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OSPF/IGRP Quick Notes

Here are some notes we jotted down that relate to OSPF. Helpful when studying for the CCIE practical exam as a primer for OSPF. If you find this helpful let us know and we will create additional notes for other routing protocols compliments of

OSPF link state, unlimited hops, classless, uses cost for metric, process # is locally significant, and

Router ospf 1

Network x.x.x.x area 0   (tells router which int will participate in OSPF)

  -or-     Network x.x.x.x area 0  (same as above except matches network)

  -or-     Network area 0 (turns OSPF on on all interfaces)


broadcast medium - ethernet, token ring, ATM.  Dont need to define neighbors

non broadcast medium frame relay.  Need to define neighbors on DR and BDR using one of two ways -

ip ospf network broadcast or point-to-multipoint -or-

neighbor x.x.x.x  and ensure all int are non broadcast by using ip ospf network non-broadcast

point-to-point (serial but not frame) No DR, separate subnet for each sub int

ip ospf network point-to-point

point-to-multipoint (frame relay from one to many) No DR, dont need neighbor cmds

ip ospf network point-to-multipoint or- neighbor x.x.x.x   -  usually one subnet for whole network


Summarize routes from one area to another at ABR (multiple ospf areas)  -  area 1 range x.x.x.x

Summarize external routes at ASBR (redist  into OSPF)  - summary address x.x.x.x

Summarize OSPF routes for redist into IGRP at ASBR   - summary address x.x.x.x


Change cost metric using ip ospf cost 10  (changes from def of 100,000,000/bps) higher bandwidth = lower cost

To change the def bandwidth calculation without using cost use ip ospf reference bandwidth ???


Virtual link to connect discontiguous areas to area 0.  area 1 virtual-link     area 1 virtual-link

            The area # is the transit area.  The IP is the router id of the neighbor  (highest loop or highest IP if no loop)

To prevent hellos and lsas from brinding up a circuit use int s0 ip ospf demand circuit

To advertise default route from ospf default information originate always  (use always if not in table)

NSSA Prevent ext type 5 lsas from being flooded from area 0 into area 2 but allows importing type 7 as ext routes

area 2 NSSA   (type 7s are translated into type 5s by the NSSA ABR then are flooded).

To advertise loopbacks run ospf on the int with network x.x.x.x or- redistribute connected route-map ian subnets

Distribute-list 1 out can only be used on ASBR

Distribute-list 1 in to filter routes on each local router.  Each router will need the dist in to filter out networks


Authentication simple

router ospf 64

area x authentication

int e0

ip ospf authentication


 IGRP distance vector, 255 hops, classful, uses metrics for route determination, AS #

Router igrp 10

Network x.x.x.x (classful)

Passive-interface e0   (listen but dont send routes)

Neighbor x.x.x.x  (use with passive-int for unicast IGRP)


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