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OSPF / PTMP no subnets

Posted: Sun Oct 04, 2020 12:22 am
by Guscht
Hi,

I discovered, that with the OSPF network-type PTMP only /32 adresses are transmitted, which results in a lost connectivity regarding the IP behind the router:

ptmp.jpg

As you can see 10.1.0.1 and 10.2.0.1 are derived from OSPF. Which means, I can not ping a host like 10.1.0.254.

White the all other network-types correct subnets are transmitted:

broadcast.jpg

Here you can see the right subnets 10.1.0.0/24 and 10.2.0.0/24 are derived from OSPF. Connectivity is possbile, and the ping gets a response.

My further findings: Only one IP per Interface is transmitted as /32 with PTMP.
Example: A Bridge with an IP 10.1.0.1/24 and 10.1.0.2/24. Only the 10.1.0.1/24 is transmitted as 10.1.0.1/32.
If I disable that IP, the second 10.1.0.2/24 is transmitted as 10.1.0.2/32. But re-enabling the first IP does not bring back the .1 IP. Per Interface (Bridge/VLAN/etc...) always the first IP that is up "wins" with PTMP.

MT-Wikis says:
Discovery on PTMP Subnets
Point-to-MultiPoint treats the network as a collection of point-to-point links.

When i interpret "point-to-point link" as as /32, then it works correct to this point.

The real problem is that in a common scenario a lot IPs are not bound to an dedicated Interface.

E. g. hosts behind a router which runs DHCP on a Bridge/VLAN/ether Interface are literally unreachable, because the IPs the DHCP hands out are not bound to an dedicated Interface of the router and wont get transmitted as /32 via OSPF/PTMP.

Is this behaviour Mikrotik specific or is this "per design" of the PTMP network-type?
I see no sense behind this logic? All it does, it results in a lost connectivity for all IPs not bound to dedicated Interface.

Regards

Re: OSPF / PTMP no subnets

Posted: Sun Oct 04, 2020 2:45 am
by xvo
MT-Wikis says:
Discovery on PTMP Subnets
Point-to-MultiPoint treats the network as a collection of point-to-point links.
Is this behaviour Mikrotik specific or is this "per design" of the PTMP network-type?
It works exactly as stated in your quote - you end up with bunch of /32 addresses.
For anything "in between" there won't be a route.
Broadcast network type is what you use in scenarios you described (for connected networks).
If there are no other routers there then put the "passive" tick.

Re: OSPF / PTMP no subnets

Posted: Sun Oct 04, 2020 3:20 am
by Guscht
Thank you!

Could you please explain the sense behind this? I see no practical reason to distribute /32 routes.
Each router can reach each router - but the hosts in the networks connected to the router cannot reach other hosts in network connected to other routers?

Id love to understand why (in which situations) Id use PtMP?

Re: OSPF / PTMP no subnets

Posted: Sun Oct 04, 2020 12:43 pm
by xvo
Could you please explain the sense behind this? I see no practical reason to distribute /32 routes.
Each router can reach each router - but the hosts in the networks connected to the router cannot reach other hosts in network connected to other routers?
It is suited fine for point-to-point links (tunnels, etc.) where there is simply nothing else except the other side of the link.
Id love to understand why (in which situations) Id use PtMP?
Same goes for point-to-multipoint - you use it when all the "clients" are other routers.
This way you get proper set of /32 routes for each point-to-point in that point-to-multipoint setup.

Once again, both modes are not suited for the networks where you have devices that don't run OSPF.

Re: OSPF / PTMP no subnets

Posted: Mon Oct 05, 2020 1:30 pm
by mrz
You use PtMP on non broadcast networks where hosts in the same L2 network cannot reach each other directly.

Re: OSPF / PTMP no subnets

Posted: Mon Oct 05, 2020 3:27 pm
by IPANetEngineer
Id love to understand why (in which situations) Id use PtMP?

PtMP was very commonly implemented about 10 to 15 years ago in Frame Relay and ATM networks that were not broadcast capable but could otherwise have more than two hosts on a Layer 2 segment.

It is sometimes used in wireless networks with more than 2 hosts in a subnet that will form an OSPF adjacency.