Migration from Cisco IGRP and EIGRP to industry-standard OSPF

woonsocketpoliticalNetworking and Communications

Oct 28, 2013 (3 years and 7 months ago)

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Migration from Cisco IGRP and EIGRP to
industry-standard OSPF
Technical white paper


Table of contents
Overview..................................................................................................................................... 2

IGRP overview .......................................................................................................................... 2

EIGRP overview ........................................................................................................................ 2

OSPF overview ......................................................................................................................... 2

Comparison between EIGRP and OSPF ....................................................................................... 3

Migration procedures .................................................................................................................... 3

Preparations ............................................................................................................................. 3

Determining the network scope ............................................................................................... 3

Checking CPU and memory utilization ..................................................................................... 3

Designing the OSPF network ................................................................................................... 3

Migration procedures ................................................................................................................ 3

OSPF configuration example ...................................................................................................... 4

Connecting an OSPF network to a Cisco IGRP/EIGRP network ........................................................... 5

Summary ..................................................................................................................................... 5

For more information ..................................................................................................................... 6





2
Overview
Open Shortest Path First (OSPF) and Enhanced Interior Gateway Routing Protocol (EIGRP) are both commonly used
Interior Gateway Protocols (IGPs).
Interior Gateway Routing Protocol (IGRP) and EIGRP are the proprietary protocols of Cisco, and have been accepted
by some customers.
OSPF was developed by the Internet Engineering Task Force (IETF). Compared with IGRP or EIGRP, OSPF features
higher efficiency, better interoperability, and faster convergence, and supports more protocol extensions. Devices
from many vendors support OSPF. OSPF has been widely used in large-scale networks and is the IGP recommended
by the IETF.
Therefore, many networks running IGRP/EIGRP need to migrate to OSPF for network expansion and adjustment.
This document describes the steps IT professionals need to follow in order to migrate from Cisco’s proprietary IGRP
and EIGRP protocols to open industry standard OSPF.
IGRP overview
Cisco developed IGRP to solve the problems of Routing Information Protocol (RIP).
IGRP is also a distance-vector routing protocol and has similar operation mechanisms with RIP, such as periodic
updates, triggered updates, split horizon, poison reverse, and hold-down timer. IGRP solves the following problems:

RIP uses hop-count as the metric, which cannot reflect the actual route cost. To solve this problem, IGRP considers
factors such as link bandwidth and delay during metric calculation.

RIP defines a maximum hop count of 16 while IGRP allows a maximum hop count of 255.

The default update interval of IGRP is 90 seconds, which is longer than the 30-second update interval of RIP. Thus,
IGRP consumes less bandwidth for route update.
Although IGRP makes some improvements, it still has the disadvantages of distance vector routing protocols, such as
slow convergence, routing loops, large bandwidth consumption, and inapplicable for large-scare networks.
Therefore, Cisco developed EIGRP to replace IGRP. The current Cisco IOS does not support IGRP anymore.
EIGRP overview
EIGRP inherits the metric calculation method of IGRP, but is totally different from IGRP in other aspects.
EIGRP is also a distance vector routing protocol, which uses an improved algorithm to solve the problems of IGRP and
RIP. EIGRP provides fast convergence and loop-free routes, and is suitable for large-scale networks.
The convergence feature of EIGRP is based on the diffusing update algorithm (DUAL). As the core of EIGRP, the DUAL
algorithm helps ensure that calculated routes are loop free.
OSPF overview
OSPF is a link-state IGP developed by the IETF. It advertises and receives link state information and uses collected link
state information to complete route calculation.
Each OSPF router generates Link State Advertisements (LSAs) that describe local topology information such as
interface and reachable neighbor information, and sends the LSAs throughout the autonomous system (AS).
Meanwhile, each OSPF router collects LSAs from other routers to compose a link state database (LSDB). An LSA
describes the network topology around a router, so the LSDB describes the entire network topology of the AS.
Each OSPF router uses the SPF algorithm to compute a shortest path tree that shows the loop-free routes to the nodes
in the AS. The router itself is the root of the tree.


3
Comparison between EIGRP and OSPF
EIGRP, which is an improvement over IGRP, is no longer used. Therefore, the following only shows the comparison
between EIGRP and OSPF.
Table 1. OSPF versus EIFRP
Item
OSPF
EIGRP
Standardization An IETF standard is mature, supported by most
vendors, and can operate in a network that
comprises devices from different vendors
A Cisco proprietary protocol, which cannot work
with devices from other vendors, has to always be
optimized—like adding support for stub—this is
not as mature as OSPF
Scope An IGP recommended by the IETF; the most
widely used routing protocol
Only used by some networks; less deployed
Algorithm SPF, which features fast convergence and
calculates loop-free routes
DUAL: The intermediate states are unknown, thus
the router may go into SIA state, and queries are
propagated throughout the entire network
Network topology Supports hierarchical network topology, and has
good manageability and scalability.
Does not support hierarchical network topology
Support for
new technologies
Supports OSPF-TE after extension Does not support TE
Migration procedures
Preparations
Determining the network scope
You need to determine the IGRP/EIGRP network scope and design OSPF areas according to the number of
IGRP/EIGRP routers.
Checking CPU and memory utilization
During migration, both IGRP/EIGRP and OSPF run on routers. Therefore, before migration, check that CPU utilization
and memory utilization on each router is less than 50% to ensure the routers can run both routing protocols.
Designing the OSPF network
Decide on the OSPF areas (for a hierarchical network, take the core layer as the backbone area that connects to
other non-backbone areas), which routers belong to the backbone area, the number of routers in each area, and the
positions of Available Bit Rate (ABR) and Autonomous System Boundary Router (ASBR) (OSPF can perform route
summarization only on ABR and ASBRs, and therefore, you need to determine the positions of the EIGRP routers that
perform summarization in the OSPF network). In addition, for broadcast and Non-Broadcast Multiple Access (NBMA)
networks, you need to determine router priorities and router IDs for Designated Router/Backup Designated Router
(DR/BDR) election.
Migration procedures
The simplest migration way is to remove IGRP/EIGRP configurations on routers and then configure OSPF on them.
But this method breaks down all the services in operation. The migration process must minimize impact on
the network.
By following correct steps, you can help ensure network connectivity and service continuity during migration.
The preferences of routing protocols are used during migration (the bigger the preference value, the lower the
priority). Follow these steps to migrate from IGRP/EIGRP to OSPF:
1. Design the OSPF network, such as determining OSPF areas and routing policies.
2. Configure OSPF on IGRP/EIGRP routers: Specify OSPF to have a bigger preference value (lower priority) than
IGRP/EIGRP, enable OSPF on relevant interfaces, and add interfaces to corresponding areas.
3. As OSPF has a lower priority than IGRP/EIGRP, OSPF routes cannot be installed in the forwarding information
base (FIB) table, and IGRP/EIGRP runs normally on routers.


