X.25 Data Link Addressing
The router supports DECnet Phase IV over X.25 and can interoperate with routers
running Digital’s implementation of DECnet Phase IV over X.25.
You set up the local and the remote DTE address with the set/define circuit
command when you set up a DECnet circuit. In the
call-userdata
parameter you
specify the local DTE address in hexadecimal octets (characters). In the
DTE-address
parameter you specify the remote address in hexadecimal octets.
Both the local and remote DTE addresses can be up to 14 hexadecimal octets in
length with two ASCII characters representing one hexadecimal octet.
Routing
DNA IV handles both forwarding of DNA IV data packets and automatic routing with
other DNA IV nodes. The router performs the following DNA IV functions:
v Announces its presence by sending hello messages on each network that has
DNA IV enabled.
v Maintains a list of adjacent DNA IV nodes from the hello packets it receives from
other DNA IV nodes.
v Exchanges routing information with other routers.
v Forwards packets between nodes.
All end and routing nodes periodically broadcast hello messages to the all-routers
multicast address. This allows each router to locate other nodes in its area.
On each broadcast network (for example, Ethernet, Token-Ring), one router
declares itself the designated router for that wire. The designated router broadcasts
its presence so that the end-nodes know to use it as their default gateway. Any
end-node sending a packet to a node not on that wire automatically sends it to the
designated router for forwarding.
In a multi-area DNA, assign priorities to routers in such a way that the designated
router is a level 2 router, or is likely to be the best next hop to commonly-used
destinations. This reduces the possibility of traffic from end-nodes having to take an
extra hop.
Routing decisions are based on a least-cost algorithm. Each link (e.g.,
point-to-point, broadcast network, hop) has a cost. Every router broadcasts (to other
routers only) its cost and the number of hops to get to every node in its area. In this
way, each router finds the cheapest path, subject to a maximum hop count.
Routing Tables
A router forwards any DNA IV data packet it receives to the proper node based on
its routing table. To maintain its routing table, a router listens to and sends level 1
updates to every node in its area. If the router’s type is set to AREA, it also
exchanges level 2 routing updates.
Each router maintains a routing table with an entry for every node (up to the
maximum address) and every possible next hop (all circuits and up to the maximum
broadcast routers). Each entry in this table contains the cost and hop to reach a
node via one circuit or next hop node. Once a second the routing table sends out a
broadcast routing timer.
Using DNA IV
Chapter 7. Using DNA IV 251
