System information

Troubleshooting DECnet 11-251
DECnet Phase IV Routing Frame Format
Figure 11-2 A DNA Phase IV Routing Layer Header
The first field in a DNA Phase IV routing header is the routing flags field, which includes:
• A return-to-sender bit that, if set, indicates that the packet is returning to the source.
• A return-to-sender-request bit that, if set, indicates that request packets should be returned to the
source if they cannot be delivered to the destination.
• An intraLAN bit, which is on by default. If the router detects that the two communicating end
systems are not on the same subnetwork, it turns the bit off.
• Other bits that indicate header format, whether padding is being used, and other functions.
The destination node and source node fields identify the network addresses of the destination nodes
and the source node.
The nodes traversed field shows the number of nodes the packet has traversed on its way to the
destination. This field allows implementation of a maximum hop count so that obsolete packets can
be removed from the network.
DECnet identifies two types of nodes: end nodes and routing nodes. Both end nodes and routing
nodes can send and receive network information, but only routing nodes can provide routing services
for other DECnet nodes.
DECnet routing decisions are based on cost, an arbitrary measure assigned by network
administrators to be used in comparing various paths through an internetwork environment. Costs
are typically based on hop count, media bandwidth, or other measures. The lower the cost, the better
the path. When network faults occur, the DECnet Phase IV routing protocol uses cost values to
recalculate the best paths to each destination. Figure 11-3 illustrates the calculation of costs in a
DECnet Phase IV routing environment.
Figure 11-3 A DECnet Phase IV Routing Protocol Cost Calculation
Routing
flags
1
Destination
node
2
Source
node
2
Nodes
traversed
Field length,
in bytes
1
Source Destination
Best path to destination
A D
C
4
87
4
5
6
2
3
23
32
A D
B E
DestinationSource