Eigrp and ospf comparison

EIGRP and OSPF Comparison



TABLE OF CONTENTS

Paragraph & Description Page

1.0 Executive Summary..................................................................................1

2.0 Introduction ..............................................................................................1
2.1 Industry Standard protocols vs. Proprietary protocols: ........................................ 1

3.0 Technical Background .............................................................................2
3.1 Types Of Routing Protocols ................................................................................. 2
3.1.1 Static Routing ........................................................................................... 2
3.1.2 Distance Vector Protocols ........................................................................ 3
3.1.3 Link-State Protocols ................................................................................. 4
3.1.4 Advanced Distance Vector Protocol ........................................................ 4
3.1.5 Path Vector Protocols ............................................................................... 5

4.0 Protocol Decision Criteria .......................................................................5
5.0 OSPF ..........................................................................................................8

6.0 EIGRP......................................................................................................17
7.0 Analysis ....................................................................................................20
8.0 Recommendation ....................................................................................21

9.0 References................................................................................................21
9.1 URLs 22
9.1.1 OSPF....................................................................................................... 22
9.1.2 EIGRP..................................................................................................... 22
9.2 Books 22
9.2.1 OSPF....................................................................................................... 22
9.2.2 EIGRP..................................................................................................... 23


1.0 Executive Summary
The <Client> network is based on the TCP/IP protocol, which permits the efficient routing of
data packets based on their IP address. Cisco routers are used at various points in the
network to control and forward the data. Alcatel OmniSwitch switch/routers are also used in
the Site 2 facilities.

At the current point a decision is being made by <Client> on whether to keep the existing
Alcatel infrastructure in the Site 2 facility or migrate that equipment to similar Cisco
equipment as exists in Site 1. The current Alcatel equipment is experiencing severe
problems such as hardware failures, power supply failures, operating system memory leaks
resulting in reboots. If the decision is made to upgrade the Alcatel switch/routers then an
evaluation will need to be made on what the proper routing protocol <Client> should be
running corporate wide will be needed. This would be an evaluation of suitability of
Enhanced Interior Gateway Routing Protocol (EIGRP) or Open Shortest Path First (OSPF).

In order for the routers to effectively and efficiently distribute data to the users in the field,
the routers must be programmed with the topology of the network. In other words, the
routers must contain a “map” of the other routers in the network and what TCP/IP devices are
connected to them.

There are a number of methods to program the routers with this information and to change
the program as the network changes. The choice of method, or routing protocol is a critical
factor in the success of the network over time. Factors that differentiate one routing protocol
from another include the speed that it adapts to topology changes (convergence), the ability
to choose the best route among multiple routes (route calculation), and the amount of
network traffic that the routing protocol creates.

Based on this evaluation of the suitability of a routing protocol for <Client>’s routed TCP/IP
network the EIGRP routing protocol should be used in the Alcatel routers are upgraded to
Cisco routers. However, if the Alcatel routers are retained for service within the Site 2
campus then <Client> has no alternative but to run OSPF throughout the organization.

2.0 Introduction
Cisco has dominated the router industry for many reasons. One of the most common reasons
is Cisco’s support for a multitude of protocols as well as features in their IOS to enhance a
router’s ability to control traffic and improve performance, and in some cases, save money. It
makes sense for a company that utilizes Cisco routers in their network, to take advantage of
the features and functionality that has helped Cisco become the leader in terms of market
share.

2.1 Industry Standard protocols vs. Proprietary protocols:

A case can be made for both standard as well as proprietary protocols.


STANDARDS PRO: A standards based protocol will theoretically allow routers of different
manufacturers to inter-operate.

STANDARDS CON: Standards based protocols require industry approval for changes.
Historically changes, as well as improvements or advancements, are rare. Changes to the
OSPF RFC have not occurred since 1986.

PROPRIETARY PRO: Owner can advance the protocol to new levels without the
agreement of a consortium of companies resulting in a protocol with the latest in
technological advancements.

PROPRIETARY CON: Protocol is not supported by other vendors requiring the
implementation of a second protocol. Use of a proprietary protocol is only an issue internally
when using multiple vendors for routers, requiring a gateway router to re-distribute routes.
This is generally not an issue with external networks, since exterior gate protocols like BGP
are used when connecting outside.

Proprietary protocol standards compliance is an issue because the <Client> network is
currently comprised of both Cisco and Alcatel routers. The Alcatel routers support RIP v1
and v2, OSPF, and BGP-4 only. They don’t support Cisco’s proprietary EIGRP.

In making a determination as to which routing protocol (stay with RIP v1/v2, OSPF, or
EIGRP) should be used, <Client> has to look at technical as well as the administrative
benefits to be derived from each. It is obvious that RIP v1 needs to be eliminated and that
decision has already been made, so a matrix of features and benefits between OSPF and
EIGRP needs to be developed.

3.0 Technical Background
3.1 Types Of Routing Protocols

3.1.1 Static Routing

The simplest form of routing is static routes. The routing information is preprogrammed by
the network administrator. When changes to the network occur, the route information must
be manually changed throughout the network.

There are a number of advantages to using static routes. Static routing is very resource
efficient, as it routing uses no additional network bandwidth, doesn't use any router CPU
cycles trying to calculate routes, and requires far less memory. It is also the most secure
form of routing protocol.

However, there are a number of disadvantages to static routing that eliminate it as a viable
alternative on the <Client> network. First and foremost, in the rapidly changing topology of
a wireless network, it is impractical for a network administrator to manually program the


routing changes as they occur. Secondly, in the case of a network failure, static routing is
usually not capable of choosing alternate paths.

