OSPF network types & OSPF network area types

OSPF network types

1, point to point

2, broadcast

3, one to multi points broadcast

4, one to multi points no broadcast

5, no broadcast

Especially, point to point and one to multi points* do not need select DR and BDR, because, one to multi points (both broadcast and no broadcast) will work in the way like several point-to-point links, there is no need to select DR and BDR. Especially, router with the highest priority will become DR and the router with the second highest priority will become BDR. In Juniper router has default priority of 128, in Cisco router has default priority of 1.

On the other side, both type 2 and type 5 need select DR and BDR in order to reduce the numbers of ospf packets (hello, lsa, etc) in the network.

Normally ospf build up adjacency by broadcasting hello packets to 224.0.0.5, but in some noethernet network (mostly in frame relay network), broadcasting is not applicable, hello packets are sent out with unique ip address of the peer side.

Network area types

1, Backbone area ( area 0 ) , can receive all LSA types information

2, Standard area, can receive type 3 info from ABR (Area border router)

3, Stub area, can receive normaly LSA type 3 info and a default route as subsitute for all external routes

4, Totally stub area, can receive only one LSA type 3 as default router towards outside of the area

5, Not so stub area, can work as stub area or totally stub area, BUT, can send external routes from  ASBR as type 5 LSA to the other area.

LSA types:

type 1:  router

type2: net route, generated and sent only by DR

type3, net summary route, generated and sent only by ABR in order to reduce the numbers of routes sent to the other area.

type4: ASBR-Summary LSA,Type 5 External LSAs are flooded to all areas and the detailed next-hop information may not be available in those other areas. This is solved by an Area Border Router flooding the information for the router (i.e. the Autonomous System Boundary Router) where the type 5 originated. The link-state ID is the router ID of the described ASBR for type 4 LSAs.

type 5:external route that send to other network area from NSSB(not so stub  area)

type 7: external route

Especially, type 1 and type 2 does not cross area border.

type 8 and type 9 are used to carry ipv6 route information,

  • SA Type 8: Link LSA
  • Link Local scope: LSA is only flooded on the local link and is further used for the LINK-LSA
  • LSA Type 9: Intra-Area LSA
  • Area Scope: LSA is only flooded throughout a single OSPF area. Used for router-LSA, Network-LSA, inter-area-prefix-LSA, inter-area-router-LSA and intra-area-prefix-LSA
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