OSPF Flashcards
Administrative Distance
110
OSPF Process stages
- Down - no hello packet recieved
- INIT - recieved hello packet but it’s router-id was not included in hello packet neighbor field.
- 2Way - We have bi-directional communication. both routers see their router-ids in hello packets
- DR/BDR election will occur.
- In NBMA and broadcast networks, the router will only become FULL with the DR and BDR. it will become DROTHER with the other routers.
- In point2point or point2multipoint networks, the routers will become FULL with other router.
- Exstart - The start of the exchange process. The routers and their DR and BDR establish a master-slave relationship and choose the initial sequence number for adjacency formation.
- Exchange - OSPF routers will exchange database descriptors (DBD) packets. DBD contains link-state advertisement (LSA) Headers only and describe the contents of the entire link-state database.
- Loading
- Full
https: //www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13685-13.html
Tables
Neighbor Table | Database Table | Routing Table
OSPF Packet types
Hello Database Description Link-State Request Link-State Update Link-State Acknowledge
Hello Packets
Every 10 seconds
Dead Timer
Every 40 Seconds
Multicast addresses
- 0.0.5 - DR will send updates to DROTHERS using this multicast address.
- 0.0.6 - DROTHERS will send updates to DR/BDR routers using this address.
Understanding OSPF Packet types in detail
https://sites.google.com/site/amitsciscozone/home/important-tips/ospf/ospf-packet-types
Metric
Open Shortest Path First (OSPF) uses “Cost” as the value of metric and uses a Reference Bandwidth of 100 Mbps for cost calculation.
The formula to calculate the cost is Reference Bandwidth divided by interface bandwidth. For example, in the case of 10 Mbps Ethernet , OSPF Metric Cost value is 100 Mbps / 10 Mbps = 10.
The default Reference Bandwidth of OSPF is 100 Mbps and the default OSPF cost formula doesn’t differentiate between interfaces with bandwidth faster than 100 Mbps. These days, 1 Gbps and 10 Gbps links are also common.
Virtual Links
- Virtual links are used to connect a dis-contiguous area to area 0.
- A logical connection built between routers.
- Virtual links are recommended for backup or temp connections.
configuring Virtual Links
Virtual links command will need to be configured on both ABRs that bound the area that we’re trying to build the link through.
Router(config)#router ospf
Router(config-router)#Area 20 virtual-link 3.3.3.3
Verify Virtual links
sh ip ospf virtual-links
DR / BDR
The designated router (DR) is responsible for updating all other routers about changes in the network. The Backup designated router (BDR) is the back up for the DR and will only assume the position or DR when the DR goes down.
Selecting the DR
1. highest priority [0-255]
2.second highest priority will be the BDR
3.default priority is 1.
4. in case of a tie, the router with the highest router-id will be the DR and the second highest router-id will be the BDR.
5. routers with priority value of 0 will never be DR or BDR
. These routers will be DROther
changing the priority value
router(config)interface x
router(config-if)ip ospf priority [0-255]
OSPF router relationships
DR/BDR —> DROTHER = FULL
DROTHER – > DROTHER = 2WAY
Link State Advertisements Types
LSA Type 1 - Router LSAs
LSA Type 2 - Network LSAs
LSA Type 3 or 4 - Summary LSAs
LSA Type 5 - Autonomous Systems External LSAs
LSA Type 6 - Multicast OSPF LSA
LSA Type 7 - Defined for Not-so-stubby areas
LSA Type 8 - External Attributes LSA for BGP
LSA Type 9,10,11 - Opaque LSAs
LSA Type 1
Router LSA
- generated by the internal router
- “o” routes in routing table
- Floods within it’s router only; does not cross ABR
LSA Type 2
Network LSA
- happens in broadcast networks
- advertised by DR of transit network
- floods within its area only; does not cross ABR
- “o” routes
LSA Type 3
Summary LSA
- used to flood network information to areas outside the originating area (inter area)
- “oIA” routes in routing table.
- Advertised by ABR of originating area.
- regenerated by subsequent ABR to flood throughout the AS
LSA Type 4
LSAs are used to advertise the address of the ASBR to all other areas.
- they are generated by the ABR of the originating router
- -regenerated by subsequent ABR to flood throughout the AS
LSA Type 5
are used to advertise routes from another autonomous system.
they are advertised and owned by the ASBR of the originated routers.
Purpose of Ospf stubs
- Reduce the processing overheads from routers.
2. Reduce the size of the routing table.
What does it mean to configure a stub area?
When an Area is configured as a stub.. it means that the ABR will no longer send LSA Type 5 advertisments to the internal routers but just a default route. External routes are replaced by a default route (o*IA)
E1/E2 routes
E1 - internal cost to ASBR added to external cost
E2 - internal cost. It is same as external cost.
Https://ipwithease.com/ospf-external-e1-and-e2-routes/
Areas which cannot be stub
- Area 0
- Area with ASBR
- Virtual Area (Area that the virtual link passes through)
How to configure stub area
Router ospf 1
Area 10 stub
What does it mean to be a Totally Stubby Area?
When an Area is configured as a totllly stubby area.. it means that the ABR will no longer send LSA Type 5 & Type 3 advertisments to the internal routers but just a default route. External routes and inter-Area routes are replaced by a default route (o*IA)
How to configure Totally Stubby Area
On All Internal Routers:
> router ospf 1
> area 10 stub
On the ABR
> routers ospf 1
>Area 10 stub no summary
NBMA
NBMA is mainly employed on networks that do not have broadcast or multicast capabilities. Frame Relay, Asynchronous Transfer Mode (ATM), home power line networking, and X.25 are some common examples of NBMA network technologies.
