topic Re: three ospf routers on same segment two sessions needed in Routing and SD-WAN

three ospf routers on same segment two sessions needed

axeleratorcisco — Mon, 25 Mar 2019 22:35:25 GMT

i have r1, r2 and r3 on the same ethernet segment

r1 needs a ospf session to r2, and r3

r2 and r3 don't need to communicate via ospf, this needs to be excluded

what is the best way of implementing this?

Re: three ospf routers on same segment two sessions needed

Peter Paluch — Wed, 25 Apr 2012 09:19:12 GMT

Hello,

I would suggest using the point-to-multipoint nonbroadcast network type in OSPF configuration. This way, you would be able to define the pair of routers to communicate with each other manually, without having the routers discover themselves automatically and building up the adjacencies.

For example:

R1:

interface FastEthernet0/0

ip ospf network point-to-multipoint non-broadcast

ip address 10.0.0.1 255.255.255.0

router ospf 1

network 10.0.0.0 0.0.0.255 area 0

neighbor 10.0.0.2

neighbor 10.0.0.3

R2:

interface FastEthernet0/0

ip ospf network point-to-multipoint non-broadcast

ip address 10.0.0.2 255.255.255.0

router ospf 1

network 10.0.0.0 0.0.0.255 area 0

neighbor 10.0.0.1

R3:

interface FastEthernet0/0

ip ospf network point-to-multipoint non-broadcast

ip address 10.0.0.3 255.255.255.255.0

router ospf 1

network 10.0.0.0 0.0.0.255

neighbor 10.0.0.1

Do you believe this would be applicable for you?

Best regards,

Peter

three ospf routers on same segment two sessions needed

axeleratorcisco — Wed, 25 Apr 2012 09:38:57 GMT

thanks

this looks nice

r1 needs to set up a vpn connection to another router behind r2, and r3

so i want r2 and r3 to advertise that peer ip address to r1, so r1 knows how to reach it via either r2 or r3.. therefore r2 and r3 need no connectivity between them

are the neighbor statements necessary in this configuration?

also can i use a 0.0.0.0 wildcard on the network statement to make it more specific?

would the ipsec configuration on the outside interface of r1 interfere somehow with this config?

thanks again

Re: three ospf routers on same segment two sessions needed

axeleratorcisco — Wed, 25 Apr 2012 18:58:07 GMT

does it matter if i set the type to non broadcast even though it's ethernet thus broadcast?

am i simply disabling unicast messaging within ospf if i configure it manually like this?

could i also create two point to point links from r1 instead of the point to multipoint so that r1 knows everything, or this a stupid thought?

Re: three ospf routers on same segment two sessions needed

axeleratorcisco — Wed, 25 Apr 2012 19:07:29 GMT

can i configure the interface on r1 as point to multipoint

and r2 and r3 as point to point, since they don't need to talk to each other?

or does it have to match on all routers, i.e. point to multipoint?

Re: three ospf routers on same segment two sessions needed

Peter Paluch — Thu, 26 Apr 2012 00:41:53 GMT

Hello,

I will try to answer all your questions in this post.

so i want r2 and r3 to advertise that peer ip address to r1, so r1 knows  how to reach it via either r2 or r3.. therefore r2 and r3 need no  connectivity between them

I see... Hmmm... Well, the configuration I have suggested will not prevent r2 and r3 from mutually learning about their networks, because r2 will tell r1 and r1 in turn will tell r3 - and vice versa. Are you OK with this?

are the neighbor statements necessary in this configuration?

Yes, they are. With non-broadcast network type, OSPF does not sent multicast Hello packets to dynamically discover neighbors. The only way for a router to know who its neighbors are on a non-broadcast network type is to define them using the neighbor statement. Without it, the adjacencies would not go up at all.

also can i use a 0.0.0.0 wildcard on the network statement to make it more specific?

Yes, of course.

would the ipsec configuration on the outside interface of r1 interfere somehow with this config?

I do not think so - but to be completely sure, I would need to see the configuration.

does it matter if i set the type to non broadcast even though it's ethernet thus broadcast?

On any real network type, you can force the OSPF to treat it as a different network type. Doing that allows you to achieve a non-standard behavior of OSPF to attain specific goals, like yours in this case. Of course, you cannot just redefine a network type arbitrarily without paying attention how the underlying network infrastructure works - the OSPF may need to be specifically tweaked to work well. For example, the NBMA network type is not really suitable for Frame Relay hub-and-spoke scenarios although it is the default network type. You often have to modify the priorities to prevent spoke routers from becoming DR/BDR, manually take care about the reachability of the spokes to each other, etc.

am i simply disabling unicast messaging within ospf if i configure it manually like this?

There is more behind it than just disabling multicast OSPF messaging. However, many things that change with the PtMP-NB network type are relevant to the internal OSPF processing and database contents, and you do not have to worry about that. What you should take into consideration is that the OSPF communication will be fully in your hands by virtue of the neighbor statements, and that routers that are not manually peered together will not be able to route packets via each other directly. This means that even if r1, r2 and r3 are on the common segment, with the configuration as I suggested, packets from r3 destined to r2 will be sent via r1.

could i also create two point to point links from r1 instead of the  point to multipoint so that r1 knows everything, or this a stupid  thought?

That's not a bad idea at all - but what it does is merely doing on physical level what we are able to do on logical OSPF level.

can i configure the interface on r1 as point to multipoint

and r2 and r3 as point to point, since they don't need to talk to each other?

or does it have to match on all routers, i.e. point to multipoint?

Man, this is like CCIE R&S Lab Exam assignment from OSPF You would at least need to tweak the timers because PtP network OSPF timers are 10/40 (Hello/Dead) while PtMP uses 30/120. After that, the combination of PtP and PtMP should work. However, this can be considered an excessive optimization with no significant advantages so I would personally discourage doing it.

Best regards,

Peter

Re: three ospf routers on same segment two sessions needed

axeleratorcisco — Thu, 26 Apr 2012 06:31:35 GMT

thanks for your lengthy reply!

well i got the issue you described above how to solve it?

r1 now has in it's routing table:

a directly connected route to the shared vlan of r1,r2,r3

an ospf route to r2

an ospf route to r3

on r2 and r3 i am redistributing bgp to this ospf instance

on r1 i get the route to the ipsec peer

but, when i do a traceroute, i get this:

r1 goes to r2

r2 goes back to r3

r3 goes back to r2

then * * * into the cloud of the provider who i am running bgp with

second traceroute from r1

r1 goes to r3

r3 goes to r2

r2 goes to r3

then * * * into the cloud

how do i prevent this extra hop?

Re: three ospf routers on same segment two sessions needed

axeleratorcisco — Thu, 26 Apr 2012 06:39:58 GMT

can i make the route via r2 more attractive, so it doesn't hop over to r3 first?

what about a dual static route configuration with the route via r2 being more attractive?

Re: three ospf routers on same segment two sessions needed

paolo bevilacqua — Thu, 26 Apr 2012 10:06:43 GMT

That's not the way OSPF works.

Router on same LAN, they all will 'communicate'.

Otherwise, do not use OSPF.

Re: three ospf routers on same segment two sessions needed

axeleratorcisco — Thu, 26 Apr 2012 14:13:02 GMT

i have not been able to get it to work

i now have a dual static route with ip sla tracking, tested this and works fine

guys, thanks alot for your input, this was quite insane!