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This chapter is from the book

This chapter is from the book

Static Routing Implementation (2.1)

As previously stated, static routes are widely used in networks today. Static routes are used in networks of all sizes, and are used along with a dynamic routing protocol. For this reason, a good understanding of static routes is a requirement for implementing routing on a network.

Reach Remote Networks (2.1.1.1)

A router can learn about remote networks in one of two ways:

  • Manually: Remote networks are manually entered into the route table using static routes.
  • Dynamically: Remote routes are automatically learned using a dynamic routing protocol.

Figure 2-1 provides a sample scenario of static routing. Figure 2-2 provides a sample scenario of dynamic routing using EIGRP.

Figure 2-1

Figure 2-1 Static and Default Route Scenario

Figure 2-2

Figure 2-2 Dynamic Routing Scenario

A network administrator can manually configure a static route to reach a specific network. Unlike a dynamic routing protocol, static routes are not automatically updated and must be manually reconfigured any time the network topology changes. A static route does not change until the administrator manually reconfigures it.

Why Use Static Routing? (2.1.1.2)

Static routing provides some advantages over dynamic routing, including:

  • Static routes are not advertised over the network, resulting in better security.
  • Static routes use less bandwidth than dynamic routing protocols, as routers do not exchange routes.
  • No CPU cycles are used to calculate and communicate routes.
  • The path a static route uses to send data is known.

Static routing has the following disadvantages:

  • Initial configuration and maintenance is time-consuming.
  • Configuration can be error-prone, especially in large networks.
  • Administrator intervention is required to maintain changing route information.
  • Does not scale well with growing networks; maintenance becomes cumbersome.
  • Requires complete knowledge of the whole network for proper implementation.

In Table 2-1, dynamic and static routing features are compared. Notice that the advantages of one method are the disadvantages of the other.

Table 2-1 Dynamic Routing Versus Static Routing

Dynamic Routing

Static Routing

Configuration Complexity

Generally independent of the network size

Increases with the network size

Topology Changes

Automatically adapts to topology changes

Administrator intervention required

Scaling

Suitable for simple and complex topologies

Suitable for simple topologies

Security

Less secure

More secure

Resource Usage

Uses CPU, memory, link bandwidth

No extra resources needed

Predictability

Route depends on the current topology

Route to destination is always the same

Static routes are useful for smaller networks with only one path to an outside network. They also provide security in a larger network for certain types of traffic or links to other networks that need more control. It is important to understand that static and dynamic routing are not mutually exclusive. Rather, most networks use a combination of dynamic routing protocols and static routes. This may result in the router having multiple paths to a destination network via static routes and dynamically learned routes. However, the administrative distance (AD) of a static route is 1. Therefore, a static route will take precedence over all dynamically learned routes.

When to Use Static Routes (2.1.1.3)

Static routing has three primary uses:

  • Providing ease of routing table maintenance in smaller networks that are not expected to grow significantly.
  • Routing to and from stub networks. A stub network is a network accessed by a single route, and the router has only one neighbor.
  • Using a single default route to represent a path to any network that does not have a more specific match with another route in the routing table. Default routes are used to send traffic to any destination beyond the next upstream router.

Figure 2-3 shows an example of a stub network connection and a default route connection. Notice in the figure that any network attached to R1 would only have one way to reach other destinations, whether to networks attached to R2, or to destinations beyond R2. This means that network 172.16.3.0 is a stub network and R1 is a stub router. Running a routing protocol between R2 and R1 is a waste of resources.

Figure 2-3

Figure 2-3 Stub Networks and Stub Routers

In this example, a static route can be configured on R2 to reach the R1 LAN. Additionally, because R1 has only one way to send out non-local traffic, a default static route can be configured on R1 to point to R2 as the next hop for all other networks.

Static Route Applications (2.1.2.1)

Static routes are most often used to connect to a specific network or to provide a Gateway of Last Resort for a stub network. They can also be used to:

  • Reduce the number of routes advertised by summarizing several contiguous networks as one static route
  • Create a backup route in case a primary route link fails

The following types of IPv4 and IPv6 static routes will be discussed:

  • Standard static route
  • Default static route
  • Summary static route
  • Floating static route

Standard Static Route (2.1.2.2)

Both IPv4 and IPv6 support the configuration of static routes. Static routes are useful when connecting to a specific remote network.

Figure 2-4 shows that R2 can be configured with a static route to reach the stub network 172.16.3.0/24.

Figure 2-4

Figure 2-4 Connecting to a Stub Network

Default Static Route (2.1.2.3)

A default static route is a route that matches all packets. A default route identifies the gateway IP address to which the router sends all IP packets that it does not have a learned or static route for. A default static route is simply a static route with 0.0.0.0/0 as the destination IPv4 address. Configuring a default static route creates a Gateway of Last Resort.

Default static routes are used:

  • When no other routes in the routing table match the packet destination IP address. In other words, when a more specific match does not exist. A common use is when connecting a company’s edge router to the ISP network.
  • When a router has only one other router to which it is connected. This condition is known as a stub router.

Refer to Figure 2-5 for a sample scenario of implementing default static routing.

Figure 2-5

Figure 2-5 Connecting to a Stub Router

Summary Static Route (2.1.2.4)

To reduce the number of routing table entries, multiple static routes can be summarized into a single summary static route if:

  • The destination networks are contiguous and can be summarized into a single network address.
  • The multiple static routes all use the same exit interface or next-hop IP address.

In Figure 2-6, R1 would require four separate static routes to reach the 172.20.0.0/16 to 172.23.0.0/16 networks. Instead, one summary static route can be configured and still provide connectivity to those networks.

Figure 2-6

Figure 2-6 Using One Summary Static Route

Floating Static Route (2.1.2.5)

Another type of static route is a floating static route. Floating static routes are static routes that are used to provide a backup path to a primary static or dynamic route, in the event of a link failure. The floating static route is only used when the primary route is not available.

To accomplish this, the floating static route is configured with a higher administrative distance than the primary route. Recall that the administrative distance represents the trustworthiness of a route. If multiple paths to the destination exist, the router will choose the path with the lowest administrative distance.

For example, assume that an administrator wants to create a floating static route as a backup to an EIGRP-learned route. The floating static route must be configured with a higher administrative distance than EIGRP. EIGRP has an administrative distance of 90. If the floating static route is configured with an administrative distance of 95, the dynamic route learned through EIGRP is preferred to the floating static route. If the EIGRP-learned route is lost, the floating static route is used in its place.

In Figure 2-7, the Branch router typically forwards all traffic to the HQ router over the private WAN link. In this example, the routers exchange route information using EIGRP. A floating static route, with an administrative distance of 91 or higher, could be configured to serve as a backup route. If the private WAN link fails and the EIGRP route disappears from the routing table, the router selects the floating static route as the best path to reach the HQ LAN.

Figure 2-7

Figure 2-7 Configuring a Backup Route

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