Hello there!
I can not leave the OSPF subject because it is too beautiful to abandon it. In this post I try to demonstrate how the LSAs work in different OSPF scenarios. In the last post I wrote what is the LSAs, which network type is available, and which router types are exist in an OSPF network. But I do not described these things. On this post I try to define these topics better.
To understand that LSAs how can be flooded on different areas you need to understand the area types. As I mentioned before, there is 5 type of area exist of OSPF.
Basically in a backbone/regular area the following LSA types will be flooded:
Type 1 Router LSA
Type 2 Network LSA
Type 3 ABR Summary LSA - if ABR exist in the area.
Type 4 ASBR Network LSA - if ASBR exist in the area.
Type 5 ASBR Summary LSA - if ASBR exist in the area.
Backbone area is special, because it is responsible for the other area can be connected each other.
And what about stub area?
Stub types of areas help us to reduce the LSDB as it possible. There is 4 type of stub area is exist:
1. Stub
In stub area the ASBR is not allowed, the following LSAs can be flooded:
Type 1 , Type 2, Type3
2. Totally stub area (TSA)
In TSA Type3 is not allowed
3. Not so stubby (NSSA)
In NSSA area there is an ASBR which is redistribute external prefix to the OSPF domain, because the Type 5 LSA is abandoned the Type 1, Type 2, Type 3 and Type 7 LSA is flooded within the area.
4. Totally NSSA
In this case the Type 3 is also filtered.
In which scenario will all LSA is going to be flooded without LSA2?
When there is no DR in the network. There is two network where the DR / BDR election is not needed:
point-to-point
point-to-multipoint
In these case routers which belong to the same OSPF area make a FULL adjacency and flood all the generated LSA-s.
 |
| Example of p2p tpology |
 |
| Example of p2m topology |
In these topologies the OSPF automatically set up its own as broadcast network, because of Ethernet protocol so we need to reconfigure the OSFP network type.
OSPF network type can configurable in IOS-XE device on globally or under the interface statement locally. In a case IOS-XR it can be configurable under the router ospf statement.
OSPF is use multicast address to send OSPF hellos to request an adjacency in a network (224.0.0.5). In which scenario will the OSPF router use uni-cast address?
The answer is the non-broadcast mode.
Why the non-broadcast mode of ospf is useful? When the routers not connected to same broadcast segment or the transport / underlay network is not support the multicast transit. (In some IPSec cases).
In this scenario, you need to configure the neighbor statement under the ospf configuration section. In this case the ospf process is need to be run on the neighbor interfaces.
As I mentioned in may last post in a case of broadcast case the DR / BDR election is mandatory step for the adjacency. What about the non-broadcast setup? It is also need to DR / BDR election process, and yes p2m non-broadcast setup the DR / BDR election is needed.
Lets look some ospf configuration template for the scenarios:
I. Make Type 1 LSA only network! What should be the configuration?
The network type between the ospf neighbor should be point-to-point. The OSPF domain should contain intra-area routes.
IOS-XE configuration template with a point-to-point topology solution
configure terminal
!
interface Lo0
ip address $ip_address $net_mask
exit
!
interface Gi$x
ip address $ip_address $net_mask
ip ospf network point-to-point
no shut
exit
!
router ospf $process_id
router_id $Lo0_ip_address #not mandatory but suggested to be used
passive-interface default
network $Lo0_ip_address $wildcard_mask area $area_id
network $Gi$x_ip_address $wildcard_mask area $area_id
no passive-interface Gi$x
!
exit
In case of IOS-XR the configuration should be:
configure terminal
!
interface Lo0
ipv4 address $ip_address/$vlsm
exit
!
interface Gi0/0/0/$x
ipv4 address $ip_address/$vlsm
no shut
!
router ospf $process_id
router-id $Lo0_ip_address #not mandatory but suggested to be used
!
area $area_id
!
interface Lo0
passive
!
interface Gi0/0/0/$x
network point-to-point
!
commit
You can verify the configuration with the following commands:
IOS-XE
show ip ospf neighbor
show ip ospf database
show ip ospf database self-originate
IOS-XR
show ospf neighbor
show ospf database
show ospf database self-originate
The last interesting fact about the ospf database that you can verify LSU with checksum... each router when generate LSA create a checksum which is should be the same for that LSA in every router.
It should be enough for today... so we can see each other on the next post!