3.1.2 Distance Vector Protocols

Distance vector protocols such as Routing Information Protocol (RIP), Interior Gateway
Routing Protocol (IGRP), Internetwork Packet Exchange (IPX) RIP, IPX Service
Advertisement Protocol (SAP), and Routing Table Maintenance Protocol (RTMP), broadcast
their complete routing table periodically, regardless of whether the routing table has changed.
This periodic advertisement varies from every 10 seconds for RTMP to every 90 seconds for
IGRP. When the network is stable, distance vector protocols behave well but waste of
bandwidth because of the periodic sending of routing table updates, even when no change
has occurred. When a failure occurs in the network, distance vector protocols do not add
excessive load to the network, but they take a long time to reconverge to an alternate path or
to flush a bad path from the network.

Distance Vector Routing protocols are dynamic. Routers that use distance vector routing
share information, or a routing map, with other routers on the network. As changes to the
network occur, the router with the change propagates the new routing information across the
entire network.

In routing based on distance-vector algorithms, routers periodically pass copies of their entire
routing table to routers that are their immediate neighbors. Each recipient of this information
adds a distance vector (it’s own distance value) to the routing table before it forwards it on to
its neighbors. This process continues in an omni directional manner among connected
routers. Eventually each router on the network learns about all the others and is able to
develop a cumulative network “map.” Each router then knows how to reach any other router,
and any other network connected to the router.

Distance vector routing provides a tremendous advantage over static routing. Routers are
able to discover the state of the network, and to propagate changes as they occur. The most
common, and most ubiquitous of distance vector routing protocols is the Routing Information
Protocol, or RIP.

However, there are also some disadvantages to distance vector routing that preclude its use
on the <Client> network:
Because distance vector routing protocols periodically transmit the entire routing table to
all immediate neighbors, they can add significant traffic. This is particularly problematic
on a wireless network with limited bandwidth.
Distance vector protocols are notoriously slow to converge, or adapt to network topology
changes. After a change to the network, and before all the routers have converged, there
is the probability of routing errors and lost data.
Distance vector routing protocols base their routing decisions on distance, or the number
of “hops” from one network to another. It does not take into consideration the speed or


bandwidth of a network path. Therefore, routers may route traffic through paths that are
suboptimum.

3.1.3 Link-State Protocols

Link-state routing protocols, such as Open Shortest Path First (OSPF), Intermediate System-
to-Intermediate System (IS-IS), and NetWare Link Services Protocol (NLSP), were designed
to address the limitations of distance vector routing protocols (slow convergence and
unnecessary bandwidth usage). Link-state protocols are more complex than distance vector
protocols, and running them adds to the router's overhead. The additional overhead (in the
form of memory utilization and bandwidth consumption when link-state protocols first start
up) constrains the number of neighbors that a router can support and the number of neighbors
that can be in an area. When the network is stable, link-state protocols minimize bandwidth
usage by sending updates only when a change occurs. A hello mechanism ascertains
reachability of neighbors. When a failure occurs in the network, link-state protocols flood
Link-State Advertisements (LSAs) throughout an area. LSAs cause every router within the
failed area to recalculate routes. The fact that LSAs need to be flooded throughout the area
in failure mode and the fact that all routers recalculate routing tables constrain the number of
neighbors that can be in an area.

Link state routing protocols, like distance vector protocols, are dynamic. They propagate
route information across networks. However, they have a number of advantages over
distance vector protocols.

One of the major advantages of link-state routing is that they calculate the best route for data
based on cost rather than distance. The algorithms used to determine cost vary from protocol
to protocol, but it is generally based on a link’s bandwidth. Thus, the router that the data
packet takes to get to its destination is optimized.

Additionally, link state protocols do not transmit their entire topology database across the
network on a periodic basis. Once the network has converged, protocol traffic is limited to
changes in specific links (link state advertisement packets) and keep-alive or “hello” packets.
Finally, convergence times for link state protocols are generally much shorter than for
distance vector protocols. A network based on link-state routing will recognize and adapt to
failures and changes much more quickly.

There are a few disadvantages to link state routing protocols that must be considered. They
are generally much more complex than either static routes or distance-vector routing. This
translates into higher implementation costs, higher CPU utilization, and greater memory
requirements.

3.1.4 Advanced Distance Vector Protocol

Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance vector
protocol that has some of the properties of link-state protocols. Enhanced IGRP addresses
the limitations of conventional distance vector routing protocols (slow convergence and high
bandwidth consumption in a steady state network). When the network is stable, Enhanced


IGRP sends updates only when a change in the network occurs. Like link-state protocols,
Enhanced IGRP uses a hello mechanism to determine the reachability of neighbors. When a
failure occurs in the network, Enhanced IGRP looks for feasible successors by sending
messages to its neighbors. The search for feasible successors can be aggressive in terms of
the traffic it generates (updates, queries and replies) to achieve convergence. This behavior
constrains the number of neighbors that are possible.

3.1.5 Path Vector Protocols

There is really only one Path Vector routing protocol and it is Border Gateway Protocol
version 4 (BGP-4). This is the primary routing protocol used on the Internet to share routing
updates between Autonomous Systems (AS). An Autonomous System is a network under a
single administrative and technical control. ASs are typically defined by the boundaries of a
single company or organizational entity. BGP-4 is typically used between Internet Service
Providers (ISPs) and between companies and the multiple ISPs they use for upstream Internet
connectivity. BGP-4 routers operate in either External BGP (EBGP) or Internal BGP (IBGP)
configurations depending on whether the connectivity is between ASs or within ASs
respectively. Since <Client> currently default routes toward their Internet points of presence
there is little reason for <Client> to use this protocol. Regardless, BGP-4 would not be used
within the corporate network and only in the future would it be used in a limited capacity at
the Internet edges of the <Client> intranet.

