Jan 18, 2012
A key thing to understand with OSPF are the different Link State Advertisements (LSA’s) and what they mean to you network configuration and design. The following table tries to summarise the important details.
[table id=17 /]
LSA types 1 and 2 are found in all areas, and are never flooded outside of an area. Whether the other types of LSAs are advertised within an area depends on the area type
If a tie between OSPF routes, then O > O*IA > E1 > E2.
OSPF : Can’t establish neighbors on secondary IP’s, secondary networks are seen as stub-networks.
– OSPF External Type1 (E1) routes
  • Include the external cost as well as the internal cost to the ASBR.
  • Used to exit the AS as close as possible to the destination.
  • Mostly used if multiple exit points out an AS exist.

– OSPF External Type2 (E2) routes:

  • Include only the external cost of the route.
  • Used to exit the AS via closest ASBR.
  • Often used with only one OSPF exit point.


Cisco routers do not support MOSPF (LSA type 6) however each time one is received a syslog message is generated. To disable you can use the following;

Router(config-router)#ignore lsa mospf

Fast Detection

To enable fast detection with sub-second hello intervals you can use the following.
What it does is sets the hold down to 1 second and then the ‘hello-multiplier’ value dictates the number of hellos to generate in the 1 second (in this example every 250ms).

Router(config-if)#ip ospf dead-interval minimal hello-multiplier 4


To get the routers to lookup DNS names for the ‘sh ospf’ commands, if no DNS available make sure router has local name resolution with ‘ip host ..’

R2(config)#ip ospf name-lookup


The equation used by OSPF is;
Reference/Bandwidth = Cost
Therefore if you want to set the cost of all FastEthernet interfaces to 70 and the others calculated proportionally rearrange the formula as;

reference = cost * bandwidth ………………………… 70 * 100,000,000 = 7,000,000,000 therefore

Router(config-router)#auto-cost reference-bandwidth 7000


When redistributing routes into OSPF the ‘subnets’ keyword will redistribute all the subneted networks, if ommited then only the classful networks will be redistributed;

Router(config-router)#redistribute connected subnets route-map ROB

Cannot summerise LSA type 3.

External routes can be summeriosed on the router that originated them.

Internal routes can be sumerised on an ABR

If the external routes are LSA 7 they MUST be summarised on ASBR

If external routes are LSA 5 they can be summarised on the router that originates then (can be ABR in NSSA or an ASBR in a normal area)

When summarising internal routes on ABR –

area xx range <ip/mask>

Summarising external routes –


When summarisation is performed on a given router IOS will auto-inject a summary route to Null0 to avoid forwarding loops

To remove the Null0 route use;

Router(config-router)no discard-route internal|external

Use the following commands on both the routers required where the area xx is the traversing area id;

Router(config-router)#area xx virtual-link

E1 vs. E2 External Routes

External routes fall under two categories, external type 1 and external type 2. The difference between the two is in the way the cost (metric) of the route is being calculated. The cost of a type 2 route is always the external cost, irrespective of the interior cost to reach that route. A type 1 cost is the addition of the external cost and the internal cost used to reach that route. A type 1 route is always preferred over a type 2 route for the same destination.

Rob Edwards

Northern (UK) chap focusing on platforms, automation, cloud and cloud native applications. Recovering network engineer, although it turns out networking is as important, if not more, now than before!