Routing information administration
To forward packets by the best possible route, NetWare routers
maintain a routing information table that holds information about
all the segments on the network.
Table 4-4
gives an
example of a routing information table (only the fields pertinent
to this discussion have been included).
Table 4-4 Portion of routing information table
---------------------------------------------------------------------------------- Network Hops to Ticks to Immediate address Net Aging number LAN segment LAN segment NIC of forwarding router status timer ---------------------------------------------------------------------------------- 00000001 1 2 A 0 00000002 1 2 B 0 FEED0038 1 20 C R 0 FEED0035 2 3 B 00001B029927 1 000000FF 2 3 A 00001B0349B2 2 FEED0036 3 4 A 00001B0349B2 2Each entry in the routing information table gives the router forwarding information for a network segment:
The method that drivers use for estimating the time delay on a segment depends on the segment type. For local segments with more than 1 megabyte per second (Token-Ring, Ethernet, Arcnet, and so on), the driver makes the assumption that delivery time is one tick. For remote segments (Tl, 64 KBps, X.25, and asynchronous), the driver will periodically poll to determine the current time delay. For example, the delay for a T1 link normally ranges from six to seven ticks. If this delay changes, the driver will inform its router. As information about the segment is passed throughout the network (by way of periodic broadcasts), routers will add any additional delay that they impose to the initial time estimate for the segment.
For NetWare versions prior to 2.16c, the routing information table keeps a list of all alternate routes for each network number in case the primary shortest route to a network number goes down. In other words, if the router can reach the segment through more than one of its NICs, it will make a record of all routes. The fastest route, the one that requires the least number of ticks, will always be used as the primary route. NetWare versions 2.16c and later maintain alternate routes only if these alternate routes require the same number of ticks to reach the segment as the primary route. This procedure reduces the size of the routing information table.