New WAN Solution, considerations for Multi-Area OSPF

[arisgiannakopoulos]

Hello,

We have 3 remote locations connected to our HQ via 2 Point-to-point circuits from the same Carrier/Provider for each location. From an OSPF perspective, each site is a different area with the 2 HQ WAN Routers playing the role of ABR. At the HQ, we use 2x3850s for WAN routers (not stacked - they are physically located in different Data-Centers). For the remote locations, we use 2x3850 stacked (circuits are connected to different switches) doing both WAN routing and local Inter-VLAN routing.

We don't have more than 1000 prefixes on the network and our network is fairly stable (failures that would affect routing and would require routing re-convergence do not happen often).

The plan is to phase out one point-to-point circuit per location and replace it with a circuit to a VPLS cloud from another provider.

So the question that arises here is, how should I configure OSPF? The VPLS cloud obviously is one big broadcast domain (one subnet) where all the Routers (1 HQ and 3 for the remote locations) are connected so I am not sure if multi-area OSPF is an option.

1. would you get rid of the other areas and migrate to a single area design?

2. if the carrier supports VLAN tagging, would you use that to configure subinterfaces, make the VPLS cloud a hub-and-spoke topology and keep the multi-area design the same?

I have attached a 2 pictures depicting our current and future state topology.

thank you

[Richard Burts · Accepted solution]

Your current network design with a separate area for each remote site is a good design. Replacing one point to point with VPLS will require some changes. Certainly changing the design to a single area is an option. For a fairly small network like yours (3 remote sites to 1 HQ with only 5 routers/switches participating in OSPF) having only a single area would not be problematic.

There is a fairly simple alternative that would keep the multi area OSPF design. Currently the HQ is the ABR with area 0 backbone at HQ and the links to the remote sites belonging to the non backbone areas. You could change the design so that the remote site router/switch is the ABR with the links connecting to HQ as part of the backbone area 0.

If the provider does support vlan tagging over the VPLS it might be the easiest solution for keeping the multi area design. With separate vlans connecting to each site I would think it would be easy to keep your current design and have each of the vlans belong to the remote site area.

HTH

Rick

[arisgiannakopoulos]

Hello Richard

thank you for your reply. I was actually considering converting the Remote Location WAN router/switch to be the ABR, my only concern about that would be that, statistically, the point where a failure could occur is the WAN circuit and I would prefer that to be within that area. I understand that if i was to choose the option to go for a single-area design, that would require a recalculation anyway.

Yesterday I was able to verify that our Carrier supports VLAN tagging so I used them to configure point to point links between all locations. That would be a quick and easy win for the first phase of our project (migrate from point-to-point WAN links to VPLS) that also gives us a very quick roll-back if there is a need.

The second phase would be to migrate the remaining point-to-point circuits to a second VPLS cloud (from a different provider). In that case I guess i could do the same (VLAN tagging - if supported from the other provider), or convert the whole topology to a single-area design.

[Richard Burts]

You are welcome. Thank you for the update. Yes I agree that if the provider does support vlan tagging that this would be the easy solution for you to implement and would require the fewest changes to your design.

HTH

Rick

[Joseph W. Doherty]

For such a small topology, I would recommend you move to a single area, either for all your 3 remotes sites retaining the HQ WAN routers as ABRs or move all your routers into the same area.

A concern I have making your remote routers ABRs, if the area zero link fails on a VPLS link, you partition area zero (I would need to think some more on it, but I think you network would be still functional, even if so, it's something to avoid).

I suspect the code on your devices is new enough it supports ISPF. Again on such a small topology, it probably doesn't matter much, but if could speed up some re-convergence calculations, i.e. somewhat further mitigating the need for using different areas.

Using VPLS VLANs to maintain you existing logical multi-area OSPF topology, would, I agree, be the easiest to convert to, but again, for such a small topology, and if you use ISPF (which should handle any edge networks changes too), I don't think you really need as many OSPF areas.

[Richard Burts]

Perhaps @Joseph W. Doherty understands something a bit differently than I do. But I do not see how a failure of the VPLS link would partition area 0. The drawing is pretty clear that each remote site has a single router with 2 links to providers. Assuming that both links are configured to be in area 0 if the VPLS link were to fail the remote site would still have an active link to area 0. Would it not? If there is something here I am not understanding I hope that Joseph will clarify.

His suggestion about ISPF is interesting and probably worth looking into. And I agree with him that for a network with 5 routers and 4 sites that using single area OSPF would not be problematic. 

HTH

Rick

[Joseph W. Doherty]

Sure Rick, I was assuming the existing p2p links would remain in the existing area. However, with your question, and your noting both WAN links would also be in area zero, your 100% correct, the remote site ABR wouldn't be partitioned (at least without losing both WAN links).

Now that we're on the same page, that design would be fine too. It offers the advantage you might, if desired, could place the remote edge networks into a "special" OSPF area or possibly summarize their addresses as provided to area zero.

[Richard Burts]

Yes much depends on assumptions that we make about what the environment will be. With the implementation of vanilla VPLS the HQ router would have a single interface with a single IP address. So it can participate in only a single OSPF area, which logically would be area 0. If the desire is to have multiple areas then it means that the remote router must be ABR for the VPLS connection. If the remote router is ABR for the VPLS connection then I believe that it is smart for the remote router to be ABR for both connections.

I agree with your further point that depending on the IP addressing plan that it might be possible to summarize routes from the remote to area 0. And if we want to think about the possibility of implementing special OSPF areas I believe that the remote areas would be excellent candidates for totally stubby areas.

HTH

Rick

[arisgiannakopoulos]

Hello!

thumbs up for mentioning that ISPF functionality of OSPF which I was not aware it existes.

Truth is that for now, I would go with the solution I already mentioned (use VLAN tags to create a Hub and spoke topology within the VPLS cloud).

However, as phase 2 of our project is to get rid of the remaining point to point links and implement a second VPLS cloud from an alternative provider, i guess migrating to a single-area design with ISPF enabled is back on the table.

thanks again for the very valuable information!

[MadonaWinslet55793]

OSPF is multiple area networks and the single area network. Cisco recommends this solution because it keps the topoloical databases . In any network design, someone must consider the WAN topology. It the new area cannot connect directly to the backbone area, two ABRs are set up to the mark. To get further details you can visit my page which is Page