USING ROUTE EXPLORER™ IN EIGRP TO OSPF/IS … · If the migration is from an EIGRP AS to a single...

TECH BRIEF

USING ROUTE EXPLORER™ IN EIGRP TO OSPF/IS- IS MIGRATIONS

Copyright © 2017, Packet Design, LLC of 10

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Table of Contents

Method 1: Overlay with AD manipulation

Method 2: Moving redistribution points

Prefix and Links Reports

Conclusion 9

Using Route Explorer™ in EIGRP to OSPF/IS-IS Migrations


One of the common reasons that network engineers discover a need for routing analytics is when they tackle migrations from one routing protocol to another. The bigger the network, the more likely you are to find scenarios you didn’t expect: Legacy implementations that were abandoned halfway through, temporary fixes that became permanent, and issues due to scale. One of the most difficult and time consuming customer projects that we encounter frequently is a migration from EIGRP to OSPF. Having the right information can help manage these types of scenarios, and that is where Route Explorer really makes life easier.

At Packet Design we have found that there are two common approaches to IGP protocol migrations, although there are nuances and hybrids for both approaches. The first method involves deploying both routing protocols concurrently on all of the routers in the network but using the higher OSPF1 Administrative Distance (AD) so that it is not used initially. (Caveat, you will need to lower the AD of EIGRP external prefixes prior to starting this method.) The second method uses overlapping redistribution points that move around the network. This paper will go into more detail about each of these methods.

So how does Route Explorer assist in each of these methods? Regardless of the method used, it is important to have an accurate picture of your network before starting the migration. This means understanding your network from a device standpoint, and also from a prefix and link perspective. Route Explorer provides this information in minutes for most networks.

Method 1 – Overlay with AD manipulation

If the migration is from an EIGRP AS to a single OSPF area or multiple OSPF areas, it is important to make sure that the external EIGRP’s AD is lowered on each router to a value below the OSPF value. Once that is done you can deploy OSPF and know that it will not change your routing. To verify this you can issue a show ip route ospf command and see if there are any OSPF routes being preferred in the routing table.

Once OSPF is configured, it is key to enable OSPF peering with Route Explorer as soon as possible so that it can record the OSPF build-out of the network. Since Route Explorer does not monitor the routing table but looks at the OSPF database instead, it will see the complete OSPF routing picture even though routers are not yet using OSPF to forward traffic.

Route Explorer’s peering allows you to see the OSPF topology, including all of the prefixes, links and routers, prior to OSPF being used. This method of migration is dependent on both IGPs existing throughout the network. An EIGRP link that is not also configured for OSPF or an EIGRP prefix that is not also being advertised by OSPF will cause an outage during the migration. If these issues go unnoticed, you may create an outage that extends beyond your maintenance window, potentially leading to a roll-back of the migration and a project delay.

1 OSPF is referenced in this paper, but the material applies equally to IS-IS.



Route Explorer can show every prefix and link in the EIGRP topology and compare them with the OSPF topology to check that each one has a corresponding entry. With comprehensive information about both protocols, you will be able to identify quickly any differences between the old and new protocols.

Once you have used Route Explorer to successfully overlay OSPF onto your EIGRP topology, you can remove EIGRP. The actual migration is done by simply removing EIGRP from the outside of the network toward the middle. This creates a single, cohesive EIGRP topology that shrinks until it is gone (see Figures 1a, 1b and 1c), being replaced by the parallel OSPF topology. As you disable EIGRP on each router (or, if you choose, you can raise both internal and external EIGRP ADs to be higher than OSPF’s to achieve the same result), you will see the routers turn red in Route Explorer’s live topology map. The map will show if you are isolating any router. Route Explorer usually takes 10-30 seconds to update the map with the EIGRP changes.

