Inject a connected LAN Network in OSPF (Redistribute connected or Passive Interface in OSPF) ?

[Chaker BCHIR]

Hello, I have this Part of Design :

2 Distribution Switches in a DC LAN (we can put 2 N7K for example).
We have an OSPF implemented with Routers and we have many Layer 2 switches on the LAN (N5Ks with VPCs).

So, let's take an SVI : Vlan 100 for example (it is one of the Vlans on our DC LAN). To announce this network in OSPF we have 2 methods :

1- Do not include in OSPF process and redistribute it into OSPF (Redistribute connected subnets ...).
2- Include the SVI in the OSPF process (ip ospf x area y) then put it in a passive state.

What is the Best Solution ? (Cisco Best Practices Design) and why ?

Thanks in advance.

[Deepak Kumar · Accepted solution]

Hi,

Q1- Do not include in OSPF process and redistribute it into OSPF (Redistribute connected subnets ...).

Ans: This is the first way of advertising routes into OSPF. When done in this way, the routes would appear external in the OSPF domain. The interfaces that would be advertise can be controlled with route-maps.

Q2- Include the SVI in the OSPF process (ip ospf x area y) then put it in a passive state.

Ans: This is the basic way of advertising routes. This command will enable the routing protocol on that interface (sending hellos, etc if no passive-interface) and advertising to other peers of the interfaces that match the network command.

If your question is which is best and why then my advice to read this article.

http://blog.ipspace.net/2011/05/ospf-and-connected-networks-to.html

Regards,

Deepak Kumar

[paul driver · Accepted solution]

Hello

I would advertised the network via the network command or from the svi interface itself, as if you redistribute then that subnet will be not only advertised as an external route but the originating rtr performing that redistribution will be also become an asbr for the area.

res

Paul

[Chaker BCHIR]

Thank you Paul and Kumar for your responses.

So, I understand that the best way is to inject the LAN Networks into OSPF (with passive interface command) in order to avoid Type 5 LSAs, isn't it ?

If this correct, could you explian please what is the difference between obtaining 10 Internal Routes (LSA 1 or 3) or 10 external routes (LSA 5) ? I see that this does not change anything (only the type of LAN networks in our routing tables) .. No ?