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                      2

Network number

Contains the network numbers of the segments of which the router is aware. The router simply matches the destination network number in the packet's IPX header with an entry in this field to get its forwarding instructions for the packet.

Hops to LAN segment

Indicates the number of routers that must be traversed to reach the network segment.

Ticks to LAN segment

Contains an estimate of the time necessary to reach the destination segment. It is used by the router in its periodic broadcasts to indicate the time necessary to deliver a packet to a node on that segment.

NIC

Indicates through which network interface card in the router the network segment can be reached

Immediate address

Contains the node number of the router that can forward packets to each segment. If the segment is directly connected to the router, this field will remain empty.

Net status

Indicates whether the segment is directly connected to the router and whether the segment is considered reliable

Aging timer

Is used to make sure that information about the segment is current

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.

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