Facts to remember about OSPF - Part4

OSPF NSSA ABR External Prefix Filtering
We want the ABR stop translating a type 7 to a type 5 from an nssa area 3 and advertise it into area 0. This can be done with the command summary and the argument not-advertise, The main difference with other filter techniques like "distribute-list" and "administrative distance" is that the prefix is eliminated from the ospf database (show ip ospf database external) so it can't be translated into type 5 and distributed into area 0 but remains in the nssa area 3

The summary must have the same mask that the prefix we are filtering.

This must be applied in the ABR NSSA not in the ASBR prefix originator

I didn't see a summary address with null 0 created in the routing table and the "show ip route" in each router inside NSSA showed the prefix as "O N2" (cause it was redistributed for another router in the nssa area (ASBR))

router ospf 1
 summary-address 160.1.10.10 255.255.255.255 not-advertise




OSPF database filtering
This is similar in operation to the passive-interface command in RIPv2. This feature allows the formation of OSPF neighbors, because hello packets are not filtered out, but it stops the advertisements of all LSAs out the interface or to the neighbor in question. This type of filtering breaks the OSPF protocol cause we will have routers inside the area with different OSPF databases

Configure R5 so that R2 cannot learn from R5, but R5 can still learn OSPF routes from R2. "database-filter" command is allowed with neighbor command  only on NBMA and point-to-multipoint networks

router ospf 1
 neighbor 155.1.0.2 database-filter all out

Configure R7 so that R9 cannot learn any OSPF routes from R7, but R7 can still learn OSPF routes from R9

R7:
interface GigabitEthernet1.79
 ip ospf database-filter all out

OSPF Stub Router Advertisement
Don't confuse with OSPF stub areas. Essentially, this feature causes the router to advertise a maximum metric for non-stub destinations, making it the worst cost path to all destinations. The result is that upon initializing the OSPF process, transit traffic will not flow through the stub router unless it is the only possible path.
Configure R4 to advertise the maximum metric value to all neighbors inside its Type-1 Router LSA.( Maximum metric in self-originated router-LSAs)

router ospf 1
 max-metric router-lsa

Example:  A_B and B_C links have been set to LSInfinity (65535) by the ‘max-metric; command on router B. So the only way to Router A will reach 3.3.3.3 will be through Link A_C

OSPF interface timers
Configure R5 and R8 to send OSPF hello packets every five seconds on VLAN 58, and wait for seven seconds before declaring a neighbor down

interface GigabitEthernet1.58
 ip ospf hello-interval 5
 ip ospf dead-interval 7

Configure R4 and R5 to send OSPF hello packets every 250ms on VLAN 45. The argument minimal set the dead interval to 1 sec, hello-multiplier states that dead interval is 4 times hello. Then hello interval will be 0.25 secs or 250ms

interface GigabitEthernet1.45

 ip ospf dead-interval minimal hello-multiplier 4

OSPF Resource Limiting

• No more than 5000 LSAs can exist in the database. (Non self-generated LSA)
• No more than 500 of these routes should be originated through redistribution.

router ospf 1
  max-lsa 5000
 redistribute maximum-prefix 500

Miscellaneous OSPF Features

• Configure R10 so that it does not generate a log message upon receipt of a Type-6 LSA advertisement.(OSPFv2 doesn't support Type 6 LSA so a error status in the log will be generated so we must stop receiving Type 6 LSA)
• Configure R10 so that it does not account for the MTU value when establishing adjacencies on its GigabitEthernet1.108 interface.(MTU mismatches are the primary reason an OSPF adjacency becomes stuck in the EXSTART state.)
• Configure R10 so that the command "show ip ospf neighbor" shows a host name and not an ip address under the column Neighbor ID

ip host R8 150.1.8.8
ip ospf name-lookup
!
interface GigabitEthernet1.108
 ip ospf mtu-ignore
!
router ospf 1
 ignore lsa mospf