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ISCOM2600G-HI (A) Series Configuration Guide
LSA type
OSPF describes link status, encrypts the information in LSA, and advertises LSA. There are 5
types of common LSAs:
Neighbor and adjacency
After being started, an OSPF router sends Hello packets out through the OSPF interface. After
receiving Hello packet, a device checks parameters (interval for sending Hello packets,
invalidation time, and area mask information) defined in the Hello packet. If it has the same
parameters, it forms a neighbor relation with the OSPF router.
A neighbor is not necessarily in an Adjacency relation, and it depends on the network type.
Only when the two devices exchange DD packets and LSAs, and synchronize to the peer
LSDB can they become in adjacency relation.
The ISCOM2600G-HI series switch supports up to 32 neighbors.
Calculating OSPF routes
OSPF calculates routes as below:
Step 1 Each OSPF router generates LSAs according to network topology, and sends LSAs to other
OSPF routers through updating packets.
Step 2 Each OSPF router collects LSAs from other OSPF routers. All LSAs form LSDB. LSA
describes network topology around the router. LSDB describes network topology of the entire
AS.
Step 3 Each OSPF route transfers LSDB to a weighted diagram, which reflects topology of the entire
network. Each OSPF router obtains the same weighted diagram.
Step 4 Each router uses the Shortest Path First (SPF) algorithm based on the weighted diagram, and
then calculates a shortest path three with itself as root. This tree provides routes to all nodes in
the AS.
Area division
When routers on a large network run OSPF, increment of routers leads to a huge LSDB which
occupies much storage space and causes the CPU to work in heavy burden. When the network
grows larger, topology changes more frequently, the network is always in oscillation status, a
large number of OSPF packets are transmitted, network bandwidth is wasted, and each change
causes recalculation of routes for all routers.
Router LSA (Type1): generated by each router, used to describe link status and cost, and
speeded in the originating area.
Network LSA (Type2), generated by the DR, used to describe link status of all routers in
this segment, advertised in the originating area.
Network Summary LSA (Type3), generated by the Area Border Router (ABR), used to
describes routes of a network segment in the area and notify other areas.
ASBR Summary LSA (Type4), generated by the ABR, used to describe routes to
Autonomous System Boundary Router (ASBR) and notify related areas.
AS External LSA (Type5), generated by the ASBR, used to describe routers out of AS
and notify all areas except Stub area.
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