OSPF passive interface - neighbour state, LSAs

Unanswered Question
Sep 25th, 2007
User Badges:

Hi all,


I want to know what happens when we configure an interface as passive ?

for eg. consider below topo :

R3--------R1---------R2 , all are in backbone area


I have configured interface in R1 which is connected R2 as passive.So, what happens then ?

Whether R1 sends hello to R2 ?

where does neighborship stop, stops it in 2-way or init ?

when R1 sends router LSA to R3, whether it also adds the link between R1 and R2 ?


Kindly clear me.


Thanks,

Vijaybabu

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 3 (1 ratings)
Loading.
ankbhasi Tue, 09/25/2007 - 01:23
User Badges:
  • Cisco Employee,

Hi Vijay,


OSPF will not send out Hello packets on a Passive interface, and thus will not establish any adjacencies with other routers on that network, even if they are running OSPF. It will, however, advertise that network's IP address on its other interfaces.


So in your case R1 will not even send out hello packets on its interface connected to R2.


HTH


Ankur


*Pls rate all helpfull post

GillieLucent Tue, 09/25/2007 - 02:28
User Badges:

Hi Ankur,


Thanks for your response.

So, for eg. if the network between R1 and R2 is 10.0.0.0/24,in that case R1 will advertise 10.0.0.0/24 in its router LSA to R3, rt ? Since there is no ospf connection between the link, so traffic can't be send through that link then whats the use of advertising that network ?


Thanks,

Vijaybabu


Richard Burts Tue, 09/25/2007 - 02:34
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Vijaybabu


You seem to think that having an interface as passive for OSPF would prevent forwarding traffic over that link. This is not the case. The subnet for the link from R1 to R2 is certainly in R1's routing table, it would be advertised to R3. And if R3 sent something to R1 whose destination was in the subnet of R1-R2 (10.0.0.0/24) then R1 certainly would forward the packet out the interface.


The concept of passive is only about OSPF processing and has no impact on forwarding traffic through that interface.


[edit] note that the passive interface would prevent R3 from learning any routes from R2 (other than the interface that connects R2 to R1).


HTH


Rick

GillieLucent Tue, 09/25/2007 - 03:32
User Badges:

Hi Rick,


I mentioned about traffic whose destination is beyond R2.

So, what is the difference between configuring a interface as passive and not including that interface in OSPF ? whats the use of adding that interface in OSPF and configuring it as passive ?


Thanks,

Vijaybabu

ankbhasi Tue, 09/25/2007 - 03:47
User Badges:
  • Cisco Employee,

Hi Vijay,


If you do not want some interface to form an adjancy but you want that interface subnet to be advertised in rest of the network you need passive interface.


Lets take your example r1 and r2 are connected to each other via switch and there are many hosts on that subnet if that subnet is not advertised to R3 and beyond how will they reach that subnet.


HTH


Ankur

Richard Burts Tue, 09/25/2007 - 03:48
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Vijaybabu


The difference is that if you do not include the interface in OSPF then that subnet does not get advertised - and no OSPF traffic is sent. If you put the interface into OSPF and make it passive then the subnet does get advertised and no OSPF traffic is sent.


There are circumstances where you want a subnet to be advertised but you may not want to run the protocol on that interface. One example would be a router with a FastEthernet interface for a subnet where there are just user PCs. You want that subnet to be advertised so the PCs are reachable but you may not want to process OSPF on that interface. What would happen if some user ran OSPF on their PC? Do you want them to be able to inject routes into your routing table?


Another example might be a router at the edge of your network. It may have an interface to devices that are not part of your network (perhaps your EBGP neighbor). You want the devices on that subnet to be reachable but you do not want them to be able to form OSPF neighbor relationship and inject routes into your routing table. Passive interface is the solution for this.


HTH


Rick

GillieLucent Tue, 09/25/2007 - 09:15
User Badges:

Hi Ankur/Rick,


Thanks for great explanation.

Rick your explanation clearly pictures the real network.


I was not able to rate your posts, I don't know why ?

Anyway thanks for your explanation.


Thanks,

VIjaybabu


Richard Burts Tue, 09/25/2007 - 10:20
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

VIjaybabu


I am glad that our explanations were helpful. I am not sure why you were not able to rate. Thanks for trying.


HTH


Rick

Actions

This Discussion