OSPF and Area 0

[JKwan]

I work for a small business with only two locations, main and branch.  We are in the midst of setting up MPLS connection to link them together.  As requested by the carrier, we provided two pairs of /30 addresses to be used on the CE and PE equipment in each location.  Also, they recommended OSPF as the routing protocol so we had to determine the area numbers to be used. 

A network drawing is attached and here is the summary of the network topology we came up with:

Site 1 (Main)

Local LAN --- (Area 1) --- CPE Device --- (Area 101) --- CE --- PE --- MPLS Cloud

• Area 1:  Local LAN & LAN interface of the CPE device
• Area 101: Segment between CPE and CE, using a pair of /30 addresses

Site 2 (Branch)

Local LAN --- (Area 2) --- CPE Device --- (Area 102) --- CE --- PE --- MPLS Cloud

• Area 2:  Local LAN & LAN interface of the CPE device
• Area 102: Segment between CPE and CE, using a pair of /30 addresses

Each location has its own Internet connection from a different provider.  At first, I thought the carrier will have Area 0 set up on their backbone but just learnt that that was not the case.  Therefore, I will have to change the area numbers in order for OSPF to work properly.

Should I change Area 1 of Site 1 to Area 0?  Or  should I suggest to the carrier to change both Area 101 & 102 to ZERO?

What other options do I have?  Which one will best suit my purpose?  I am not familiar with OSPF.  Your suggestion and advice will be very much appreciated.

[Georg Pauwen]

Hello,

first of all, I wonder if OSPF is really the best solution here. How many routers do you have in each LAN ?

Either way, if you have to use OSPF, and you want multiple areas, you need area 0. The best place for area 0 is probably the main site, and if you make the branch site area 1, you keep the design as simple as possible.

That said, what control do you have over the CPE device, is that provider-managed, or can you yourself configure OSPF on these devices ?

[JKwan]

I agree that OSPF might not be the best choice as these two sites just have only 1 router each for connecting to its own ISP for Internet access.  The main site, however, has a DMZ running web servers but this is also handled by the Watchguard firewall/router.  The WAN carrier recommended OSPF based on simplicity (no need to maintain static routes) and potential for connecting more branches.

CE is a Cisco 4000 series router provided by the carrier.  We have no access to it.  CPE is our own equipment, which is a layer 3 switch with one of the gigabit ports set up as a routed interface for connecting to CE.  We have full control of this Cisco 3750 switch.

>>The best place for area 0 is probably the main site, and if you make the branch site area 1, you keep the design as simple as possible.

I will probably change the Local LAN in the main site to Area 0 and make the branch local LAN area 1 as suggested.

Site 1 (Main)

Local LAN --- (Area 0) --- CPE Device --- (Area 101) --- CE --- PE --- MPLS Cloud

• Area 0:  Local LAN & LAN interface of the CPE device
• Area 101: Segment between CPE and CE, using a pair of /30 addresses

Site 2 (Branch)

Local LAN --- (Area 1) --- CPE Device --- (Area 102) --- CE --- PE --- MPLS Cloud

• Area 1:  Local LAN & LAN interface of the CPE device
• Area 102: Segment between CPE and CE, using a pair of /30 addresses

[Georg Pauwen]

Hello,

that won't work. Each area needs to be connected to the backbone area (0). That is why I proposed only two areas, area 0 at the main site, and area 1 at the branch.

[JKwan]

Given that both interfaces (LAN and routed) of the CPE will be assigned to area 0, I will have to get the carrier to change the connecting interface to the same area.

[paul driver]

Hello

Just have the one ospf area then no need for backbone area 0 or anything else like changing the cpe area?

[Meheretab Mengistu]

Hi,

If you have MPLS cloud and running OSPF for CE-PE, the MPLS cloud is treated as a superbackbone area. You don’t have to worry about it.

HTH, 

Meheretab 

[Georg Pauwen · Accepted solution]

According to the original post:

--> At first, I thought the carrier will have Area 0 set up on their backbone but just learnt that that was not the case.

I guess it needs to be checked again with the ISP if the backbone is or is not area 0...

[JKwan]

When I checked with the provider yesterday, I was told that they did not have an area 0 set up.  They use BGP on the MPLS cloud.  I am not sure if he understood my question correctly.

With the initial design, the laptop was not able to reach the CE of the branch.  Afterward, I changed area 1 of the main site to 0 as below, that seemed to work.  

Site 1 (Main)

  Local LAN --- (Area 0) --- CPE Device --- (Area 101) --- CE --- PE --- MPLS Cloud

  Area 0: Local LAN to internal interface of the CPE

  Area 101 Network segment between CPE device and CE router
Site 2 (Branch)

  Local LAN --- (Area 1) --- CPE Device --- (Area 102) --- CE --- PE --- MPLS Cloud  

  Area 1: Local LAN to internal interface of the CPE
  Area 102: Network segment between CPE device and CE router

Still I am skeptical if the above design will work problem free. I am trying to reach another tech to get clarification on the area numbers used in the ISP backbone.  If there is no area 0 in the backbone, I might go with George's suggestion of using just 2 area numbers, 0 & 1.  However, I am not sure if I understand it correctly.  Please review the design below and comment:

Site 1 (Main)

  Local LAN --- (Area 0) --- CPE Device --- (Area 0) --- CE --- PE --- MPLS Cloud

  Area 0: Local LAN to internal interface of the CPE

  Area 0: Network segment between CPE device and CE router
Site 2 (Branch)

  Local LAN --- (Area 1) --- CPE Device --- (Area 1) --- CE --- PE --- MPLS Cloud  

  Area 1: Local LAN to internal interface of the CPE
  Area 1: Network segment between CPE device and CE router

Would the above design work if IPS backbone does not have an area 0?

[Georg Pauwen]

Hello,

the lastest design will work. Just make sure (obviously) that one of the CPE routers is the ABR, that is, that it has an interface in area 0 and area 1.

[Meheretab Mengistu]

Since there is MPLS L3VPN, it will be treated as Superbackbone (Area 0). One of the solutions is to implement it as follows:

Site 1 (Main)

  Local LAN --- (Area 101) --- CPE Device --- (Area 101) --- CE --- PE --- MPLS Cloud

 
Site 2 (Branch)

  Local LAN --- (Area 102) --- CPE Device --- (Area 102) --- CE --- PE --- MPLS Cloud  

Please read to RFC 4577 when you get a chance. 

HTH,

Meheretab

[JKwan]

Here is the reply from the ISP:

    >>There is OSPF between CPE and CE and we are redistributing using BGP in cloud. We do not have OSPF in our network.

So the answer is negative to the question of having area 0 in the ISP backbone.

In that case, we have to think about putting in an area 0 somewhere or making the two sites one single OSPF area (if that is feasible). Suggestions?

[paul driver · Accepted solution]

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@JKwan wrote:

In that case, we have to think about putting in an area 0 somewhere or making the two sites one single OSPF area (if that is feasible).,<---- go for the latter?

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[paul driver]

Hello

Why re-invent the wheel , just have the one area in each site?

[JKwan]

Paul,

Do you mean something like using Area 0 in Site 1 and Area 1 in Site 2 such as below?

Site 1 (Main)

  Local LAN --- (Area 0) --- CPE Device --- (Area 0) --- CE --- PE --- MPLS Cloud

 
Site 2 (Branch)

  Local LAN --- (Area 1) --- CPE Device --- (Area 1) --- CE --- PE --- MPLS Cloud