4.0 Protocol Decision Criteria
In order to conduct a proper evaluation <Client>’s requirements for a routing protocol should
be documented.

Simplicity of configuration is a significant requirement for <Client>’s selection of a routing
protocol. It must be easy to configure and easy to maintain. <Client>’s IT resources are
currently stretched thinly and complexity of a routing protocol is a primary consideration.
Currently within the Cisco portions of the <Client> network OSPF is being used but only
with all routers being in a single Area 0. This was done for simplicity and to reduce the
complexity of configuring Area Border Routers (ABRs). However, the entire advantage of
OSPF’s hierarchy is not being taken advantage of.

<Client> is using RFC1918 addresses internally such that Site 1 Arizona and
Western regions of the company uses the 172.16.0.0/12 while Site 2 and other parts of the
company use 10.0.0.0/8. The 192.168.0.0/16 is being used for the internal side of the
Internet portals. When considering a TCP/IP routing protocol the IP addressing plays a
significant role in the decision and engineering process. Therefore, it is a requirement that
<Client> use a routing protocol that supports Variable Length Subnet Masking (VLSM).
The RIP version 1 that is being used in Site 2 in a classful routing protocol that does not
support VLSM and it has already been determined that RIP needs to be phased out.

The Site 1 Arizona and the Western regions of the company use Cisco routers while the Site
2 and Eastern regions of the company use Alcatel OmniSwitch switch/routers. Therefore,


only the Cisco portions of the network can use EIGRP because of its proprietary nature. This
issue has already been mentioned previously in this document.

There are some networks within the <Client> enterprise network that use IPX for some
applications. The IPX protocol is used only in Site 2 and a few other locations. The use of
IPX is being deprecated and will be eliminated soon. The questions in exactly when this will
be complete. It should be mentioned that to accomplish this IPX routing within the current
<Client> network IPX for RIP is being used. The EIGRP protocol has the ability to support
not only IP, but IPX and AppleTalk with a single routing protocol. This provides added
functionality combined with simplicity.

Below is a list of criteria that should be considered by <Client> during the routing protocol
selection process.

ADMINISTRATIVE CRITERIA
COST-There is a cost associated with any implementation. The cost in this instance
is the labor needed to implement the protocol.

EASE OF IMPLEMENTATION-The ease of implementation is important because
it is also tied into the cost of the manpower and skills required to implement.

SPEED OF IMPLEMENTATION-The importance of speed is to get to a point of
stability in the <Client> network as soon as possible.

TECHNOLOGICAL CRITERIA
SECURITY- Controlling access to network resources is a primary concern. Some
routing protocols provide techniques that can be used as part of a security strategy.

With some routing protocols, you can insert a filter on the routes being advertised so
that certain routes are not advertised in some parts of the network.

Some routing protocols can authenticate routers that run the same protocol.
Authentication mechanisms are protocol specific and generally weak. In spite of this,
it is worthwhile to take advantage of the techniques that exist. Authentication can
increase network stability by preventing unauthorized routers or hosts from
participating in the routing protocol, whether those devices are attempting to
participate accidentally or deliberately.

CONVERGENCE- When network topology changes, network traffic must reroute
quickly. The phrase "convergence time" describes the time it takes a router to start
using a new route after a topology changes.

Routers must do three things after a topology changes:

Detect the change


Select a new route

Propagate the changed route information

ROUTE SELECTION- Routing protocols compare route metrics to select the best
route from a group of possible routes. Route metrics are computed by assigning a
characteristic or set of characteristics to each physical network. The metric for the
route is an aggregation of the characteristics of each physical network in the route.

SCALABILITY- The ability to extend your internetwork is determined, in part, by
the scaling characteristics of the routing protocols used and the quality of the network
design.

Network scalability is limited by two factors: operational issues and technical issues.
Typically, operational issues are more significant than technical issues. Operational
scaling concerns encourage the use of large areas or protocols that do not require
hierarchical structures. When hierarchical protocols are required, technical scaling
concerns promote the use of small areas.

ROUTE SUMMARIZATION-. With summarization, routers can reduce some sets
of routes to a single advertisement, reducing both the load on the router and the
perceived complexity of the network. The importance of route summarization
increases with network size.

MEMORY- Routing protocols use memory to store routing tables and topology
information. Route summarization cuts memory consumption for all routing
protocols. Keeping areas small reduces the memory consumption for hierarchical
routing protocols.

CPU REQUIREMENTS- CPU usage is protocol dependent. Some protocols use
CPU cycles to compare new routes to existing routes. Other protocols use CPU cycles
to regenerate routing tables after a topology change. In most cases, the latter
technique will use more CPU cycles than the former. For link-state protocols, keeping
areas small and using summarization reduces CPU requirements by reducing the
effect of a topology change and by decreasing the number of routes that must be
recomputed after a topology change.

BANDWIDTH REQUIREMENTS- Bandwidth usage is also protocol dependent.
Three key issues determine the amount of bandwidth a routing protocol consumes:

When routing information is sent---Periodic updates are sent at regular intervals.
Flash updates are sent only when a change occurs.

