Eigrp to Ospf

EIGRP to OSPF Migration Strategies

White Paper


Juniper Networks, Inc. 1194 North Mathilda Avenue Sunnyvale, CA 94089 USA 408 745 2000 or 888 JUNIPER www.juniper.net Part Number: 350053-001

Contents Contents ................................................................................................................................................2 List of Figures.......................................................................................................................................2 Introduction..........................................................................................................................................4 Evolution of IP Networks and IP Routing Protocols ......................................................................4

EIGRP ...........................................................................................................................................4 OSPF .............................................................................................................................................5

OSPF Design Methodology ................................................................................................................5 EIGRP to OSPF Pre-Migration Tasks ................................................................................................5

Network Discovery, Cleanup, and Design..............................................................................5 Determining Router CPU and Memory Utilization Requirements......................................6

EIGRP Topologies and Migration Strategies....................................................................................6 OSPF Overlay Model..................................................................................................................6 Route Redistribution Model ......................................................................................................7 Integrated Model ........................................................................................................................8 Classification of OSPF Migration Efficiency ...........................................................................8 Classification of IP Networks....................................................................................................9

Conclusion ............................................................................................................................................9

List of Figures Figure 1: Sample EIGRP Network....................................................................................................7

Copyright © 2005, Juniper Networks, Inc.


Introduction Enterprise data networks are evolving into IP-only data networks. In this IP environment, Open Shortest Path First (OSPF) and Enhanced Interior Gateway Routing Protocol (EIGRP) are two popular interior gateway routing protocols (IGRPs) used by large enterprises. EIGRP is Cisco’s proprietary protocol and provides link-to-link protocol-level security to avoid unauthorized access to routing tables in Cisco routers. OSPF is an open standards-based and a complete link-state routing protocol, which means that a table or database in the router stores all of the state information for each link in the network. The latest Internet Engineering Task Force (IETF) OSPF specification, OSPF version 2, is IETF Standard 0054 (April 1998). This type of link-state protocol is more efficient, more scalable, interoperable, and typically provides superior convergence. For these reasons, enterprises that currently have EIGRP running in their networks commonly migrate to OSPF. This paper addresses the migration from EIGRP to OSPF and important migration considerations.

Evolution of IP Networks and IP Routing Protocols The first IP networks were used for military or research applications. Mostly static routes were used and routing protocols were based on distance vector algorithms, such as Routing Information Protocol (RIP). There was little concern for IP address depletion or IP address summarization. There was no hierarchy in the routing protocol. Reliability of the physical layer was not hidden and its flapping disrupted in the network. Applications were text-based and not delay-sensitive and a single autonomous domain managed the network.

The IP networks of today carry mostly commercial traffic. Multimedia applications used today are very delay-sensitive and hence, routing protocols must provide stability, and security, and converge quickly. Today, the Internet is divided into many autonomous systems or domains. Each domain uses its own internal routing protocol. The IP routing protocols in each domain must be carefully designed in order to build a stable and scalable network. Physical instabilities and IP address planning and summarization both inside and between autonomous domains must be taken into account.

EIGRP

EIGRP is a Cisco-proprietary distance vector Interior Gateway Routing Protocol (IGRP) that allows routers to exchange vector updates that represent link distances. These updates are non-periodic, partial and bounded, as generally represented in link state-like functions. The EIGRP updates are based on a diffusing computational algorithm that provides advantages of network resource utilization, loop topology avoidance, link bandwidth conservation, and multiple network-layer protocol support (IP, IP eXchange (IPX), and AppleTalk (AT)) over previous generation distance vector routing protocols such as RIP. These characteristics allow EIGRP to be used effectively for small-to-medium-scale IP networks.

However, the proprietary status of EIGRP presents an inherent limitation to its universal use. In addition, several other notable deficiencies, such as its inability to be used in a hierarchical arrangement, which precludes its use in large IP networks, and its lack of support for MultiProtocol Layer Switching-Traffic Engineering (MPLS-TE) also restrict its overall networking utility.

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OSPF

OSPF is a link state IGRP and was developed as an open standard application, primarily to overcome the limitations of existing distance vector protocols. It is widely deployed and has been available and used for a number of years. (The original OSPF specification, Request for Comment 1131, October 1988.)

OSPF differs from EIGRP in that it can be used in hierarchical arrangements, thereby supporting scalability to very large IP networks. It also offers the advantages of capabilities such as On Demand Routing (ODR), acknowledged communications, authentication, quick convergence due to use of the Dykstra algorithm, Variable Length Subnet Mask (VLSM), route summarization for internal and external routes, and MPLS-TE.

