OSPF ROUTE notes and fundamentals

OSPF – > Open shortest Path first.



Capabilities and other attributes…


Fast convergence not as fast as EIGRP

Link state routing protocol.

Uses Djikstra code to find paths.

Uses cost as metric.

Does not perform Summerization by default.  Only ABR / ASBR

All areas must connect to area 0 unless connected through VRF

All routers have a copy of LSDB

Cisco suggests an area should have no more than 50 routers.



Numbers to remember..

Multicast Address is

DR/BDR Multicast =

Port 89  packets.. Its own layer 4 Protocal sends Lsacks

Administrative distance = 110

Cost is determined the following way – > 100/link bandwith in MBPs for T1 100/1.54=64 Cost

Every 30 minutes Neighbor Routers send LSA’s to check on their neighbors with a Higher Seq #

Hello packets are 10 seconds by default, To find neighbors to check the neighbor router.

Hello packets are sent 30 seconds for NBMA networks

Dead timer is 4x the Hello packet all the time.

Router Priority by default is always 1




Different Type OSPF Packets.

1.)   LSA – > Updates about individual Routes….. Normaly in LSU’s they contain Multiple LSA’s.  So a LSU can come over after a LSR, it will have LSA’s for network and network.




2.)   DBD – > Data base Description Checks for Database Syncrhonization -> Contains a summary of LSDB, Which includes all known RIDS and their last sequence #.

3.)   LSR – > Link state request – > requests information about networks…

4.)   LSU -> Contains the full LSA enteries.  Multiple LSA entries can fit one OSPF Update packet.. Replies to the LSR

5.)   Lsack – > Acknoledges other packet types..  Making it reliable.



Biggest difference between the link state and the Distance vector is Distance vector relys completely on the neighbor adjacent router to give the router its best path to a destination.


All Link state routers keep track of the following information…


OSPF Neighbor Table – > OSPF neighbor table or Its adjacent neighbor table.


OSPF topology Table – > All other Routers in a topology table, in their Area or in the next area if its a ABR.


Routing Table – > Topology table including ALL paths from every single router to every network.


How a OSPF Router becomes a neighbor with another OSPF router….


1.)Down state – > Router sends hello packets on the Mulitcast address of


Hello packet contains the following..



Subnet Mask


Authentication if it uses it

Stub flag


2.)INIT state – > Receiving router checks the hello packet(is it in the same Area, is the dead timers / hello timers are they the same at that point the router is in its up state)


Checks the following…

Hello / Dead Intervals

Checks Subnet Masks

Area ID

Authentication password if it uses it

Stub area flag




3.)Two way state – > When the router receives all the packets it sends a Unicast reply,


4.)   EX Start – >

Starts the DR / BDR election





4)After it is in that Up state, the two routers exchange their LSDB.  Now the routers are in a FULL relationship.

5.)Router then fowards any new LSA’s to neighbor routers.


OSPF Neighboring States.


Down – No OSPF yet sends out the Hello packet at this point on the multi cast

Initiate – Hello packet has been received

two way – Router see its on RID in a hello packet

Exstart – DR / BDR election process or it finds it.

Exchange – DBD / LSDB has been exhanged

Loading – Exhange of LSRS and LSUS to make the LSDB

Full – Neighbors are fully adjacent at this point.


Convergence process.


If the following happens.. ——–> Router1 > ———Router 17> ———–> Router 2(DR)


Router 1 will then send the multi cast for the bdr and DR at that point to tell them it needs to flood LSU’s for flooding LSA’s.


At that point the DR sends a ACK telling Router 1 it received aa LSU from it.


Next the Router 2 / DR sends a Multicast LSU to other routers on the normal Multicast.  Each Router responds with a Lsack.


If a router is connected to another area it will first forward the LSU to its DR in that area.










Advantages of using different Areas to break OSPF down further…


Reduced Overhead with SPF calculations – > Each area within itself with do calculations only for that areas, it will not, go over to the next area and flood that area with any LSA’s.


Smaller routing tables – > Since the ABR /ASBR summerizes routes it makes the routing table smaller so it makes chosing a path easier.


Reduced LSU overhead – > Instead of sending multiple LSU’s the ABR / ASBR summerizes a route.