What routing information is sent---Complete updates contain all routing
information. Partial updates contain only changed information.


Where routing information is sent---Flooded updates are sent to all routers.
Bounded updates are sent only to routers that are affected by a change.

Note: These three issues also affect CPU usage.

5.0 OSPF
OSPF is an Interior Gateway Protocol (IGP) developed for use in Internet Protocol (IP)-
based internetworks. As an IGP, OSPF distributes routing information between routers
belonging to a single autonomous system (AS). An AS is a group of routers exchanging
routing information via a common routing protocol. The OSPF protocol is based on shortest-
path-first, or link-state, technology.

Two design activities are critically important to a successful OSPF implementation:

Definition of area boundaries

Address assignment

Ensuring that these activities are properly planned and executed will make all the difference
in an OSPF implementation. Each is addressed in more detail with the discussions that
follow. These discussions are divided into six sections:

OSPF Network Topology

OSPF Addressing and Route Summarization

OSPF Route Selection

OSPF Convergence

OSPF Network Scalability

OSPF Security

OSPF Network Topology

OSPF works best in a hierarchical routing environment. The first and most important
decision when designing an OSPF network is to determine which routers and links are to be
included in the backbone and which are to be included in each area.

There are several important guidelines to consider when designing an OSPF topology:

The number of routers in an area---OSPF uses a CPU-intensive algorithm. The number of
calculations that must be performed given n link-state packets is proportional to n log n.
As a result, the larger and more unstable the area, the greater the likelihood for


performance problems associated with routing protocol recalculation. Generally, an area
should have no more than 50 routers. Areas with unstable links should be smaller.

The number of neighbors for any one router---OSPF floods all link-state changes to all
routers in an area. Routers with many neighbors have the most work to do when link-state
changes occur. In general, any one router should have no more than 60 neighbors.

The number of areas supported by any one router---A router must run the link-state
algorithm for each link-state change that occurs for every area in which the router resides.
Every area border router is in at least two areas (the backbone and one area). In general,
to maximize stability, one router should not be in more than three areas.

Designated router selection---In general, the designated router and backup designated
router on a local-area network (LAN) have the most OSPF work to do. It is a good idea to
select routers that are not already heavily loaded with CPU-intensive activities to be the
designated router and backup designated router. In addition, it is generally not a good
idea to select the same router to be designated router on many LANs simultaneously.

Backbone Considerations

Stability and redundancy are the most important criteria for the backbone. Keeping the size
of the backbone reasonable increases stability. This is caused by the fact that every router in
the backbone needs to re-compute its routes after every link-state change. Keeping the
backbone small reduces the likelihood of a change and reduces the amount of CPU cycles
required to re-compute routes. As a general rule, each area (including the backbone) should
contain no more than 50 routers. If link quality is high and the number of routes is small, the
number of routers can be increased.

Redundancy is important in the backbone to prevent partition when a link fails. Good
backbones are designed so that no single link failure can cause a partition.

OSPF backbones must be contiguous. All routers in the backbone should be directly
connected to other backbone routers. OSPF includes the concept of virtual links. A virtual
link creates a path between two area border routers (an area border router is a router connects
an area to the backbone) that are not directly connected. A virtual link can be used to heal a
partitioned backbone. However, it is not a good idea to design an OSPF network to require
the use of virtual links. The stability of a virtual link is determined by the stability of the
underlying area. This dependency can make troubleshooting more difficult. In addition,
virtual links cannot run across stub areas. See the section "Backbone-to-Area Route
Advertisement," later in this chapter for a detailed discussion of stub areas.

Avoid placing hosts (such as workstations, file servers or other shared resources) in the
backbone area. Keeping hosts out of the backbone area simplifies internetwork expansion
and creates a more stable environment.

Area Considerations


Individual areas must be contiguous. In this context, a contiguous area is one in which a
continuous path can be traced from any router in an area to any other router in the same area.
This does not mean that all routers must share a common network media. It is not possible to
use virtual links to connect a partitioned area. Ideally, areas should be richly connected
internally to prevent partitioning.

The two most critical aspects of area design follow:

Determining how the area is addressed

Determining how the area is connected to the backbone

Areas should have a contiguous set of network and/or subnet addresses. Without a
contiguous address space, it is not possible to implement route summarization. The
routers that connect an area to the backbone are called area border routers. Areas can have a
single area border router or they can have multiple area border routers. In general, it is
desirable to have more than one area border router per area to minimize the chance of the
area becoming disconnected from the backbone.

When creating large-scale OSPF internetworks, the definition of areas and assignment of
resources within areas must be done with a pragmatic view of your internetwork. The
following are general rules that will help ensure that your internetwork remains flexible and
provides the kind of performance needed to deliver reliable resource access.

Consider physical proximity when defining areas---If a particular location is densely
connected, create an area specifically for nodes at that location.

Reduce the maximum size of areas if links are unstable---If your internetwork includes
unstable links, consider implementing smaller areas to reduce the effects of route
flapping. Whenever a route is lost or comes online, each affected area must converge on a
new topology. The Dykstra algorithm will run on all the affected routers. By segmenting
your internetwork into smaller areas, you can isolate unstable links and deliver more
reliable overall service.