OSPF Design Methodology The basic design principles for OSPF networks center on the creation of an optimal hierarchical structure, incorporating appropriate redundancy measures, ensuring stability of the routers and links, and providing for IP address summarization.

A typical OSPF network is designed to map the two hierarchical levels permitted by the protocol to physical arrangements comprised of a three-level configuration: core, distribution, and access. Additional design tasks include selection of Area Border Router (ABR) locations, providing for Internet connectivity, and accommodating router-associated network scaling issues such as the number of routers in an area, the number of neighbors, and the number of router areas.

Other miscellaneous design criteria include:

The selection of a designated router (DR). The DR should be one of the most powerful routers on the network and can only be the DR on one of its interfaces.

Redundancy and partition avoidance. Redundant links to the backbone is recommended. Avoiding partition or isolation is recommended by connecting all of the members of an area with redundant connections to more than one ABR.

EIGRP to OSPF Pre-Migration Tasks There are many possible strategies for successful and transparent EIGRP to OSPF migration. These strategies can be grouped into three proven models: overlay, route redistribution, and integrated. The selection of a model for a particular network depends on many factors. However, prior to any migration, there are pre-migration tasks to complete.

Network Discovery, Cleanup, and Design

Before migrating from EIGRP to OSPF, there should be a comprehensive understanding of the current network including network topology, IP addressing for the backbone and edge infrastructure, the current number of routers and links, the Central Processing Unit (CPU) and router memory, where traffic enters the network, the different network traffic types, and the network traffic flow. An understanding of the current network helps to determine the OSPF design.

Copyright © 2005, Juniper Networks, Inc. 5


IP addressing issues should be resolved by removing any ad-hoc assigned IP addresses and by grouping contiguous subnets so that network summarization is the most efficient. A solid IP addressing design will ensure a cleaner transition to OSPF.

Once there is a satisfactory addressing scheme in place, create the OSPF topology design. The OSPF topology design will include the type and number of routers in the backbone area, the ABRs and Autonomous System Border Routers (ASBRs). The routers in the broadcast domain and NonBroadcast MultiAccess (NBMA) clouds should be configured to specify DR selection. Normally, a router with the highest Router ID (RID) is the elected DR on a segment and the router with the second highest router ID is the elected Backup DR (BDR). In order for OSPF to function properly, the DR and BDR routers should have full connectivity to all other routers in the area.

A prefix aggregation plan should be developed to align network prefixes with ABRs. The goal is to minimize routes in a router’s routing table using route summarization. The number of routes needing summarization is dependent on the IP addressing scheme. The cleaner the IP addressing scheme, the cleaner the routers’ routing tables. If the network is carrying external routes, route summarization is done on the ASBRs as well as on the ABRs.

Determining Router CPU and Memory Utilization Requirements

Checking the CPU load and memory utilization of all the routers in the network running EIGRP is a necessary task prior to migration. If the CPU load is 50% or less and the memory utilization is 60% or less, load OSPF on the routers in parallel with EIGRP, and set the OSPF administrative distance to 255 to activate OPSF. If the CPU load on the routers running EIGRP is over 50%, then monitor the CPU load while loading OSPF or wait to load OSPF during the cutover when EIGRP is removed from the router. If the memory utilization of the router is over 60%, consider upgrading the memory. If upgrading the memory is not an option, monitor the memory utilization while loading OSPF or wait to load OSPF during the cutover.

Selecting places that will incur the least amount of disruption such as during a downtime maintenance window to beginS the cutover. Changing the OSPF administrative distance back to 110, the default, is a necessary step. During the transition, monitoring the CPU, memory, and routing selection ensures proper functioning. Once OSPF has been running for a predetermined time, removing the router EIGRP statements completes the migration.

EIGRP Topologies and Migration Strategies EIGRP is not hierarchical, but it can be forced to behave hierarchically. In this case, the migration from EIGRP to OSPF is easier. If the current EIGRP network is flat and cannot be forced to run hierarchically, then the OSPF migration plan may require building a network hierarchically-based on OSPF design requirements and principles. When the EIGRP network is flat, the migration would require using an integrated model where route redistribution is required between EIGRP and OSPF.

OSPF Overlay Model

The basis for using the Overlay Model is to enable OSPF on EIGRP enabled-routers. Protocol preferences (administrative distances) are taken into account so only EIGRP is used during this transition. The protocols operate without being aware of each other. The routers contain databases for each protocol, but only install EIGRP routes into the routing table.



One key consideration of this model is network stability, and in particular, the stability of EIGRP in the network. For instance, if two EIGRP neighbors lose adjacency due to the instability of the EIGRP protocol, yet the OSPF adjacency remains, then some routes use OSPF while other routes use EIGRP. If both EIGRP and OSPF are active, this causes routing loops, as EIGRP and OSPF may not agree on the best path.

