Basic Ospf Concepts - H3C S7500 Series Operation Manual

Operation Manual – Routing Protocol
H3C S7500 Series Ethernet Switches
Each OSPF router maintains a link state database (LSDB), which describes the
topology of the whole AS. Based on the network topology around itself, each
router generates link state advertisements (LSAs) and sends them to other
routers in update packets. The LSAs a router receives from other routers form
the LSDB of the router.
An LSA describes the network topology around a router, whereas an LSDB
describes the network topology of the whole network. Routers can easily
transform the LSDB to a weighted and directed map, which reflects the topology
of the whole network. All routers have the same map.
Each router uses the shortest path first (SPF) algorithm to calculate the shortest
path tree that shows the routes to the nodes in the autonomous system. The
router itself is the root of the tree. External routes are leaf nodes, which are
marked by the routers from which they are advertised to record information
outside the AS. Each router maintains a different routing table.
Furthermore, to enable individual routers to broadcast their local status information
(such as information about available interface and reachable neighbor) to the whole
AS, routers in the AS must establish adjacencies among them. In this case, a route
change on any router will result in multiple transmissions, which are unnecessary and
bandwidth consuming. To solve this problem, designated router (DR) and backup
designated router (BDR) are defined in OSPF. For details about DR and BDR, see
section 4.1.4 III. "DR and
OSPF supports interface-based packet authentication to guarantee the security of
route calculation. In addition, it transmits and receives packets in multicast (224.0.0.5
and 224.0.0.6).

4.1.3 Basic OSPF Concepts

I. Router ID
To run OSPF, a router must have a router ID. A router ID can be configured manually.
If no router ID is configured, the system will automatically select an IP address from
the IP addresses of the interfaces as the router ID. A router ID is selected in the
following way: if loopback interface addresses are configured, the system chooses
the latest configured IP address as the router ID; if no loopback interface is configured,
the first configured IP address among the IP addresses of other interfaces will be the
router ID.
II. DR and BDR
For details, see section
III. Area
If all the routers on an ever-growing huge network run OSPF, the large number of
routers will result in an enormous LSDB, which will consume an enormous storage
BDR".
4.1.4 III. "DR and BDR".
4-2
Chapter 4 OSPF Configuration