OSPF Addressing and Route Summarization

Address assignment and route summarization are inextricably linked when designing OSPF
internetworks. To create a scalable OSPF internetwork, you should implement route
summarization. To create an environment capable of supporting route summarization, you
must implement an effective hierarchical addressing scheme. The addressing structure that
you implement can have a profound impact on the performance and scalability of your OSPF
internetwork. The following sections discuss OSPF route summarization and three
addressing options:


Separate network numbers for each area

NetworkInformationCenter (NIC)-authorized address areas created using bit-wise
subnetting and VLSM

Private addressing, with a "demilitarized zone" (DMZ) buffer to the official Internet
world

Note: You should keep your addressing scheme as simple as possible, but be wary of
oversimplifying your address assignment scheme. Although simplicity in addressing saves
time later when operating and troubleshooting your network, taking short cuts can have
certain severe consequences. In building a scalable addressing environment, use a structured
approach. If necessary, use bit-wise subnetting---but make sure that route summarization can
be accomplished at the area border routers.

OSPF Route Summarization

Route summarization is extremely desirable for a reliable and scalable OSPF internetwork.
The effectiveness of route summarization, and your OSPF implementation in general, hinges
on the addressing scheme that you adopt. Summarization in an OSPF internetwork occurs
between each area and the backbone area. Summarization must be configured manually in
OSPF.

When planning your OSPF internetwork, consider the following issues:

Be sure that your network addressing scheme is configured so that the range of subnets
assigned within an area is contiguous.

Create an address space that will permit you to split areas easily as your network grows. If
possible, assign subnets according to simple octet boundaries. If you cannot assign
addresses in an easy-to-remember and easy-to-divide manner, be sure to have a thoroughly
defined addressing structure. If you know how your entire address space is assigned (or
will be assigned), you can plan for changes more effectively.

Plan ahead for the addition of new routers to your OSPF environment. Be sure that new
routers are inserted appropriately as area, backbone, or border routers. Because the addition
of new routers creates a new topology, inserting new routers can cause unexpected routing
changes (and possibly performance changes) when your OSPF topology is recomputed.

Separate Address Structures for Each Area

One of the simplest ways to allocate addresses in OSPF is to assign a separate network
number for each area. With this scheme, you create a backbone and multiple areas, and
assign a separate IP network number to each area.

The following are some clear benefits of assigning separate address structures to each area:


Address assignment is relatively easy to remember.

Configuration of routers is relatively easy and mistakes are less likely.

Network operations are streamlined because each area has a simple, unique network
number.

Bit-Wise Subnetting and VLSM

Bit-wise subnetting and variable-length subnetwork masks (VLSMs) can be used in
combination to save address space. Consider a hypothetical network where a Class B address
is subdivided using an area mask and distributed among 16 areas.

Route Summarization Techniques

Route summarization is particularly important in an OSPF environment because it increases
the stability of the network. If route summarization is being used, routes within an area that
change do not need to be changed in the backbone or in other areas.

Route summarization addresses two important questions of route information distribution:

What information does the backbone need to know about each area? The answer to this
question focuses attention on area-to-backbone routing information.

What information does each area need to know about the backbone and other areas? The
answer to this question focuses attention on backbone-to-area routing information.

Area-to-Backbone Route Advertisement

There are several key considerations when setting up your OSPF areas for proper
summarization:

OSPF route summarization occurs in the area border routers.

OSPF supports VLSM, so it is possible to summarize on any bit boundary in a network or
subnet address.

OSPF requires manual summarization. As you design the areas, you need to determine
summarization at each area border router.

Backbone-to-Area Route Advertisement

There are four potential types of routing information in an area:


Default. If an explicit route cannot be found for a given IP network or subnetwork, the
router will forward the packet to the destination specified in the default route.

Intra-area routes. Explicit network or subnet routes must be carried for all networks or
subnets inside an area.

Inter-area routes. Areas may carry explicit network or subnet routes for networks or
subnets that are in this AS but not in this area.

External routes. When different AS’s exchange routing information, the routes they
exchange are referred to as external routes.

In general, it is desirable to restrict routing information in any area to the minimal set that the
area needs.

There are three types of areas, and they are defined in accordance with the routing
information that is used in them:

Non-stub areas---Non-stub areas carry a default route, static routes, intra-area routes,
inter-area routes and external routes. An area must be a non-stub area when it contains a
router that uses both OSPF and any other protocol, such as the Routing Information
Protocol (RIP). Such a router is known as an autonomous system border router (ASBR).
An area must also be a non-stub area when a virtual link is configured across the area.
Non-stub areas are the most resource-intensive type of area.

Stub areas---Stub areas carry a default route, intra-area routes and inter-area routes, but
they do not carry external routes. Stub areas are recommended for areas that have only
one area border router and they are often useful in areas with multiple area border routers.
See "Controlling Inter-area Traffic," later in this chapter for a detailed discussion of the
design trade-offs in areas with multiple area border routers. There are two restrictions on
the use of stub areas: virtual links cannot be configured across them, and they cannot
contain an ASBR.

Stub areas without summaries---Software releases 9.1(11), 9.21(2), and 10.0(1) and later
support stub areas without summaries, allowing you to create areas that carry only a
default route and intra-area routes. Stub areas without summaries do not carry inter-area
routes or external routes. This type of area is recommended for simple configurations
where a single router connects an area to the backbone.

OSPF Route Selection

When designing an OSPF internetwork for efficient route selection, consider three important
topics:

Tuning OSPF Metrics


Controlling Inter-area Traffic

Load Balancing in OSPF Internetworks

Tuning OSPF Metrics

The default value for OSPF metrics is based on bandwidth. The following characteristics
show how OSPF metrics are generated:

Each link is given a metric value based on its bandwidth. The metric for a specific link is
the inverse of the bandwidth for that link. Link metrics are normalized to give Fast
Ethernet a metric of 1. The metric for a route is the sum of the metrics for all the links in
the route.

