Overview
- Link-state protocol.
- Uses Dijkstra which calculates the shortest path tree in each area.
- OSPF v2 for IPv4 and OSPFv3 for IPv6.
- LSDB = link-state database.
- Area 0 = backbone, all inter-area traffic goes via area 0.
- Directly attached routers (i.e. in the same broadcast domain) form adjacencies using hello packets:
- DR and BDR controls traffic between neighbours
- 224.0.0.5 = all SPF/link-state routers
- 224.0.0.6 = all DR routers
- Does not use TCP/UDP, but IP protocol 0x89.
Area Types
- Backbone: all other areas connect to it, so it must be (logically) contiguous.
- Stub: contains intra-, inter-area and default routes, but no external routes.
- Totally stubby: contains intra-area and default routes only.
- NSSA: contains intra-, default and external routes:
- within NSSA, ASBR sends type 7 LSAs.
- at ABR, sends type 5 LSAs to rest of domain.
Router Types
- ABR: connects one or more areas to area 0.
- ASBR: connected to more than one AS, and typically runs BGP as well.
- IR: internal router (inside an area).
- BR: backbone router (inside area 0).
Path Preference
- Path cost/metric is usually associated with link speed
- 4 metric types:
- intra-area
- inter-area
- external type 1 = external + internal path cost
- external type 2 = external path cost
DR/BDR
- If priority = 0 then never DR/BDR.
- Highest priority wins, then highest logical IP address, then highest active interface IP address.
- When DR fails, BDR becomes new DR and another BDR is elected.
- New routers with higher priority do not take over DR/BDR unless there is a DR/BDR failure.
LSA Types
| Type |
LSA |
Description |
| 1 |
Router |
Intra-area (generated by all routers) |
| 2 |
Network |
Intra-area (generated by DR on NBMA networks) |
| 3 |
Summary |
Routes to networks (generated by ABR for intra-AS routes) |
| 4 |
Summary |
Routes to ASBRs (generated by ABR for intra-AS routes) |
| 5 |
AS external |
Routes to external destinations (generated by ASBR) |
| 7 |
|
NSSA external link state |
| 8 |
|
External attributes for BGP |
| 9,10,11 |
|
Opaque LSA |
OSPF-TE
- Extension to OSPF.
- Additional properties:
- max reservable bandwidth
- unreserved bandwidth
- available bandwidth
IOS Configuration Example
auto-cost reference-bandwidth <...>
area 0.0.0.0 authentication message-digest
passive-interface default
no passive-interface <...>
network x.x.x.x y.y.y.y area z.z.z.z
JUNOS Configuration Example
from [src/dst address|src/dst port]
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 Updating... Ċ Robert Larsen, 4 Jan 2011, 01:05
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