Area Terminology


Two types of networks that exists Within a OSPF network…


backbone – > also known as Area 0

regular non backbone area – > Any other area that is not that backbone that is connected via area 0.


Internal router – > Routers that have al of their interfaces in the same area.  All routers within the area have the same LSDBs…. that meens they all have the same LSDBs for their networks, and the ABR / ASBR does the summerization… so for example area 1 is going to know about all its routes plus maybe is comming out of another router to area 0 to another router.

Backbone router – > Router that sit in Area 0.


Area Border Router – > Routers that have interfaces connected to area 0 and the directly attached network.  So for example ABR from              Area 0—-R1—–R2—area 15


Autonomous System Boundary Router – > Routers that have at least one interface connecting to a different AS possibly a exit to the internet or possibly another IGP.




Designated Router – > DR – > 1 per every shared segment

BDR – > really does not do much other than back it up incase of failure

Drother – > everything else.


Point to Point – > Obviously has no DR


How are Dr / BDR’s Selected?


1st has to do with the OSPF priority Default is one. The highest priority wins for DR second is BDR

the tie breaker is the Router ID.


All Drothers to each other are ALWAYS in a 2 way state, the only full relationship other than a P2P in a Braodcast network is 2 way.


Full relationships are both for the DR and BDR.



How to reditribute Static Routes from a ASBR to all other routers…


Has to be done after setting OSPF process…


Router OSPF 1

redistribute static Subnets(OSPF tries to summarize automatically) metric 200 (Gives a metric of 200) Metric-type 2(sets it as E2)




OSPF External Types.

E1 – > Increment their Metric – > So all routers will Add the Cost per router, it will increase

E2 – > Does not Increase the metric – > so this will keep the cost metric at 200 per prior example


OSPF In BroadCast Mode -> Ethernet / Token Ring.

DR/BDR elections

Single 10 Second Hellos

2 multicast address



OSPF In Point to Point Networks…


Both are in FULL mode

Hellos 10 seconds




Frame Relay / ATM


5 Different ways of Using OSPF



No Multicast or Broadcasts

Psudo Broadcasting – When a broadcast is sent through the cloud.  Its more like a unicast thrugh the DLCI




How To make OSPF work without connecting to Area 0 With Virtual Links.


Depends on the RID


Router ospf 1


Area 1 virtual-link – > RID

the rid has to be the ABR/ASBR

The virtual link has to be configured on both sides


LSA Types..

type 1 – hello LSA

type 2 – DR /  BDR

type 3 – ABR

type 4 – ASBR – > These are from summarized

type 5 –  External Routes


Stub Area – > Blocks Type 5 LSA

Totally Stuby – > block type 3,4,5 LSA


Stub Area


Simple Configuration.


Router OSPf 1

(config-router) area 1 Stub

The reasons for blocking LSAs would be to get routes to be filtered in that area.


So for example if you configured R3 as regular ethernet setup and had

O E2192.168.1.0

O E2


If you set it up as a stub network it would then just set


O* via the ABR


Just giving it a external path that does not give any information about the external routes.                        via ABR since the ABR knows the entire route.

This is best to shrink the routing table and give less over head if there are a ton of external routes.


Totally Stuby Area – > Blocks almost everything LSA 3,4,5.. Cisco Propriatary



the only router that has to be a Cisco Device is the ABR.


(config-router) area 1 stub no-summary

^^ on the ABR


the regular directly connected router has to be just a stub area.


This filters practically everything, should only have

O*IA via ABR…


Since it blocks lsa 345.. .. the only other LSAs which are avaliable there would be the Dr and the hello packets




Not So Stubby Area


this is confusing….


Passes external routes through a type 7 LSA


If a Router is connected through a OSPF router to a entire area, and it is also a ASBR to something like EIGRP RIP static etc etc…


The router which is directly connected to the NssA router takes that area as a LSA 5…


Area 1 Nssa


The ABR would have to configured as a


area 1 nssa no-s






OSPF Commands


SH ip proto

SH ip OSPF Database

Router-id – > sets RID

Clear IP ospf Proccess

sh ip ospf int

ip ospf priority(interface)0 – sets its to drother automatically

Area 10 range -> Will Summarize that route for the area.


(ospf-router)auto-cost referrence bandwith 1000 – > divides it by 1000 instead of 100





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