Note: In some cases, your network might implement a media type that is faster than the
fastest default media configurable for OSPF (Fast Ethernet). An example of a faster media is
ATM. By default, a faster media will be assigned a cost equal to the cost of an Fast Ethernet
link---a link-state metric cost of 1. Given an environment with both Fast Ethernet and a faster
media type, you must manually configure link costs to configure the faster link with a lower
metric. Configure any Fast Ethernet link with a cost greater than 1, and the faster link with a
cost less than the assigned Fast Ethernet link cost. Use the “ipospf cost” interface
configuration command to modify link-state cost.

When route summarization is enabled, OSPF uses the metric of the best route in the
summary.

There are two forms of external metrics: type 1 and type 2. Using an external type 1
metric results in routes adding the internal OSPF metric to the external route metric.
External type 2 metrics do not add the internal metric to external routes. The external
type 1 metric is generally preferred. If you have more than one external connection, either
metric can affect how multiple paths are used.

Controlling Inter-area Traffic

When an area has only a single area border router, all traffic that does not belong in the area
will be sent to the area border router.

In areas that have multiple area border routers, two choices are available for traffic that needs
to leave the area:

Use the area border router closest to the originator of the traffic. (Traffic leaves the area
as soon as possible.)

Use the area border router closest to the destination of the traffic. (Traffic leaves the area
as late as possible.)


If the area border routers inject only the default route, the traffic goes to the area border
router that is closest to the source of the traffic. Generally, this behavior is desirable because
the backbone typically has higher bandwidth lines available. However, if you want the traffic
to use the area border router that is nearest the destination (so that traffic leaves the area as
late as possible), the area border routers should inject summaries into the area instead of just
injecting the default route.
Most network designers prefer to avoid asymmetric routing (that is, using a different path for
packets that are going from A to B than for those packets that are going from B to A.) It is
important to understand how routing occurs between areas to avoid asymmetric routing.

Load Balancing in OSPF Internetworks

Internetwork topologies are typically designed to provide redundant routes in order to
prevent a partitioned network. Redundancy is also useful to provide additional bandwidth for
high traffic areas. If equal-cost paths between nodes exist, Cisco routers automatically load
balance in an OSPF environment.

OSPF Convergence

One of the most attractive features about OSPF is the ability to quickly adapt to topology
changes.

There are two components to routing convergence:

Detection of topology changes---OSPF uses two mechanisms to detect topology changes.
Interface status changes (such as carrier failure on a serial link) is the first mechanism.
The second mechanism is failure of OSPF to receive a hello packet from its neighbor
within a timing window called a dead timer. Once this timer expires, the router assumes
the neighbor is down. The dead timer is configured using the ipospf dead-interval
interface configuration command. The default value of the dead timer is four times the
value of the Hello interval. That results in a dead timer default of 40 seconds for
broadcast networks and 2 minutes for nonbroadcast networks.

Recalculation of routes---Once a failure has been detected, the router that detected the
failure sends a link-state packet with the change information to all routers in the area. All
the routers recalculate all of their routes using the Dykstra (or SPF) algorithm. The time
required to run the algorithm depends on a combination of the size of the area and the
number of routes in the database.

OSPF Network Scalability

Your ability to scale an OSPF internetwork depends on your overall network structure and
addressing scheme. As outlined in the preceding discussions concerning network topology
and route summarization, adopting a hierarchical addressing environment and a structured
address assignment will be the most important factors in determining the scalability of your
internetwork.


Network scalability is affected by operational and technical considerations:

Operationally, OSPF networks should be designed so that areas do not need to be split to
accommodate growth. Address space should be reserved to permit the addition of new
areas.

Technically, scaling is determined by the utilization of three resources: memory, CPU,
and bandwidth.

Memory

An OSPF router stores all of the link states for all of the areas that it is in. In addition, it can
store summaries and externals. Careful use of summarization and stub areas can reduce
memory use substantially.

CPU

An OSPF router uses CPU cycles whenever a link-state change occurs. Keeping areas small
and using summarization dramatically reduces CPU use and creates a more stable
environment for OSPF.

Bandwidth

OSPF sends partial updates when a link-state change occurs. The updates are flooded to all
routers in the area. In a quiet network, OSPF is a quiet protocol. In a network with substantial
topology changes, OSPF minimizes the amount of bandwidth used.

OSPF Security

Two kinds of security are applicable to routing protocols:

Controlling the routers that participate in an OSPF network

OSPF contains an optional authentication field. All routers within an area must agree on the
value of the authentication field. Because OSPF is a standard protocol available on many
platforms, including some hosts, using the authentication field prevents the inadvertent
startup of OSPF in an uncontrolled platform on your network and reduces the potential for
instability.

Controlling the routing information that routers exchange

All routers must have the same data within an OSPF area. As a result, it is not possible to use
route filters in an OSPF network to provide security.


6.0 EIGRP
The Enhanced Interior Gateway Routing Protocol (Enhanced IGRP) is a routing protocol
developed by Cisco Systems and introduced with Software Release 9.21 and Cisco
Internetworking Operating System (Cisco IOS) Software Release 10.0. Enhanced IGRP
combines the advantages of distance vector protocols, such as IGRP, with the advantages of
link-state protocols, such as Open Shortest Path First (OSPF). Enhanced IGRP uses the
Diffusing Update Algorithm (DUAL) to achieve convergence quickly.

