Understanding OSPF Terminology and Operation-1
OSPF Router Types
How OSPF routers exchange information is based on:
- The function of the router.
- The type of LSAs it can forward.
- The type of area it resides in.
OSPF routers may function as either:
- Internal router
- Backbone router
- Area Border Router (ABR)
- Autonomous System Boundary Router (ASBR)
- A router can exist as more than one router type.
Routers that have all their interfaces within the same area.
Internal routers in the same area:
- Have identical LSDBs.
- Run a single copy of the routing algorithm.
OSPF design rules require that all areas be connected to a single backbone area (Area 0).
- Area 0 is also known as Area 0.0.0.0
An Area 0 router is referred to as a backbone router.
- Depending on where it resides in Area 0, it may also be called an Internal router, an ABR, or an ASBR.
Area Border Router (ABR)
Routers with interfaces attached to multiple areas and responsible for:
- Joining areas together.
- Maintaining separate link-state databases for each area.
- Routing traffic destined to/arriving from other areas.
- Summarizing information about each area connected and flooding the information through area 0 to the other areas connected.
- An area can have one or more ABR.
ABR cannot send LSU’s to other areas until the entire intra-area is synchronized.
Autonomous System Boundary Router (ASBR)
Routers that have at least one interface connected to another AS, such as a non-OSPF network.
Routers support redistribution.
- They can import non-OSPF network information to the OSPF network.
Should reside in the backbone area.
OSPF Router Types
Routers A, B, C, D and E are backbone routers.
- Backbone routers make up Area 0.
Routers C, D and E are area border routers (ABRs).
- ABRs attach all other areas to Area 0.
Routers A, B, F, G, and H are internal routers.
- Internal routers are completely within an area and do not interconnect to any other area or autonomous system (AS).
DR and BDR Routers
To reduce the amount of OSPF traffic on multiaccess broadcast networks such as Ethernet, OSPF elects:
- A Designated Router (DR)
- A Backup Designated Router (BDR)
The DR is responsible for updating all other OSPF routers (called DROTHERs) when a change occurs in the multiaccess network.
- The BDR monitors the DR and takes over should the DR fail.
A router connected to multiple broadcast networks can be a DR on one segment and a regular (DROTHER) router on another segment.
OSPF Metric Calculation
OSPF Cost Formula
Cost = 100,000,000 / Bandwidth (bps) For example:
- 10BaseT= 100,000,000 / 10,000,000= 10
- T1= 100,000,000 / 1,544,000= 64