Chapter 2 terms or words to know

Use STATIC route to...

* Connect to a specific network * Connect a stub router * Summarize routing table entries * Create a backup route

Default Static Route

A default route is a route that matches all packets and is used by the router if a packet does not match any other, more specific route in the routing table. A default route can be dynamically learned or statically configured. 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. In this situation, the router is known as a stub router. Refer to the figure for a stub network default route scenario.

what is a stub network

A stub network, or pocket network, is a somewhat casual term describing a computer network, or part of an internetwork, with no knowledge of other networks, that will typically send much or all of its non-local traffic out via a single path, with the network aware only of a default route to non-local destinations.

Floating Static Route

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. 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 the figure, 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.

Static Route Applications

As shown in the figure, 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

what does EIGRP stand for

Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance-vector routing protocol that is used on a computer network for automating routing decisions and configuration. The protocol was designed by Cisco Systems as a proprietary protocol, available only on Cisco routers. ADDITION (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.)

Configure a Fully Specified Static Route

Fully Specified Static Route In a fully specified static route, both the exit interface and the next-hop IP address are specified. This is another type of static route that is used in older IOSs, prior to CEF. This form of static route is used when the exit interface is a multi-access interface and it is necessary to explicitly identify the next hop. The next hop must be directly connected to the specified exit interface. Suppose that the network link between R1 and R2 is an Ethernet link and that the GigabitEthernet 0/1 interface of R1 is connected to that network, as shown in Figure 1. CEF is not enabled. To eliminate the recursive lookup, a directly connected static route can be implemented using the following command: R1(config)# ip route 192.168.2.0 255.255.255.0 GigabitEthernet 0/1 However, this may cause unexpected or inconsistent results. The difference between an Ethernet multi-access network and a point-to-point serial network is that a point-to-point serial network has only one other device on that network, the router at the other end of the link. With Ethernet networks, there may be many different devices sharing the same multi-access network, including hosts and even multiple routers. By only designating the Ethernet exit interface in the static route, the router will not have sufficient information to determine which device is the next-hop device. R1 knows that the packet needs to be encapsulated in an Ethernet frame and sent out the GigabitEthernet 0/1 interface. However, R1 does not know the next-hop IPv4 address; therefore, it cannot determine the destination MAC address for the Ethernet frame. Depending upon the topology and the configurations on other routers, this static route may or may not work. It is recommended that when the exit interface is an Ethernet network, that a fully specified static route is used, including both the exit interface and the next-hop address. As shown in Figure 2, when forwarding packets to R2, the exit interface is GigabitEthernet 0/1 and the next-hop IPv4 address is 172.16.2.2. Note: With the use of CEF, a fully specified static route is no longer necessary. A static route using a next-hop address should be used. Use the Syntax Checker in Figure 3 and 4 to configure and verify fully specified static routes on R2 and R3.

Configure a Next-Hop Static Route

In a next-hop static route, only the next-hop IP address is specified. The exit interface is derived from the next hop. For example, in Figure 1, three next-hop static routes are configured on R1 using the IP address of the next hop, R2. Before any packet is forwarded by a router, the routing table process must determine the exit interface to use to forward the packet. This is known as route resolvability. Figure 2 details the basic packet forwarding process in the routing table for R1. When a packet is destined for the 192.168.2.0/24 network, R1: 1. Looks for a match in the routing table and finds that it has to forward the packets to the next-hop IPv4 address 172.16.2.2, as indicated by the label 1 in the figure. Every route that references only a next-hop IPv4 address and does not reference an exit interface must have the next-hop IPv4 address resolved using another route in the routing table with an exit interface. 2. R1 must now determine how to reach 172.16.2.2; therefore, it searches a second time for a 172.16.2.2 match. In this case, the IPv4 address matches the route for the directly connected network 172.16.2.0/24 with the exit interface Serial 0/0/0, as indicated by the label 2 in the figure. This lookup tells the routing table process that this packet is forwarded out of that interface. It actually takes two routing table lookup processes to forward any packet to the 192.168.2.0/24 network. When the router performs multiple lookups in the routing table before forwarding a packet, it is performing a process known as a recursive lookup. Because recursive lookups consume router resources, they should be avoided when possible. A recursive static route is valid (that is, it is a candidate for insertion in the routing table) only when the specified next hop resolves, either directly or indirectly, to a valid exit interface. If the exit interface is "down" or "administratively down", then the static route will not be installed in the routing table. Use the Syntax Checker in Figures 3 and 4 to configure and verify next-hop static routes on R2 and R3.

Directly connected static route

Only the router exit interface is specified

Manually

Remote networks are manually entered into the route table using static routes.

Dynamically

Remote routes are automatically learned using a dynamic routing protocol.

ip route Command

Static routes are configured using the ip route global configuration command. The basic syntax for the command is shown in the figure. The following parameters are required to configure static routing: network-address - Destination network address of the remote network to be added to the routing table, often this is referred to as the prefix. subnet-mask - Subnet mask, or just mask, of the remote network to be added to the routing table. The subnet mask can be modified to summarize a group of networks. One or both of the following parameters must also be used: ip-address - The IP address of the connecting router to use to forward the packet to the remote destination network. Commonly referred to as the next hop. exit-intf - The outgoing interface to use to forward the packet to the next hop. The distance parameter is used to create a floating static route by setting an administrative distance that is higher than a dynamically learned route.

When to Use Static Routes

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. The figure 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. 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.

what are static routes

Static routing is a form of routing that occurs when a router uses a manually-configured routing entry, rather than information from a dynamic routing traffic. ... Unlike dynamic routing, static routes are fixed and do not change if the network is changed or reconfigured.

Fully specified static route

The next-hop IP address and exit interface are specified

Summary Static Route

To reduce the number of routing table entries, multiple static routes can be summarized into a single 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 the figure, 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. For example, I can represent all four networks: * 172.20.0.0/16 *172.21.0.0/16 *172.22.0.0/16 *172.23.0.0/16 Using a single summary route: *172.20.0.0/14

Static vs Dynamic

n the figure, dynamic and static routing features are compared. Notice that the advantages of one method are the disadvantages of the other. 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, recall that the administrative distance (AD) value is a measure of the preference of route sources. Route sources with low AD values are preferred over routes sources with higher AD values. The AD value for a static route is 1. Therefore, a static route will take precedence over all dynamically learned routes, which will have higher AD values.

An ISP (network)

An ISP (Internet service provider) is a company that provides individuals and other companies access to the Internet and other related services such as Web site building and virtual hosting.

Next-Hop Options

Next-Hop Options In this example, Figures 1 to 3 display the routing tables of R1, R2, and R3. Notice that each router has entries only for directly connected networks and their associated local addresses. None of the routers have any knowledge of any networks beyond their directly connected interfaces. For example, R1 has no knowledge of networks: 172.16.1.0/24 - LAN on R2 192.168.1.0/24 - Serial network between R2 and R3 192.168.2.0/24 - LAN on R3 Figure 4 displays a successful ping from R1 to R2. Figure 5 displays an unsuccessful ping to the R3 LAN. This is because R1 does not have an entry in its routing table for the R3 LAN network.

Next-hop route

Only the next-hop IP address is specified

Why Use Static Routing?

Static routes are not advertised over the network, resulting in better security. Static routes use less bandwidth than dynamic routing protocols, 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 is 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.