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For routing between routers come work properly in one internetwork, routers must have knowledge of other network IDs or it is in configured v a default route. On large internetworks, the routing tables need to be kept so the the traffic always travels along optimal paths. Exactly how the routing tables space maintained specifies the distinction between static and dynamic routing.

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Static routing is no really a routing protocol. Revolution routing is merely the procedure of manually beginning routes right into a device’s routing table via a configuration file that is loaded when the routing maker starts up. As an alternative, this routes can be gone into by a network administrator that configures the paths manually. Since these manually configured routes don’t adjust after they space configured (unless a human changes them) lock are dubbed ‘static’ routes.

Static routing is the simplest type of routing, however it is a hands-on process.

Use revolution routing once you have actually very few devices to configure (router how to with every network segment to which that is not directly attached. Think about the network presented in figure 5-1. This network has three routers connecting 5 network segment together. Clearly, the just path from Router1 come a host on 172.16.3.0/24 goes with Router2. Likewise, the only path to hosts on subnet 172.16.4.0/24 goes v Router3.

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A tiny network using static routing

Here space the relevant fragments of the construction of these three routers using static routing. Keep in mind that every router configuration have to contain a static route for the networks attached to the other routers, however not those straight attached come the router itself.

Configuration because that Router1:

hostname router1

!

interface ethernet 0

ip resolve 172.16.1.1 255.255.255.0

!

interface ethernet 1

ip attend to 172.16.2.1 255.255.255.0

!

ip course 172.16.3.0 255.255.255.0 172.16.1.2

ip path 172.16.4.0 255.255.255.0 172.16.1.2

ip course 172.16.5.0 255.255.255.0 172.16.1.2

Configuration for Router2:

hostname router2

!

interface ethernet 0

ip deal with 172.16.1.2 255.255.255.0

!

interface ethernet 1

ip attend to 172.16.3.1 255.255.255.0

!

interface ethernet 2

ip attend to 172.16.5.1 255.255.255.0

!

ip course 172.16.2.0 255.255.255.0 172.16.1.1

ip route 172.16.4.0 255.255.255.0 172.16.3.2

Configuration for Router3:

hostname router3

!

interface ethernet 0

ip attend to 172.16.3.2 255.255.255.0

!

interface ethernet 1

ip deal with 172.16.4.1 255.255.255.0

!

ip route 172.16.1.0 255.255.255.0 172.16.3.1

ip path 172.16.2.0 255.255.255.0 172.16.3.1

ip path 172.16.5.0 255.255.255.0 172.16.3.1

Each construction starts by giving the router a name, and also then specifies an IP address and subnet mask because that each the the router’s interfaces. For example, ~ above Router2, the 2nd Ethernet interface (ethernet 1) is assigned the IP attend to 172.16.3.1, and also the netmask 255.255.255.0. Comparable commands specifying addresses and netmasks for neighborhood interfaces will show up in every router configurations, nevertheless of whether a router is participating in a dynamic routing protocol. If the configurations stopped here, each router would have entries in that routing table because that the networks attached to its own interfaces, and also no others. But the configurations walk on to define static courses showing just how to with networks attached come the other routers. Each ip path statement specifies a static path to the destination indicated by the network number and mask by way of the router IP resolve listed. For example, the very first ip course statement in Router3’s configuration specifies a route to 172.16.1.0, with a mask the 255.255.255.0 (172.16.1.0/24) via the router at 172.16.3.1. The static routes, along with the the neighborhood interfaces, let each router populate its routing table with information around each destination and the following hop to with it.

While this tiny network doesn’t require many static courses to accomplish complete connectivity, and also Router1 and also Router3 might get by v static default paths pointing come Router2, that is basic to see that the configurations because that a huge network with hundreds of network segments and routers would certainly be really complex.

And the flaw of revolution Routing

While static routing has advantages over dynamic routing, the is not without the disadvantages. The price of its simplicity is a lack of scalability. For five network segment on three routers, computer an ideal route indigenous every router to every location is no difficult. However, countless networks are much larger. Consider what the routing can look like for a network v 200 network segments interconnected by much more than a dozen routers. To implement static routing, friend would need to compute the next hop for each network segment because that each router, or more than 2,400 routes! together you have the right to see, the job of precomputing routing tables conveniently becomes a burden, and is vulnerable to errors.

Of course, you can argue the this computation need only take place once, when the network is an initial built. Yet what happens when a network segment moves, or is added? if the computation might be reasonably easy, come implement the change, girlfriend would need to update the configuration for every router on the network. If you miss one, in the finest case, segments attached to that router will be unable to reach the moved or included segment. In the worst case, you’ll create a routing loop the affects countless routers.

