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 CCIE4u.com

OSPF – link state, unlimited hops, classless, uses cost for metric, process # is locally significant, 224.0.0.5 and 224.0.0.6

Router ospf 1

Network x.x.x.x 0.0.0.0 area 0   (tells router which int will participate in OSPF)

  -or-     Network x.x.x.x 0.0.0.31 area 0  (same as above except matches network)

  -or-     Network 0.0.0.0 255.255.255.255 area 0 (turns OSPF on on all interfaces)

broadcast medium - ethernet, token ring, ATM.  Don’t 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, don’t 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 255.255.0.0

Summarize external routes at ASBR (redist  into OSPF)  - summary address x.x.x.x 255.255.0.0

Summarize OSPF routes for redist into IGRP at ASBR   - summary address x.x.x.x 255.255.0.0

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 2.2.2.2     area 1 virtual-link 1.1.1.1

            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 hello’s and lsa’s 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 0.0.0.0 not in table)

NSSA – Prevent ext type 5 lsa’s from being flooded from area 0 into area 2 but allows importing type 7 as ext routes

area 2 NSSA   (type 7’s are translated into type 5’s 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 don’t send routes)

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

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