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The data link layer passes a negative advice indicator to IP when it is unable to
transmit data to a first-hop (directly-connected) gateway. In most cases, negative
advice from the data link layer means that the Address Resolution Process (ARP)
processing performed by the data link layer was unable to resolve the location of
first-hop gateway on the directly connected physical network. (ARP is not
performed on all physical network types. Some physical network types, such as
X.25, use an alternative scheme for this purpose.)
Negative advice, whether from TCP or the data link layer, is always expressed in
terms of the first-hop gateway. Dead gateway processing on a given host only
attempts to verify the first-hop gateway. However, gateways also carry out their
own dead gateway processing for other adjacent gateways. In this way, all of the
gateways along the path to a destination host are taken care of.
How IP Responds to Negative Advice
When receiving negative advice from TCP or the data link layer concerning a next
hop gateway, IP marks all routes that use this gateway as suspect. IP attempts to
deliver data destined for the suspect gateway via routes that use other gateways (if
any are configured). Next, an IP process is started that uses periodic PING requests
to attempt to contact the suspect next-hop gateway. If the suspect gateway
continues to be unresponsive for an extended period of time, the frequency of the
PING requests is reduced.
When any PING response is received from a suspect gateway, the gateway is
considered active and the routes are restored.
Notes about IP Responses to Negative Advice:
1. If an ICMP redirect message is received during dead gateway processing,
routes to a suspect gateway may be temporarily restored. However, dead
gateway PING processing is not interrupted, and subsequent negative advice
forces the IP routing table back to its previously adjusted state.
2. Responses from user-initiated PINGs can also indicate that a suspect gateway is
active.
3. Negative advice is not passed from the UDP or RAW IP protocol machines.
Applications using these protocols must use other mechanisms to detect and
respond to apparent network problems. However, data link layer-negative
advice is still used to manage problems with the first-hop gateway.
Multihoming Function
A multihomed host has multiple IP addresses, which we may think of as logical
interfaces. These logical interfaces may be associated with one or more physical
interfaces, and these physical interfaces may be connected to the same or different
networks.
The iSeries TCP/IP implementation supports multihoming. This allows you to
specify either a single interface or multiple interfaces for a line description. You
can have your server appear as any one or combination of the following scenarios:
v A single host on a network over a communications line
v Multiple hosts on the same network over the same communications line
v Multiple hosts on the same network over multiple communications lines
v Multiple hosts on different networks over the same communications line
v Multiple hosts on different networks over multiple communications lines
56 OS/400 TCP/IP Configuration and Reference V5R1