Ospf enhanced concepts, definitions and configurations, Below I am trying to show you some of the main important concepts about OSPF routing.
OSPF cost = 100 / BW_in_MBps
Ospf messages :
DBD Database description
LSR Link state request
LSA Link state Advertisement
LSU Link state update
LSAck Link state acknowledge
Ospf routers :
DR : Designated router to send all change updates to others
BDR : Backup Designated router
router ospf 1
network ?
router-id ?
int s0/0
ip ospf priority ?
ip ospf ? # instead of using network command
Force a router not to be a DR or BDR :
ip ospf priority 0 #means it can not be a DR or BDR
neighbor ? priority ? # priority 0 is the same thing
Show commands :
show ip ospf database
show ip ospf topology
OSPF network Types :
Broadcast , multiaccess networks (ethernet,token ring)
Point to Point (T1 CAS , ISDN BRI/PRI)
None-Broadcast Multiaccess (NBMA) (Frame relay , ATM)
OSPF over NBMA modes :
None Broadcase (NBMA) – RFC -
by default – have to staticly define neighbors , one subnet , like as lan environment , DR/BDR must have full connectivity to all others
Point to Multipoint – RFC
don’t have to define neighbors
Point to Point – Cisco
threads every single subinterface as a link , need to configure frame relay as point to point with subinterfaces
Broadcast – Cisco
thread like a lan (everyhting need to be connected and see each other)
Point to Multipoint , Non Broadcase – Cisco
Same as Point to multipoint – RFC but you have to specify the neighbors
int s0/0
ip ospf network ? # to change the mode
On a Border router :
router ospf 1
area 1 range ? # to summarize routes from different areas to each other
router ospf 1
summary-address ? # to advertise routes on an ASBR router
redistribute rip subnets ?
OSPF LSA Types :
Type 1 : Router LSA (advertise about one network)
Type 2 : Network LSA (DR generated , advertise about all of the routers )
Type 3 : Summary LSA (ABR summary route , summarize routes)
Type 4 : Summary LSA (ASBR Location , ip address of ASBR on the network)
Type 5 : External LSA (ASBR Summary route )
Virtual links :
router ospf 1
area 1 virtual-link router-ID # to specify a tunnel when an area is not directly connected to area 0
Stub and Totally stub :
Stub : Blocks Type 5 LSAs from Everything
router ospf 1
area 2 stub
Totally stub : Block type 3,4,5 LSAa from everything
router ospf 1
area 1 stub no-summary
Not-So-Stubby area : passes external routes through via Type 7 LSA , those will be convertef to type 5 once they reach the backbone ,
used when an external network is connected to stub or totally stub router
router ospf 1
area 2 nssa
redistribute rip
router ospf 1
are 1 nssa no-autosummary
redistribute rip
OSPF Authentication :
int s0/0
ip ospf authentication
ip ospf authentication-key ?
or
int s0/0
ip ospf authenticaion
ip ospf message-digest-key 1 md5 ?
OSPF External route types :
type 1 : incerements metric on any hop with the cost of that hop
type 2 : only one metric value will be sent
Default cost of a hop on OSPF and how to change it :
cost=100/BW
router ospf 1
auto-cost reference-bandwidth ? # must be the same on all routers
router ospf 1
max-lsa ? # to specify a limit for LSA packets , will block if it reaches the limit for 5 mins
router ospf 1
default-information originate always # to advertise router as a default route for all routers
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