Finally, due to the fact that static routing is, through definition, static, it cannot use redundant network links to adapt to a failure in the network. Consider what would take place to ours network if we add secondary interface to Router3 and connected it to 172.16.2.0/24, however left routing unchanged. If Router2 fails, Router3 would be unable to it is adapted to the change in the network topology and also would tho be can not to with hosts top top 172.16.1.0/24. This inability to it is adapted to network failures, also when redundant paths are available, and the problems linked with scaling, space the major motivations behind dynamic routing.

Dynamic Routing

Dynamic routing protocols are supported by software program applications running on the routing an equipment (the router) which dynamically discover network destinations and also how to gain to castle and also advertise those destinations to various other routers. This advertisement role allows all the routers to learn around all the destination networks the exist and also how come to those networks.

A router making use of dynamic routing will ‘learn’ the paths to every networks that space directly associated to the device. Next, the router will discover routes from various other routers that run the very same routing protocol (RIP, RIP2, EIGRP, OSPF, IS-IS, BGP etc). Every router will then type through it’s perform of routes and also select one or much more ‘best’ paths for every network location the router knows or has learned.

Dynamic routing protocols will certainly then distribution this ‘best route’ info to other routers to run the very same routing protocol, thereby prolonging the details on what networks exist and also can be reached. This gives dynamic routing protocols the capability to it is adapted to reasonable network topology changes, tools failures or network outages ‘on the fly’.

Advantages of Dynamic Routing

The chief benefits of dynamic routing over revolution routing space scalability and also adaptability. A dynamically routed network have the right to grow more quickly and larger, and also is maybe to it is adapted to changes in the network topology brought around by this expansion or through the fail of one or more network components.

With a dynamic routing protocol, routers learn about the network topology by communicating with various other routers. Every router announces the presence, and also the paths it has available, to the various other routers on the network. Therefore, if you include a new router, or add second segment come an currently router, the various other routers will hear about the addition and change their routing tables accordingly. Friend don’t have to reconfigure the routers to tell them the the network has actually changed. Similarly, if you relocate a network segment, the various other routers will hear about the change. Girlfriend only need to change the construction of the router (or routers) that attach the segment the moved. This reduce the chance that errors will certainly occur.

The ability to learn about changes to the network’s configuration has implications past adding brand-new segments or moving old ones. It also way that the network can change to failures. If a network has redundant paths, then a partial network failure appears to the routers as if some segments acquired moved (they are now reached via alternate paths), and also some segments have actually been eliminated from the network (they are now unreachable). In short, there’s no actual difference between a network failure and a configuration change. Dynamic routing permits the network to proceed functioning, maybe in a degraded fashion, as soon as a partial failure occurs.

And the defect of Dynamic Routing

I would be a liar if ns told you that dynamic routing has actually no disadvantages. Chief among the disadvantages is rise in complexity. Communicating information around network topology is not as an easy as saying, “Hey, I can reach the adhering to destinations…” every router participating in the dynamic routing protocol must decide specifically what information to send to other routers; an ext important, it must attempt to select the best route because that reaching other destinations from the candidates it learns about from various other routers. In addition, if a router is walking to adapt to transforms in the network, it should be prepared to eliminate old or unusable details from that routing table. Exactly how it identify what is old or unusable adds come the complexity of the routing protocol. Unfortunately, the better a protocol handles the various different instances in a network, the more complex it is likely to be. This complexity tends to result in errors in the protocol’s implementation, or differences in how vendors interpret the protocol.

In order to interact information about the topology that the network, routers have to periodically send messages to each various other using a dynamic routing protocol. This messages should be sent across network segments just like any kind of other packets. Yet unlike various other packets in the network, these packets perform not contain any type of information come or from a user. Instead, they contain info that is only useful to the routers. Thus, from the users’ suggest of view, these packets room pure overhead. Top top a low-speed link, this messages can consume much of the accessible bandwidth, specifically if the network is big or unstable.

Finally, part or every one of the devices in a network might be unable to speak any type of dynamic routing protocol, or they may not speak a typical protocol. If the is the case, static routing may be your just option.

With every the disadvantages provided of both static and also dynamic routing, you might be wondering what the best an option is. Only you can say for certain what is finest for her network, yet there is a reasonable center ground that boundaries the complexity of dynamic routing there is no sacrificing that scalability. This center ground is a hybrid scheme, in which component of the network uses static routing and part uses dynamic routing.

Hybrid Routing Schemes

In a hybrid routing scheme, some parts of the network usage static routing, and also some components use dynamic routing. Which components use revolution or dynamic routing is not important, and also many options are possible. Among the most common hybrid schemes is to usage static routing top top the fringes of the network (what ns have dubbed the access networks) and also to usage dynamic routing in the core and distribution networks. The benefit of using static routing in the access networks is the these networks space where her user makers are typically located; this machines regularly have tiny or no assistance for dynamic routing. Additionally, access networks regularly have just one or 2 router attachments, so the load of configuring revolution routing is limited. That may also be feasible to specify nothing an ext than a default path on these stub networks. Since of the limited connections to these networks, you generally don’t should reconfigure routing top top a stub network once it gets moved to a new place in the network.