Enhanced IGRP includes support for IP, Novell NetWare, and AppleTalk. The discussion on
Enhanced IGRP covers the following topics:

Enhanced IGRP Network Topology

Enhanced IGRP Addressing

Enhanced IGRP Route Summarization

Enhanced IGRP Route Selection

Enhanced IGRP Convergence

Enhanced IGRP Network Scalability

Enhanced IGRP Security

Enhanced IGRP Network Topology

Enhanced IGRP uses a nonhierarchical (or flat) topology by default. Enhanced IGRP
automatically summarizes subnet routes of directly connected networks at a network number
boundary. This automatic summarization is sufficient for most IP networks. See the section
"Enhanced IGRP Route Summarization" later in this chapter for more detail.

Enhanced IGRP Addressing

The first step in designing an Enhanced IGRP network is to decide on how to address the
network. In many cases, a company is assigned a single NIC address (such as a Class B
network address) to be allocated in a corporate internetwork. Bit-wise subnetting and
variable-length subnetwork masks (VLSM’s) can be used in combination to save address
space. Enhanced IGRP for IP supports the use of VLSM’s.

Enhanced IGRP Route Summarization

With Enhanced IGRP, subnet routes of directly connected networks are automatically
summarized at network number boundaries. In addition, a network administrator can


configure route summarization at any interface with any bit boundary, allowing ranges of
networks to be summarized arbitrarily.

Enhanced IGRP Route Selection

Routing protocols compare route metrics to select the best route from a group of possible
routes. The following factors are important to understand when designing an Enhanced IGRP
internetwork.

Enhanced IGRP uses the same vector of metrics as IGRP. Separate metric values are
assigned for bandwidth, delay, reliability and load. By default, Enhanced IGRP computes the
metric for a route by using the minimum bandwidth of each hop in the path and adding a
media-specific delay for each hop. The metrics used by Enhanced IGRP are as follows:

Bandwidth-Bandwidth is deduced from the interface type. Bandwidth can be modified
with the bandwidth command.

Delay-Each media type has a propagation delay associated with it. Modifying delay is
very useful to optimize routing in network with satellite links. Delay can be modified
with the delay command.

Reliability-Reliability is dynamically computed as a rolling weighted average over five
seconds.

Load-Load is dynamically computed as a rolling weighted average over five seconds.

When Enhanced IGRP summarizes a group of routes, it uses the metric of the best route in
the summary as the metric for the summary.

Enhanced IGRP Convergence

Enhanced IGRP implements a new convergence algorithm known as DUAL (Diffusing
Update Algorithm). DUAL uses two techniques that allow Enhanced IGRP to converge very
quickly. First, each Enhanced IGRP router stores its neighbors routing tables. This allows the
router to use a new route to a destination instantly if another feasible route is known. If no
feasible route is known based upon the routing information previously learned from its
neighbors, a router running Enhanced IGRP becomes active for that destination and sends a
query to each of its neighbors asking for an alternate route to the destination. These queries
propagate until an alternate route is found. Routers that are not affected by a topology change
remain passive and do not need to be involved in the query and response.

A router using Enhanced IGRP receives full routing tables from its neighbors when it first
communicates with the neighbors. Thereafter, only changes to the routing tables are sent and
only to routers that are affected by the change. A successor is a neighboring router that is
currently being used for packet forwarding, provides the least cost route to the destination,
and is not part of a routing loop. Information in the routing table is based on feasible


successors. Feasible successor routes can be used in case the existing route fails. Feasible
successors provide the next least-cost path without introducing routing loops.

The routing table keeps a list of the computed costs of reaching networks. The topology table
keeps a list of all routes advertised by neighbors. For each network, the router keeps the real
cost of getting to that network and also keeps the advertised cost from its neighbor. In the
event of a failure, convergence is instant if a feasible successor can be found. A neighbor is a
feasible successor if it meets the feasibility condition set by DUAL. DUAL finds feasible
successors by the performing the following computations:

Enhanced IGRP Network Scalability

Network scalability is limited by two factors: operational issues and technical issues.
Operationally, Enhanced IGRP provides easy configuration and growth. Technically,
Enhanced IGRP uses resources at less than a linear rate with the growth of a network.

Memory

A router running Enhanced IGRP stores all routes advertised by neighbors so that it can
adapt quickly to alternate routes. The more neighbors a router has, the more memory a router
uses. Enhanced IGRP automatic route aggregation bounds the routing table growth naturally.
Additional bounding is possible with manual route aggregation.

CPU

Enhanced IGRP uses the DUAL algorithm to provide fast convergence. DUAL re-computes
only routes, which are affected by a topology change. DUAL is not computationally
complex, so it does not require a lot of CPU.

Bandwidth

Enhanced IGRP uses partial updates. Partial updates are generated only when a change
occurs; only the changed information is sent, and this changed information is sent only to the
routers affected. Because of this, Enhanced IGRP is very efficient in its usage of bandwidth.
Some additional bandwidth is used by Enhanced IGRP's HELLO protocol to maintain
adjacencies between neighboring routers.

Enhanced IGRP Security

Enhanced IGRP is available only on Cisco routers. This prevents accidental or malicious
routing disruption caused by hosts in a network.

In addition, route filters can be set up on any interface to prevent learning or propagating
routing information inappropriately.


7.0 Analysis
Now that the <Client> requirements as well as the technical merits and downfalls of the
routing protocols have been defined an analysis needs to be conducted of this information.