4
4. After OSPF completes convergence (display OSPF routes on routers and ensure that OSPF routes are correct),
change the preference of OSPF or IGRP/EIGRP so that OSPF has a higher priority than IGRP/EIGRP. Then OSPF
routes replaces IGRP/EIGRP routes to guide packet forwarding.
5. Remove IGRP/EIGRP configurations on routers. You can change the preference of OSPF to the default value as
needed (the modification is not needed if only OSPF is used). Then the migration process completes.
There is very little probability that routing loops and black hole routes occur during migration. The configuration and
routing information of IGRP/EIGRP are saved. Therefore, you can restore IGRP/EIGRP configuration as needed before
performing step 5. If the network is very large, make sure that the routers have enough CPU and memory resources to
complete migration. You can use scripts and the Network Management System (NMS) to implement automatic
configuration.
OSPF configuration example

Figure 1. Network diagram


Configure OSPF on the routers: Divide the network into three OSPF areas, and configure Router A and Router B as
ABRs to forward routes between areas.
# Configure Router A
<RouterA> system-view
[RouterA] ospf
[RouterA-ospf-1] area 0
[RouterA-ospf-1-area-0.0.0.0] network 10.1.1.0 0.0.0.255
[RouterA-ospf-1-area-0.0.0.0] quit
[RouterA-ospf-1] area 1
[RouterA-ospf-1-area-0.0.0.1] network 10.2.1.0 0.0.0.255
[RouterA-ospf-1-area-0.0.0.1] quit
[RouterA-ospf-1] quit
Area 0
Eth1/1
10.2.1.2/24
Eth1/2
10.4.1.1/24
Eth1/1
10.1.1.1/24
Eth1/1
10.1.1.2/24
Eth1/2
10.2.1.1/24
Eth1/2
10.3.1.1/24
Eth1/1
10.3.1.2/24
Eth1/2
10.5.1.2/24
Router BRouter A
Router C
Area 1 Area 2
Router D


5
# Configure Router B
<RouterB> system-view
[RouterB] ospf
[RouterB-ospf-1] area 0
[RouterB-ospf-1-area-0.0.0.0] network 10.1.1.0 0.0.0.255
[RouterB-ospf-1-area-0.0.0.0] quit
[RouterB-ospf-1] area 2
[RouterB-ospf-1-area-0.0.0.2] network 10.3.1.0 0.0.0.255
[RouterB-ospf-1-area-0.0.0.2] quit
[RouterB-ospf-1] quit
# Configure Router C
<RouterC> system-view
[RouterC] ospf
[RouterC-ospf-1] area 1
[RouterC-ospf-1-area-0.0.0.1] network 10.2.1.0 0.0.0.255
[RouterC-ospf-1-area-0.0.0.1] network 10.4.1.0 0.0.0.255
[RouterC-ospf-1-area-0.0.0.1] quit
[RouterC-ospf-1] quit
# Configure Router D
<RouterD> system-view
[RouterD] ospf
[RouterD-ospf-1] area 2
[RouterD-ospf-1-area-0.0.0.2] network 10.3.1.0 0.0.0.255
[RouterD-ospf-1-area-0.0.0.2] network 10.5.1.0 0.0.0.255
[RouterD-ospf-1-area-0.0.0.2] quit
[RouterD-ospf-1] quit
Connecting an OSPF network to a Cisco IGRP/EIGRP network
If you do not want to modify IGRP/EIGRP configurations, you can configure OSPF only on routers.
Routers form an OSPF routing domain that communicates with the IGRP/EIGRP routing domain through ASBRs.
Because the ASBRs must run both IGRP/EIGRP and OSPF, only Cisco routers can act as the ASBRs. Then, configure
IGRP/EIGRP and OSPF to redistribute routes from each other on the ASBRs. The ASBR routers must have high
performance because they forward all inter-routing domain traffic. To avoid a single point of failure, configure two or
more ASBRs.
Summary
It is recommended that you migrate from Cisco IGRP/EIGRP to OSPF. Through migration, you can implement the
hierarchical network architecture of OSPF, and avoid the ASBR bottleneck in the solution of connecting an OSPF
network to an IGRP/EIGRP network.



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4AA3-9292ENW, Created March 2012