an ICMP ‘‘time exceeded’’
message. When
traceroute receives an ‘‘invalid port’’ message, it knows that it
reached the remote destination. ( traceroute operates by intentionally using an
invalid port.) When traceroute receives this message, it knows it has reached
the destination host and terminates the trace. In this way, traceroute develops a
list of gateways starting at one hop away, and increasing one hop at a time until
the remote host is reached.
For more information about using traceroute , see Appendix A.
1–26 Troubleshooting Techniques and Tools