The Open Shortest Path First Protocol is an “open standard.” This means that it can be
implemented on any platform, from any vendor or manufacturer. This is an advantage over
Enhanced Interior Gateway Protocol, which is a proprietary standard from Cisco. However,
this is the only clear advantage of OSPF over EIGRP.
As previously stated, OSPF is designed primarily for hierarchical networks with a clearly
defined backbone area. This is clearly not the case in the <Client> network. In addition,
when compared to EIGRP, OSPF uses more bandwidth to propagate its topology requires
more router CPU time and memory. OSPF is also more difficult, and therefore more costly,
to implement that EIGRP.

Enhanced Interior Gateway Protocol is a proprietary routing protocol developed by Cisco and
used exclusively in their routing products. Although it is often lumped in with OSPF as a
link state protocol, it is actually a hybrid; containing the best elements of both link state and
distance vector protocols.

EIGRP, as stated previously, has several advantages over OSPF when used in the <Client>
network. A brief summarization of these advantages include:

Improved router memory and CPU utilization when compared to OSPF
Intelligent bandwidth control – EIGRP takes into consideration the available bandwidth
when determining the rate at which it will transmit updates. Interfaces can also be
configured to use a certain (maximum) percentage of the bandwidth, so that even during
routing topology computations, a defined portion of the link capacity remains available
for data traffic.
EIGRP does not require a hierarchical network design to operate efficiently. It will
automatically summarize routes where applicable.
Unlike OSPF, which only takes bandwidth into consideration when calculating the cost
of a route, EIGRP can be configured to use bandwidth, delay, reliability, and load when
calculating optimum routes. This has proven to be a valuable consideration in a wireless
environment.
EIGRP has greater control on timing issues, such as hold times and hello intervals, than
does OSPF. This allows greater flexibility with wireless connections, where these
intervals must be fine-tuned to a particular device or bandwidth.
EIGRP is less complex and has less cost (manpower and time) involved in configuration
and administration.
Although EIGRP is proprietary, it can communicate and redistribute routing information
with other routing protocols, such as OSPF. This is accomplished through router
redistribution or using an exterior routing protocol such as BGP.

Given all of this data and analysis a table is used to consolidate the issues and synthesize


Issue EIGRP OSPF
Ease of Implementation Easy, but remember “no Complicated
auto-summary”
Support of IPX and Yes No
AppleTalk
Standards-based Cisco Proprietary IETF Open Standard
Hierarchical Design No – summary statements Yes – hierarchy is part of
on interfaces the design
VLSM Support Yes Yes
Protocol Type Enhanced Distance Vector LinkState
Routing Metrics Combination of bandwidth, Link
delay, reliability and load 10^8/Interface_Bandwidth
CPU Requirements Lower CPU and memory Higher CPU and memory
requirements requirements
Maturity Since 1986 Since 1986
Stability Excellent Excellent

The cells that are highlighted in green are attributes that are advantageous given <Client>’s
requirements.

8.0 Recommendation
Lucent Technologies Worldwide Services feels confident in strongly recommending a
migration to Enhanced IGRP if an all Cisco network is deployed. OSPF only supports IP
where as EIGRP supports IP, IPX, and Apple Talk. The routers can be set up to support
OSPF for IP and EIGRP for IPX, but it is not clear what advantage there would be to that
configuration. Based on the following criteria:

Protocols used at <Client> (IP, IPX, but no AppleTalk)
The time and effort it takes to implement
Requirement for VLSM support
Time to maintain and support
Cisco versus Alcatel equipment

Lucent believes that EIGRP can be implemented much quicker than OSPF as well as provide
the functionality that will provide the stabilization <Client> is looking for. After
implementing EIGRP, and <Client> still has a desire to use OSPF, it can be accomplished in
a time frame that will be much more manageable because the environment will be more
stable.

At this time, EIGRP is the clear choice for the <Client> network. It is faster and easier to
implement, it is more configurable, and performs better in a wireless environment.

9.0 References


9.1 URLs

Designing Large Scale IP Networks:
http://www.cisco.com/univercd/cc/td/doc/cisintwk/idg4/nd2003.htm

9.1.1 OSPF

http://www.cisco.com/cgi-bin/Support/PSP/psp_view.pl?p=Internetworking:OSPF

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/ospf.htm

http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/12cgcr/np1_c/1cprt1/1cos
pf.htm

Networkers 2000:
http://www.cisco.com/networkers/nw00/pres/#2204

Troubleshooting OSPF:

9.1.2 EIGRP

http://www.cisco.com/cgi-bin/Support/PSP/psp_view.pl?p=Internetworking:EIGRP

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/en_igrp.htm

http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/12cgcr/np1_c/1cprt1/1cei
grp.htm

Networkers 2000:


9.2 Books

Routing TCP/IP Volume I (CCIE Professional Development), by Jeff Doyle, Cisco Press,
ISBN: 1578700418

9.2.1 OSPF

OSPF Network Design Solutions, by Tom Thomas, Cisco Press, ISBN: 1578700469

OSPF Anatomy of an Internet Routing Protocol, by John T Moy, Addison Wesley, ISBN:
0201634724


9.2.2 EIGRP

EIGRP Network Design Solutions: The Definitive Resource for EIGRP Design, Deployment,
and Operation
by Ivan Pepelnjak, Cisco Press, ISBN: 1578701651

EIGRP for IP: Basic Operation and Configuration
by Alvaro Retana, et al, Addison Wesley, ISBN: 0201657732

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