i am having an OSPF network..based on IP ..
we r migrating to MPLS for deploying new services.., and i am in the phase of designing the new network..
my question is , is there any restrictions on using OSPF as an IGP while deplying MPLS..as i heared that ISIS is better if i will impelemnt MPLS..
take into consideration that we will implement MPLS-VPN, Traffic Engineering, QoS.
and if i am already having my OSPF network..with its areas , what benefits will i have if i change to ISIS..??
Can any one help me in this issue..
I am going through a similar excercise as yourself. I am opting for ISIS. I have used this protocol for a number of years now so am pretty much comfortable with it. One major advantage of ISIS over OSPF is that using ISIS you dont require to have your traffic go through your backbone at all times. ISIS eleviates the area 0 problem that OSPF has. Imagine that every time your traffic levels increase on your PE-s you need to upgrade your core instead of identifiying your traffic patterns and connect 2 PEs together as you can with ISIS. ISIS uses TLV fields to identify things like MPLS-TE and is very easy for Cisco and other vendors to make changes to these TLVs whereas OSPF uses LSA's and takes an age to get a new LSA added to the OSPF protocol. This is why you are seeing new features always coming out months, sometimes years, on ISIS before you see them in OSPF. I could ramble on for sometime over the benefits of using ISIS over OSPF (area 0 size Cisco say 50 routers using OSPF - I have a network which is 100+ routers in one single ISIS area and cant remember the last time I seen a fault on the network). It all comes down to - are you prepared to come out of your comfort zone and configure and design ISIS if you are unsure if it. If you are then go for it and use ISIS. If you require more info please email me on email@example.com.
To clarify your statement "One major advantage of ISIS over OSPF is that using ISIS you dont require to have your traffic go through your backbone at all times. ISIS eleviates the area 0 problem that OSPF has", are you saying that because ISIS has no Area Identifier as in OSPF, to merge two L1 areas you just need to physically connect them, as opposed to OSPF, where you would also need to change the Area IDs of one of the areas that is being merged?
Yes this is correct. The two routers that you would "join" together would become Layer 2 or Layer 1/2 routers. The traffic would then flow along this shortest path rather than eating up bandwidth within your core and also in most cases causing you to add expensive cards into GSRs and the like.
I think one more benifit especially when you are going for MPLS-TE is that one ISIS area can grow verry big > 500 routers. As such you will never run into problems of implementing traffic engineering across multiple area's since that is hell.
The problem when implementing TE in a multi-area backbone is that the area's on themself no longer have a complete view of the whole network. As such implementing TE is getting pretty hard.
On reason more for thinking of implementing ISIS.
I would question whether there is a valid reason/explanation why ISIS can have more routers in one area than OSPF. I think this is only based on an old paper by Cisco that mentioned 20 routers per area, that was written over 5 years ago. With the CPU and memory in routers today this figure of 20 could grow to 500. I think the size is not so much based on CPU/memory now, but on the level of risk due to the complexity (i.e. not using areas to simplify the design) of the network.
In summary, for 500 routers in an area, ISIS and OSPF both have to build a 500 node SPF tree, so there is no reason I can see why it can be argued that ISIS can have more routers in an area than OSPF?
so..this means that when implementing MPLS-TE while having OSPF multiple areas, this wil be hard ..or will cause problems....am i right..??
what about merging all my OSPF areas to one single area..take into consideration that i only have 3 areas+ area0..
number of routers in all OSPF Domain will range from 150 to 200 Router..
is that good..??
and how many routers can a single area hold..??
Yes, you are correct, MPLS-TE with multiple areas is hard.
However, my point is simply that if with ISIS you can put 500 routers in one area, then with OSPF you should be able to put 500 routers in one area, since the CPU and memory requirements are almost identical. The only difference may come down to vendor implementations, and whether their code has allowed for this many routers. Personally, I believe in hierarchical design, so in the absence of MPLS-TE would use areas with ISIS or with OSPF, to reduce the failure domain and the complexity.
Thanks for the Info..
is there any documnets that talks about the restrictions or the difficulties that faces TE while having OSPF multiple areas..??
or why is there difficulty in implementing TE with multipl OSPF areas..??
The problem is like I told in my previous post because the routers in one area only have detailed information about that specific area, so to run the CSPF (constrained based SPF) it will be much harder since it needs info from the ABR which sits at the border and has the view on multiple area's.
If you want I have a presentation on this from the "MPLS deployment forum" from last year. Send me a mail and I will forward it to you. (firstname.lastname@example.org).
We had a similar case once a time.....but the choice was in favor of OSPF for some reasons:
1- It's much easier and familiar for most of us
2- simply you can collapse all yr 3 areas into one area (area 0) without no scalability fears......an area can occupy up to 400/500 routers as far as I can remember.
3- Also the decision to use OSPF/IS-IS is related to no. of PEs you'll have in yr network .....cause as you know the IGP (either OSPF/ISIS) is used for routing bet the PEs and Ps and then from the RT the LDP database can be populated.
So as far as I know the use of OSPF is fair enough.
4- Till now,we haven't implemented MPLS TE....so I can't help you concerning OSPF constraints with MPLS TE.
For further discussions....contact me at
We have one customer using OSPF between CE-PE. This is for isdn backup purposes. But then, only 28 (or 27 ..not sure correct me if i'm wrong) ospf process can be configured inside a PE. That means, If I have c7500 series, i cannot have more than 27 different customers, different vrf. 2 Cisco senior engineers told me the same thing.
BGP seems to be most popular routing after static, sure static is the best choise, simple to configure but then, we need dynamic routing for isdn to kick and do the backup.