OSPF Routing - Foundation Concepts 1


Link State Routing Protocols

Two Types:
IS-IS
OSPF

All other routing protocols are distance vector (yup even big bad Mr. Internet BGP)

Maintains 3 Tables (just like EIGRP, but with a difference)

NEIGHBOUR TABLE
tracks neighbours all neighbour it has relationships with and exchange routes with and directly connected to (ONLY FORMS NEIGHBOURS WITH ROUTERS IN SAME AREA ONLY)

TOPOLOGY TABLE
every link state protocol maintains this table, but this is a ROAD MAP for that AREA, so they see the whole layout (all links), all the routers in that router

EIGRP, does have this too, - but its not a roadmap of everything in the area, it is just a list of what the neighbour has past on ....."routing by rumour"

ROUTING TABLE
Best path


Uses Dijkstra's shortest path first (SPF) algorithm

Advantage
Each router knows everypath and knows the best path with that area (network added or removed, they all know about it)

Disadvantage
Can be an intense process (CPU) (so make sure the areas dont get too BIG) as each router has to make decisions on whole area


Sends triggered updates to announce network changes (unlike RIP sending whole routing table)


Sends periodic updates (LS refresh) on long intervals, this is the whole routing table to its neighbour if after 30 minutes there has been no activity (so pokes neighbour and says, just in case you missed it, this is what i see road map as)




OSPF AREA DESIGN AND TERMS

All areas must connect to area 0 (1 interface)

All routers in an area have the same topology table (convergence/load gets more the bigger the area)

Goal: localise updates within an area (Area 2 does not have to both the backbone etc or cross a slow WAN link)

Requires a hierarchical design (i.e Area 2 10.1.0.0, 10.2.0.0., area 3 has 172.16.0.0, 172.17.0.0. ....) as this is the whole pupose of an area (contiguous network/discontiguous network), keep updates local and this is achived by summarisation


All Areas are considered to be in the OSPF AS (divided by the ABR's,) whereas the ASBR is the AS Boundary Router ...Now these two types of routers are the only place you can do summarisation ...(unlike good ole EIGRP, where you can summarise on any interface on any router)


 
UNDERSTANDING OSPF NEIGHBOUR RELATIONSHIPS

RID
Only changes when you restart the OSPF process or reboot the router

dead timers are x4 the hello (if you take the hello message interval over NBMA network, the dead time is 2 minutes! ... these can be tweaked and should be tweaked, 2 minutes is WAY TO LONG!)



3. This Hello msg .... is considered the "DOWN STATE"

4. Upon recieving the HELLO; we enter the  "INIT STATE" - the routers haven't formed neighbourship as of yet, but it is checking..

5. Is the "2-WAY STATE"

6. We enter the "EXSTART STATE"- Exchange Start state, this means the HELLO packets agree, lets being exchanging our link state databases

7.  Is the "LOADING STATE" state

Which leads on to CONCEPTS PART II