EIGRP and OSPF Comparison
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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 Network Scalability
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.
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.
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
Network Information Center (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 “ip ospf 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.
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 ip ospf 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.
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.
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.
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.
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.
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.
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.
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.
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.
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
Ease of Implementation
Easy, but remember “no auto-summary”
Support of IPX and AppleTalk
IETF Open Standard
No – summary statements on interfaces
Yes – hierarchy is part of the design
Enhanced Distance Vector
Combination of bandwidth, delay, reliability and load
Lower CPU and memory requirements
Higher CPU and memory requirements
The cells that are highlighted in green are attributes that are advantageous given <Client>’s requirements.
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.
Designing Large Scale IP Networks:
Routing TCP/IP Volume I (CCIE Professional Development), by Jeff Doyle, Cisco Press, ISBN: 1578700418
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
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