Some clients may have a complex network requiring multiple routes versus a single gateway entry, commonly known as a *DFTROUTE. In these complex environments, a particular router (also known as a gateway) may be preferred over another router. Or, a system may have a particular TCP/IP interface and ethernet line they prefer to use over other interfaces and lines when it comes to remote connections. One of the common concerns with these complex setups is, "How do I know which route is truly being used?"

The information below assumes the following TCP/IP configuration on the system:

CFGTCP, Option 1 shows:

Work with TCP/IP Interfaces
System: MYIBMBOX
Type options, press Enter.
1=Add 2=Change 4=Remove 5=Display 9=Start 10=End

Internet Subnet Line Line
Opt Address Mask Description Type
10.10.10.1 255.255.255.0 ETHLINE1 *ELAN
10.10.10.2 255.255.255.0 ETHLINE2 *ELAN
10.10.10.3 255.255.255.0 ETHLINE3 *ELAN

CFGTCP, Option 2 shows:

Work with TCP/IP Routes
System: MYIBMBOX
Type options, press Enter.
1=Add 2=Change 4=Remove 5=Display

Route Subnet Next Preferred
Opt Destination Mask Hop Interface

*DFTROUTE *NONE 10.10.10.254 10.10.10.1
*DFTROUTE *NONE 10.10.10.254 10.10.10.2
*DFTROUTE *NONE 10.10.10.254 10.10.10.3


Let us assume the above three routes were added with the following commands:

ADDTCPRTE RTEDEST(*DFTROUTE) SUBNETMASK(*NONE) NEXTHOP('10.10.10.254') BINDIFC('10.10.10.1') DUPRTEPTY(8)
ADDTCPRTE RTEDEST(*DFTROUTE) SUBNETMASK(*NONE) NEXTHOP('10.10.10.254') BINDIFC('10.10.10.2') DUPRTEPTY(7)
ADDTCPRTE RTEDEST(*DFTROUTE) SUBNETMASK(*NONE) NEXTHOP('10.10.10.254') BINDIFC('10.10.10.3') DUPRTEPTY(6)

With these commands, the higher duplicate route priority will take precedence when the IBM i needs to do route lookup and communicate back to a host (or initiate traffic). In this case, the "Order Used" would be the route with DUPRTEPTY(8), then DUPRTEPTY(7), followed by DUPRTEPTY(6). When the system receives a connection from a remote host, or initiates a connection to a remote host, there is a route lookup function performed. Assuming no other, more specific, route applies, the system will end up choosing a *DFTROUTE to communicate. In the above setup, the route chosen by the system would be the first one with DUPRTEPTY(8). The only way this route would not be used is if the route were unavailable.

An item of interest when it comes to routing with this environment: If a remote client attempts to connect to the system on either the 10.10.10.2 or 10.10.10.3 interface, the system will still route "out" the top route with DUPRTEPTY(8). However, we will respond (tag the packet) with the 10.10.10.2 or 10.10.10.3 address (because that was the address the packet was originally intended for).