Raisecom ISCOM2600G-HI (A) Series Configuration Manual page 263

Raisecom
ISCOM2600G-HI (A) Series Configuration Guide
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Router ID
To run OSPF, a router must have a router ID which is a 32-bit symbol-free integer. The router
ID can uniquely identify a router in an AS.
The router ID can be elected by the system or manually configured. The election
rules are as below:
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DR/BDR
In a broadcast network, any two routers need to exchange routing information. Thus route
change on a router causes multiple transmissions, which wastes bandwidth resources. To solve
this problem, OSPF defines the Designated Router (DR), which receives information from all
routers and then advertises link status.
When the DR fails due to a fault, OSPF use a Backup Designated Router (BDR) to avoid
incorrect calculation of routes in DR re-election time. Thus a BDR is elected while the
corresponding DR is elected. The BDR establishes adjacency relation with all routers in the
network segment and exchanges route information with them. When the DR fails, the BDR
immediately becomes the DR. Then, a new BDR is elected, but this does not impact route
calculation.
On a network running OSPF, a router not DR nor BDR is called DR Other. A DR Other
establishes adjacency relation with DR and BDR only rather than another DR Other, as shown
Figure 5-5. It reduces the number of relations between routers in the broadcast network and
NBMA network, reduces network traffic, and saves bandwidth resource.
Point-to-Point (P2P): when the data link layer protocol is PPP or High-Level Data Link
Control (HDLC), OSPF takes network type as P2P by default. In such networks, OSPF
sends protocol packets in multicast mode (multicast address: 224.0.0.5).
If there are loopback interfaces configured with IP address, choose the maximum
IP address of loopback interface as the router ID.
If there are loopback interfaces without IP addresses, choose the maximum IP
address of IP interface as the router ID.
If the IP address is used by other OSPF process, it cannot be used by this OSPF
process.
If no IP address is configured, the route ID cannot be elected, the process cannot
be created; you have to manually configure the router ID.
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