The route protocol preference is a key migration requirement. When a router learns EIGRP routes and OSPF routes, Cisco routers select the best routes―the ones with the lower administrative distance values. External routes to the EIGRP domain have a different administrative distance than the internal routes. Juniper implements a similar feature that distinguishes routes learned from multiple protocols. Juniper's route distinguisher is based on preference values. The routes with lower preference values are injected into the forwarding table. The Juniper router distinguishes the internal and external OSPF routes with preference values in addition to the protocols distinguisher.

Figure 1 shows a sample EIGRP network topology for a simple migration to OSPF. Notice the core, distribution, and access groupings. The routers in the access area are considered the weakest links because they cannot run both EIGRP and OSPF. The migration can be done from core to access or from access to core.

Figure 1: Sample EIGRP Network

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Route Redistribution Model

The Route Redistribution Model is used for migration when the existing EIGRP network is flat and cannot be forced to behave in a hierarchical fashion. In this approach, the network’s distribution and access layers are separated to form the future OSPF areas. The gateway routers are configured as ABRs to run OSPF and EIGRP on a parallel basis. As in the previously described Overlay Model, the routers are set for EIGRP preferences. The network is migrated by successive



hot cuts of internal routers and redistribution of associated network sectors to OSPF. In the final stage of the migration process, backbone routers are hot cut to OSPF and EIGRP is completely removed from the network.

The Route Redistribution approach is good for a relatively small network. The issues of this approach are:

Migration is localized and stability can be judged only at that level.

It requires a lot more planning in the IP address space assignment because of the mutual redistribution and is prone to human error.

Once the entire network is migrated to OSPF, backing out is very difficult.

Routers that cannot run both protocols and are connected to the backbone will have to wait until migration of the backbone is complete.

Integrated Model

The Integrated Model is used to integrate flat EIGRP networks that cannot be converted to a hierarchical configuration. In this approach, a new network core, which runs OSPF, is built and connected to the EIGRP domain at one or more routers capable of running both EIGRP and OSPF. Portions of the EIGRP domain are then migrated to the OSPF backbone on a sequential basis. It should be noted that this approach also requires a considerable amount of IP re-addressing to ensure that the new network is hierarchical and more manageable.

Here are the issues with this approach.

It is a very expensive migration strategy.

It requires extra equipment and network resources to build a parallel network, at least for the OSPF backbone.

It requires IP re-addressing and may even require additional public address space from the American Registry of Internet Numbers (ARIN). The public address space can be returned to ARIN once the migration is complete.

Careful planning and summarization techniques need to be in place since it is a one-way redistribution.

Classification of OSPF Migration Efficiency

Unless the network is designed with a hierarchical structure, migration of the network from EIGRP to OSPF may require re-addressing and/or reconfiguration of the network topology to achieve the most benefits. However, due to the complexity of the network and to the difficulties to accomplish these tasks, the benefits achieved from optimization may not outweigh the effort necessary to reach it. The following list correlates network modifications to OSPF design efficiency:

1. No change to network – least efficient.

2. Change IP addressing – somewhat efficient.

3. Change topology – somewhat efficient.

4. Change IP addressing and topology – most efficient.



Classification of IP Networks

Each company may have additional considerations, other than those mentioned in this paper, to address during migration based on the type of IP network and its projected growth. A company should evaluate the classification of its IP network based on the size and breadth of the network. The size and breadth can be divided into the four categories: international IP network, nationwide IP network, regional IP network, and campus IP network.

Conclusion Converting a network from EIGRP to OSPF will create a more efficient, scalable, and interoperable network. However, the migration must be well planned to ensure success. Careful consideration of the basic design for the OSPF network is of utmost importance. The design of the OSPF network should incorporate redundancy measures and be based on a hierarchical structure. It must ensure stability of the routers and links and provide for IP address summarization.

Before beginning the migration process, there are key tasks that are important to complete. It is important to have a comprehensive understanding of the current network, including topology, IP addressing, the number of routers and links employed, the CPU and router memory available, and the type of traffic carried, where it enters the network, and how it flows. In addition, it is important to clean up any IP addressing issues that might reduce the efficiency of the network route summarization. Checking CPU load and memory utilization of all the routers in the network running EIGRP is a crucial pre-migration task. Moreover, sometimes memory upgrades need to be made prior to cutover to ensure successful migration.

Once all pre-migration tasks have been completed, choosing the migration model that best suits your company’s needs becomes an important consideration. The OSPF Overlay Model, Route Redistribution Model, and Integrated Model offer varying approaches to migration that conform to networks of varying sizes, designs, and projected future needs.


Eigrp to Ospf

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