OSPF: Design Consideration

OSPF Design Guidelines
The OSPF protocol, as defined in RFC 1583 & RFC 2178 provides a high end solution for designing and implementing wan solutions for any organisation. But RFC 2178 is very good while designing OSPF network. It is advanced or updated version of RFC 1583. While designing OSPF network two important things need to be remembered. One is finalising the area boundary & another is assigning ip addresses in a proper and well structured manner. While revamping any cloud one must take care of both.

OSPF Network Topology
While designing OSPF network one should remember to maintain the following cited points:-
a) Hierarchical structure which includes area 0 works with the other area and other areas will work around it.
b) The number of routes in particular area.
c) Number of area to be connected with ABR.
d) Selection of DR on multi-access network.

Number of routers in an area
It is always recommended that OSPF area should not have more than 40 – 50 numbers of routers because SPF algorithm is very CPU intensive. But it doesn’t mean that we cannot increase the number of routers in an area, the all depends on the stability of links. The more the links will be stable less SPF will run. One can check the SPF by issuing show ip ospf command. In it SPF last triggered timer is given.

Number of areas connected with an ABR
It is always recommended that an ABR(One interface is in area 0) should not have more than 2 or 3 areas. Because for every area router has to maintain the LSD for every area and it is very cpu intensive process. The more you increase the number of areas lower the performance you get from the router.

Selection Of DR router
On broadcast or lan network DR router should be selected by issuing the priority command with 255 and the router should be enough capable to handle the SPF calculations because it is the one who will heading the whole routers on that lan. It is also recommend that a DR router should work only for one broadcast domain not for multiple.


regards
shivlu jain

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