ospf design for core to dist

Unanswered Question
Sep 10th, 2008

Hi all, Im in process of puttinga distribution layer into my network, whicj is routed to the core. My question is, should I put it all in the same area? or would I have different areas? what is the benefit of having these different areas?

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (1 ratings)
Giuseppe Larosa Wed, 09/10/2008 - 01:29

Hello Carl,

using multiple areas provides the following main benefits:

- allows for reduction of SPF recalculations if a topology changes happens in area 5 only routers that have at least one interface in area 5 will need to run SPF again

- the area border routers connecting area 0 and area 5 have the chance to perform area summarization with the area range command so reducing the number of prefixes needed in the OSPF domain and helping stability: this can be combined with first point: if a subnet member of an aggregate fails this info does not travel out area 5 and routers out area 5 will not perform any recalculation.

- allows for control of what routing info to send into or out an area: using area based filters or playing with the area type like using stub or no so stubby area you can accomodate cases where a prefix is needed inside area 7 but shouldn't go out of area 7.

Without using areas you cannot perform summarization of internal routes.

the requirement is that all areas need to connect to area 0 and traffic between two areas is sent via area 0.

I would use a multi-area approach because it provides you options that can be useful in the future and provides better scalability.

Hope to help


carl_townshend Wed, 09/10/2008 - 03:04

hi there, what is the command to summarise ospf? do you do it on the interface or under the router config?


Marwan ALshawi Wed, 09/10/2008 - 03:12

if u have ABR which is the router between two areas

lets say u have area 2 that has

u can sumrized like

for example

router ospf 1

area 2 range

and in the routing table of nieghbors this route will apear like

0 IA

if helpful Rate

carl_townshend Wed, 09/10/2008 - 03:56

Hi there, do you have to use the range command?

can you not use the summary address command ?

Marwan ALshawi Wed, 09/10/2008 - 04:05

first sorry i put the

IPs wrong by mistake

bsed on the four networks the summary shoud looks like

area 2 range

and will apear on the routing table as

/21 because they are four networks

about ur question it is good question

in ospf there are two trypes of summarization intra area sumarization which is the one i sent u this one between ospf areas and the is a comman ip summary address this one used with ospf ASBR which summarize routes from external routing like rip, eigrp into ospf

this is the deffrnece between the use of those two commands

good luck

if helpful Rate

syedsadiq Thu, 09/11/2008 - 00:20

Dear Carl,

The decision to have multiple areas depends on multiple factors.

1. Number of links in an area.

2. Size of routing table

3. Number of L3 devices.

4. SPF calculations...

carl_townshend Thu, 09/11/2008 - 02:17

Hi There

The design is this

20 stacks of layer 2 switches, these connect to an access distribution layer, the dist layer will be there gateway. There will be about 15 vlans on there. this will then connect to the core routers. there will also be a server access/dist layer which connects to the core, there will be around 6 vlans on them dist switches.

what would be best here ?

syedsadiq Thu, 09/11/2008 - 02:38


Can you explain your complete network. The information provided is very minimal. I could make out that there are very few L3 links. Is OSPF already deployed in your network? If Yes then what is the architecture? If it is not deployed, Then Why are you looking for OSPF for the described network ?

carl_townshend Thu, 09/11/2008 - 03:43

there will me minimal layer 3 links to the core, probably something like an etherchannel to each core from the distribution switches, we do not use ospf or any other routing protocol at the moment, however its something we want to do.

Another quick question, if we did have our core as area 0 and the distribution switches in another area, where would we put both areas, on the core or the dist switches, ie which would be the abr's?



Marwan ALshawi Thu, 09/11/2008 - 03:48


the ABR shoul dbe the one between two areas

like access switches--dist--SVIs in area 1--- core uplinks network area 0---

so the dist is the ABR

and the summary will be from area 1

like the example i have given to u the core will see only summary route from area 1 for less CPU and route calculations

good luck

and hope this helpful

syedsadiq Thu, 09/11/2008 - 04:15


I suggest you to go with Single area as you have very few L2 links. How ever for smaller setups, it is not recommanded to go with a routing protocol. You can very well have a highly available network (By using floating static routes or IP SLA configured) with static routing as well. If you want to go with Multi area, you need to freeze the scope of the area first, that which devices will be in area 0 and which will be in other areas.

carl_townshend Thu, 09/11/2008 - 05:49

hi there, thanks for your input,

what is the recommendations for knowing when to have another area rather than one, ie number of links/subnets etc?

Also is it possible to redistribute another companys ospf into ours, how would we do this if both companys have a backbone area?


This Discussion