Underlay Scale

[dm2020]

Hi All,

I've just been watching Cisco Live session "Deploying Your First Cisco SD-Access Project - BRKENS-2824" (great session by the way) and I have some questions about underlay scale.

I'm currently working on a deployment that will have 2 x C9500 combined CP/border nodes and about 500 switches in the underlay (intermediate and edge). I was planning on using LAN Automation to establish the underlay, however the presenter in the above states that only 250 switches are supported in a single L2-Only IS-IS area and if more than 250 switches are needed then the underlay should be deployed manually using multiple areas.

I'm trying to work out what my options are as we are sold on using LAN automation to automate the underlay. Can we modify IS-IS post LAN automation to create multiple areas? Or is it advised that if a site needs this level of scale then the underlay should be deployed manually?

[jedolphi]

Hi, OSPF with area support **should be** added to LAN Automation towards end of this year, for more specfic detail please talk to your customer's Cisco account team. If you must LAN Automate earlier and/or must use ISIS, then after LAN Automation has concluded you can indeed modify the NET ID and level configuration to create areas, LAN Automation will not change your custom NET ID or level settings. Changing NET ID on an LAN Automated switch can get a little tricky  because it will flap ISIS. Some customers have used EEM scripts to modify ISIS on access switches post-LAN-Automation. I would advise to test the procedure carefully in pilot enviroment before moving to production so you don't run into a scenario that causes switches to be disconnected from ISIS and become unreachable/unmanagable.

[dm2020]

Hi @jedolphi 

Thanks for the response. We are going to start configuring the underlay now so ufortunatley cannot wait for OSPF support to be introduced. Out of interest, will ISIS multi area support also be introduced with LAN automation or will this only be with OSPF?

We dont have to use LAN automation or use ISIS, this is what has been suggested to make the most of the automation capabilities of Catalyst Center. This is for a greenfield network so trying to determine the best approach to support a high quantly of switches in the underlay. From what I see I currently have 3 options. Is that correct?

1) LAN Automation using ISIS. Modify areas post automation where needed

2) Manual deployment using ISIS with areas where needed

3) Manual deployment using OSPF with areas where needed

[Andrii Oliinyk]

good question is "where is it needed". & then u need to ask yourself "why do i use multiarea" (see my another question to @jedolphi below)?

[jedolphi]

No plans for LAN Automation of ISIS areas, sorry. Yes I agree with the three options you presented. (Technically speaking you could also use EIGRP or static routes, but I realise that's out of context). You could automate switch onboarding with areas using CatC PNP. Some customers have created some very advanced PNP processes to build routed underlay with areas.

[Andrii Oliinyk]

Hi Jerom
it's clear that in the background of not using many routers in the single ISIS area the same topic is laying - prevent network from convergence degradation. inter-area summarization is an essence of multiple areas whether it's IS-IS or OSPF. But within single Fabric we need /32 RLOCs populating the underlay's RIB. Introducing multi-area either with OSPF or IS-IS confuses on the topic of how we keep /32 RLOCs from summarisation. Unless we create multi-site with SDA transits. But it makes multi-area IGP in underlay senseless.

[jedolphi]

Hi Andy. A single fabric site can have up to 1200x Fabric Nodes e.g. 2x BN, 2x CP and 1196x Edge Nodes. It would not be wise to put all 1200 in a single IGP area, especially if there is lower end switches. We can leak /32s between ISIS areas/levels or OSPF areas. The areas contrain LSP/LSA flooding which promotes faster IGP convergence.