IBM SC41-5420-04 Tcp/ip Configuration And Reference

Also see for i Series: Cabling guide User guide Manual Handbook Handbook

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The data link layer passes a negative advice indicator to IP when it is unable totransmit data to a first-hop (directly-connected) gateway. In most cases, negativeadvice 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 offirst-hop gateway on the directly connected physical network. (ARP is notperformed on all physical network types. Some physical network types, such asX.25, use an alternative scheme for this purpose.)Negative advice, whether from TCP or the data link layer, is always expressed interms of the first-hop gateway. Dead gateway processing on a given host onlyattempts to verify the first-hop gateway. However, gateways also carry out theirown dead gateway processing for other adjacent gateways. In this way, all of thegateways along the path to a destination host are taken care of.How IP Responds to Negative AdviceWhen receiving negative advice from TCP or the data link layer concerning a nexthop gateway, IP marks all routes that use this gateway as suspect. IP attempts todeliver data destined for the suspect gateway via routes that use other gateways (ifany are configured). Next, an IP process is started that uses periodic PING requeststo attempt to contact the suspect next-hop gateway. If the suspect gatewaycontinues to be unresponsive for an extended period of time, the frequency of thePING requests is reduced.When any PING response is received from a suspect gateway, the gateway isconsidered 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, deadgateway PING processing is not interrupted, and subsequent negative adviceforces the IP routing table back to its previously adjusted state.2. Responses from user-initiated PINGs can also indicate that a suspect gateway isactive.3. Negative advice is not passed from the UDP or RAW IP protocol machines.Applications using these protocols must use other mechanisms to detect andrespond to apparent network problems. However, data link layer-negativeadvice is still used to manage problems with the first-hop gateway.Multihoming FunctionA multihomed host has multiple IP addresses, which we may think of as logicalinterfaces. These logical interfaces may be associated with one or more physicalinterfaces, and these physical interfaces may be connected to the same or differentnetworks.The iSeries TCP/IP implementation supports multihoming. This allows you tospecify either a single interface or multiple interfaces for a line description. Youcan have your server appear as any one or combination of the following scenarios:v A single host on a network over a communications linev Multiple hosts on the same network over the same communications linev Multiple hosts on the same network over multiple communications linesv Multiple hosts on different networks over the same communications linev Multiple hosts on different networks over multiple communications lines56 OS/400 TCP/IP Configuration and Reference V5R1