Figure 1a. EIGRP network before starting the migration to OSPF



Figure 1b. EIGRP being disabled during the migration process

Figure 1c. Later during the migration process



During this process, you won’t see any changes at all from an OSPF perspective, and the map will look the same as it did before the changes. The OSPF database should not change during the migration. As you finish up, you should ensure that no prefixes were isolated. Route Explorer’s “List Prefixes” report will list each prefix at least twice, once for each protocol. OSPF will show the prefix as up and EIGRP will show it as down. If you see any prefix entries that list EIGRP but not OSPF, these will be the prefixes that were not configured in OSPF. This may be due to missing static routes or redistribution from static routes to OSPF. The report shows the router on which prefixes should be configured, making it easier to find and fix the issue. Route Explorer shows exactly which prefixes are in each protocol, which ones are missing, and where each prefix was originated. This information is key to a successful routing protocol migration.

Method 2: Moving redistribution points

The second protocol migration method involves moving the network boundary between OSPF and EIGRP and gradually migrating the network, layer by layer. To model this type of migration accurately, you should know the first migration point where you are introducing OSPF, and your desired last EIGRP node. It is important to peer Route Explorer with OSPF as soon as possible and ensure EIGRP is recorded until you are done. For help, refer to the Administrator’s Guide or contact the Packet Design Technical Support team. Assuming you are already recording EIGRP, begin by adding a GRE tunnel to the first router you are migrating to OSPF (as long as it is not the same one where you are recording

Figure 2a. EIGRP topology map at the start of the OSPF migration



EIGRP) and ensure you have a neighbor relationship formed with it. It is also a good idea to change the Route Explorer peering to the last router that will have EIGRP disabled. This will allow continuous recording throughout the conversion.

In the EIGRP map shown in Figure 2a and the OSPF map shown in Figure 2b below, Veronica is the first router to be converted to OSPF and Westjefferson is the redistribution point.

At this point, Westjefferson is doing redistribution between OSPF and EIGRP. Generally speaking, you should have a route map of these redistributions so that you can tag routes coming from each protocol and ensure that they are not re-advertised to their originating protocol. If this happens, one system will experience a lot of protocol churn and ever-increasing metrics, typically. With Route Explorer you will see the two routers (or more) that are experiencing this and where you need to check for proper tagging or filtering.

In the example, the next step may be to extend OSPF to the Richard router which will then be the only router running both OSPF and EIGRP. See Figures 3a and 3b.

Figure 3a. EIGRP topology map as OSPF routing is extended to Richard

Figure 2b. OSPF topology map at the start of the

migration



Packet Design recommends that you open Route Explorer in two windows side by side so that you can see the changes as they happen and verify the topologies look correct.

To complete the migration to OSPF you could migrate sections of the network during maintenance windows while minimizing the number of redistribution points. The more redistribution points you use, the greater the effort and likelihood of mistakes being made that affect your routing topology.

Prefix and Links Reports

For both methods there are several Route Explorer reports, as well as some alerts, that you may use. For alerts, we recommend configuring Prefix Alerts for all user subnets before starting the migration. Setting up one alert for a down condition and another for an up condition will produce a lot of alarms but will warn you if they don’t match one for one. Your network management system should be able to correlate these data and show a list of just the currently active alarms.

For reporting, Packet Design recommends checking the List Prefixes report and ensuring that all prefixes are learned by a routing protocol both before and after a maintenance activity. For Method 1 above, you would want to ensure that all prefixes that were ever learned by EIGRP are learned by OSPF. For Method 2, it is important that the prefixes are learned by both routing protocols; one of them will be the active protocol and the other will be the protocol that is redistributed.

Lastly, the List Routers report is helpful as well. For Method 1, you want to ensure that there are EIGRP and OSPF entries for every router prior to shutting off EIGRP. For Method 2, you want to see each router enabled for one routing protocol, except for your redistribution points which should be enabled for both.

Figure 3b. OSPF topology map as OSPF routing is extended to Richard



Conclusion

Changing IGP protocols is never an easy task and we have heard from customers about many missteps and false starts when executing these projects. Many of them had to roll back or put the project on hold while they investigated what caused service disruptions. Packet Design customers find that using routing analytics in the Route Explorer system is the best way to avoid these problems. Deep forensics of every single IGP routing event with operational alerts help users know if something is not working as expected, and which router is either advertising, or not advertising, a critical prefix.



To learn more about Packet Design and the Explorer Suite, please visit www.packetdesign.com

USING ROUTE EXPLORER™ IN EIGRP TO OSPF/IS … · If the migration is from an EIGRP AS to a single...

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