Ospf Route Calculation; Basic Ospf Concepts - Huawei Quidway S3900 Series Operation Manual

Operation Manual – Routing Protocol
Quidway S3900 Series Ethernet Switches-Release 1510

4.1.2 OSPF Route Calculation

Taking no account of area partition, the routing calculation process of the OSPF
protocol is as follows:
Each OSPF-capable router maintains a link state database (LSDB), which
describes the topology of the whole AS. According to the network topology
around itself, each router generates a link state advertisement (LSA). Routers on
the network exchange LSAs with each other by transmitting protocol packets.
Thus, each router receives the LSAs of other routers and all these LSAs 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 directed map, which actually reflects the
topology of the whole network. Obviously, all routers get exactly the same map.
A router uses the shortest path first (SPF) algorithm to calculate the shortest path
tree with itself as the root. The tree shows the routes to the nodes in the
autonomous system. External routes are leaf nodes, which are marked with the
routers from which they are advertised to record information outside the AS.
Obviously, the routing tables obtained by different routers are different.
Furthermore, to enable individual routers to broadcast their local status information
(such as available interface information and reachable neighbor information) to the
whole AS, routers in the AS should establish neighboring relationship among them. In
this case, the route changes on any router will result in multiple transmissions, which
are unnecessary and waste the precious bandwidth resources. 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 BDR".
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. If no router ID is configured, the system
will automatically select an IP address from the IP addresses of the current interfaces
as the router ID. A router ID is selected in the following way: if there exists loopback
interface addresses, the system chooses the loopback address with the greatest IP
address value as the router ID; if no loopback interface address is configured, the IP
address of the physical interface (for a switch, the VLAN interface address) that was
first configured and is UP will be the router ID.
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Chapter 4 OSPF Configuration