IBM SC30-3865-04 - Manual (Page 325)

v Source ID
v Set of area addresses that it services
v IS type (L2)
v System IDs and the cost of reaching IS adjacencies
v If applicable, the system ID of the pseudonode
v Address prefixes for ISs located in an external domain
L2 Link State Update (pseudonode)
The L2 pseudonode LSU is multicast over the interface and propagated to all L2
ISs located outside the subnetwork. Any L2 non-pseudonode IS located on the
same subnetwork that receives the LSU relays the LSU to all L2s located outside
the subnetwork. The L2 IS places the following information in its LSU:
v Source ID
v IS type (L2)
v System IDs and metrics for non-pseudonode ISs located on the same
subnetwork
Attached and Unattached L2 IS
An attached L2 IS is a router that knows of other areas. An unattached L2 IS is a
router that does not know of any areas other than its own.
When routing, an unattached L2 IS routes packets to the closest attached L2 IS.
Routing Tables
An L1-only IS uses one routing table, the level 1 routing table. An L2-only IS
contains three routing tables: an L2 area-address routing table, an L2
internal-metric reachable-address-prefix routing table, and an L2 external-metric
reachable-address-prefix routing table. An L1/L2 IS contains the L1 routing table
and all L2 routing tables. The routing table entries are built from information in the
link state database.
L1 Routing
The following summarizes L1 routing:
1. An L1 IS receives a packet and compares the area address portion of the
destination address in the header of the packet to the set of area addresses in
the router.
2. If the packet is destined for the router’s area, the router extracts the system ID
from the address. Searching for a match, the router compares the system ID to
the system IDs in the L1 routing table.
3. If a match occurs, the IS routes the packet to the ES or the next hop IS. If no
match occurs, the packet is dropped.
4. If the packet is not destined for this area, the L1 forwards the packet to the
nearest L2 IS or if this router is an L1/L2 IS, it checks its L2 routing tables as
described in the next section. If the L1 cannot determine where to route the
packet, the packet is dropped.
Using OSI/DECnet V
Chapter 9. Using OSI/DECnet V 295