On the various other hand, distribution and core networks regularly have plenty of router connections, and also therefore numerous different routes to maintain. Therefore, routers in these contents of the network typically can’t acquire by through a default route. Routers (and hosts) in the main parts of the network need finish routing details for the whole network. Furthermore, routers in the core and also distribution networks usually have to be informed of transforms in the connectivity of access networks. While it is certainly feasible to inform each router manually once an change occurs, that is usually easier and much more practical to enable a dynamic routing protocol come propagate the changes.

Another advantage of utilizing static paths in your accessibility networks is control. Relying on the framework of your network administration, you could not be able to control what wake up in access networks. These might be handled by departmental LAN coordinators that carry out not report come you. Once this is the case, it is often much easier to configure static routing with the access networks, and also only operation the dynamic routing protocol between routers and also machines end which girlfriend have straight control. Since a dynamic routing protocol indicates a specific level of trust, that is often safer not to use a dynamic routing protocol v such departmentally controlled access networks. In short, when managing routers no under your direct control, that is finest to usage static routing wherein you can, and to usage dynamic routing just where friend must.

Another kind of hybrid routing framework to consider is based on network bandwidth worries rather than on governmental control. In this type of hybrid structure, you would run dynamic routing protocols on her high-speed LAN and also WAN links, and use static routing for connections that cross lower-speed links. As an example, consider a multi-campus network at a university. Within each campus, there may be multiple routers, interconnected through Ethernets, FDDI, and other high-speed links. However, the link in between the campuses may well be a 56 kbps WAN link. If the two campuses have actually separate network address spaces, the is unlikely the the routes in between the campuses will certainly change. In this case, the only benefit to utilizing dynamic routing in between the campuses is that packets that cannot be yielded to a maker on the other campus due to the fact that of an inner network failure deserve to be stopped prior to they traverse the low-speed link. However, this is the impressive case. The normal instance is that interior network procedure is stable.

With this in mind, it is reasonable to ask exactly how much bandwidth the dynamic routing protocol is taking on the low-speed link. While most routing protocols are designed to minimize the affect of the routing updates on the link, the overhead is non-zero. It may be better to usage dynamic routing within every campus, but to usage static routing for the inter-campus link, whereby bandwidth is at a premium.

Static routing between campuses linked by a slow link may also be worthwhile if the campuses share a usual address an are (perhaps a single class B-sized network aggregate). However, more work might be necessary. The difficulty with a typical address room is that transforms on one campus typically need to be visible to the dynamic routing protocol top top the various other campus. One systems to this trouble is to divide the address room between the 2 (or more) campuses so the each is its own smaller aggregate, as displayed in the complying with table. If you can divide the deal with space, you deserve to simplify revolution routing in between the campuses by working v the aggregates.

Two campuses share a common attend to space

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In the figure, Router1 only demands a static route for the accumulation 172.16.128.0/17. Likewise, Router5 only requirements a static course for the accumulation 172.16.0.0/17. This allows each campus come allocate subnets in their section of the common address space independently, and yet save static routing between the campuses simple.

A final trouble with our multi-campus hybrid routing scheme occurs when the inter-campus connect moves between routers. Because that example, what happens if the attach is relocated from Router1 come Router3? Router configurations on Campus 2 are unaffected, because the very same IP addresses because that the serial link can proceed to be used, and also the changes necessary on Campus 1 require not be extensive. Clearly, Router1 and Router3 must know of the change. ~ all, also without routing to deal with, they need to have interface configuration alters made. However does Router2 should be reconfigured to understand that the link to Campus 2 has moved? no if the routers are all configured to encompass information around their static routes in the dynamic routing protocol.

Most routers have actually some capability to propagate information from one routing source to another. We generally think in state of exchanging routing information in between two dynamic routing protocols, however passing information around static routes right into a dynamic routing protocol is really simply a unique case. Us won’t enter the details right here of how this is done because it will be much easier to describe once we have actually talked about the dynamic routing protocols and how come configure them, yet you have to be mindful that consisting of static courses in her dynamic routing protocol to update is an easy way to confine construction of static routes to a minimum that routers.

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Now the we have actually covered static, dynamic, and hybrid routing schemes, girlfriend should have actually a setup for where you would like to use dynamic routing, and also where you will usage static routing. The next step is to think more about dynamic routing protocols, and decide i beg your pardon are suitable to use.