OSPF to IS-IS Migration

There are many reasons of OSPF to IS-IS migration, specifically for the Service Provider networks. Some of these reasons are shared later in the case study.

OSPF to IS-IS migration can be done in three ways.

In this article I will share, ‘  ship in the night approach ‘ which relies on having both routing protocols on the devices at the same time.

Please note that, this approach may not be fit if there is not enough CPU, memory or link capacity.

OSPF to IS-IS Migration Case Study: 


Fastnet is a fictitious service provider, which has had some security problems with their internal IGP routing recently. They want to deploy IPv6 on their network very soon but they don’t want to run two different routing protocol (one for existing IPv4 and new IPv6 architecture).
They currently have OSPF routing protocol deployed in their network. Fastnet knows that adding a new feature in IS-IS by the vendors is faster than OSPF in general.

Also thanks to the TLV structure of IS-IS, when additional features are needed, IS-IS is easily extendable.
Since the majority of service providers historically use IS-IS for their core IGP routing protocol, Fastnet decided to migrate their IGP from OSPF to IS-IS.
Please provide a migration plan for Fastnet for smooth transition. Fastnet will plan all of their activity during a maintenance window.

Fastnet has been using flat OSPF design, but they want flexible IS-IS design which will allow easier migration to multi-level IS-IS in the future if it is required.

Please identify the best possible IS-IS design option and provide a detailed migration plan for smooth OSPF to IS-IS migration.


OSPF to IS-IS Migration Case Study Analyze


If they would continue with OSPF, since they want to deploy IPv6, company would require to deploy OSPFv3 as well. So, they would have to manage two routing protocols. IS-IS has been by eight of the nine largest Service Providers and it has been proved to be scalable.

Not only scalability but extensibility, easier troubleshooting, faster convergence, and simplicity are the other advantages compare to OSPF. This has been explained in OSPF vs. IS-IS comparison post.

Important keyword to decide IS-IS design is, ‘  they want flexible IS-IS design which will allow easier migration to multi-level IS-IS in the future if it is required’.

They should start to deploy IS-IS level 2 on their network. It is a best practice to deploy IS-IS Level 2 on the network from day one. It allows easier migration.

That’s why our migration will be from OSPFv2 to IS-IS Level 2 for this company network.


OSPF to IS-IS Migration Plan
Below are the steps for the migration. A “ship in the night” approach will be used. Both routing protocols will be running on the network at the same time during migration.

1. Verify OSPF configuration and operation.

In any migration, first step should be the documentation of the existing situation. I will explain step by step what should be done.

  • You should check if there are any routing table instabilities.
  • Ensure that next-hop values for the BGP are valid and reachable.
  • Check OSPF routing table and record the number of prefixes.
  • Check OSPF database entries, record the number of database entry for backbone area only since this is flat OSPF design migration.
  • If there is IP overlaps or any other possible problems, it is good idea to start migration with a clean infrastructure, that’s why all the possible mistakes can be fixed in this stage.

2. Deploy IS-IS over entire network.

  • IS-IS admin distance is higher than OSPF in many platforms, leave as-is in this step.
  • Use wide metrics for IS-IS. This will allow IPv6, MPLS Traffic Engineering, and new extensions.This company requires IPv6 specifically but thanks to extendibility of IS-IS, if they need MPLS Traffic Engineering in the future, they won’t require new protocol deployment.
  • Deploy L2-only IS-IS since flexibility is desired by the company. L2-only reduces the resource requirements on the routers and allows easier migration to multi-level IS-IS.
  • Some vendor equipment may enable both Level 1 and Level 2 IS-IS routing (Example: Cisco) when IS-IS is turned on. Thus in this stage you need to disable IS-IS Level 1 per router so none of the interface runs Level 1 IS-IS.
  • Deploy both IPv4 and IPv6 on the links. IS-IS will run separate SPF per IP address family.
  • Deploy IS-IS passive interface at the edge links; these links should be carried in IBGP. Prefix suppression can be used to carry infrastructure links in IBGP, but these are not a requirement of Fastnet.These are some IS-IS best practice design steps.
  •  Make sure the IS-IS LSDB (Link state database) is consistent with OSPF Link state database
  • Make sure the number of routing neighbor is consistent with the OSPF
  • Make sure the number of routing table entry is the same with OSPF.

3. Set OSPF admin distance to be higher than IS-IS.

  • Increase the AD of OSPF across the entire network.
  • IS-IS Administrative distance is higher than OSPF by default. When step 2 is completed, in this step, we increase the Administrative distance of OSPF to be higher than IS-IS to start using IS-IS path according the routing table.
  • We don’t change IS-IS Administrative distance because when we remove OSPF from the network, IS-IS will remain and using the routing protocol with  default administrative distance is best practice since it provides consistent operation.

4. Check OSPF leftovers.

  • In this step all the prefixes in the routing table should be learned by IS-IS. If there are any OSPF prefixes, we should find out why they are there. You can compare the “show IP OSPF neighbor” with “show IS-IS neighbor”; should be the same number of neighbors for both.
  • If the number is not the same, fix the problem.

5. Remove OSPF from entire network.

  • All the OSPF related command can be removed.
  • If there is interface-specific configuration such as Traffic Engineering (metric/cost), authentication should be removed as well.

6. Verify IGP and related operations.

  • Entire network should be functioning over IS-IS. Verify IBGP sessions.
  • Verify MPLS sessions
  • Verify customer and edge link prefixes.All the verification steps for IS-IS and the other protocols which interact with IS-IS should be checked to verify whether they are functioning properly.


Further OSPF to IS-IS Migration Study Resources

AOL Backbone Migration

Migrating from OSPF to IS-IS Protocol

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