Static Routing: Configuration and Troubleshooting

Chapter 1: Static Routing

• Static routing implementation involves the manual entry of remote networks into a router's routing table.
• Static and default routes must be configured.
• A review of CIDR (Classless Inter-Domain Routing) and VLSM (Variable Length Subnet Masking) is required.
• Summary and floating static routes are required.
• Troubleshooting of static and default route issues should be performed.
• Advantages and disadvantages of static routing are: better security, less bandwidth usage, and known data paths, but time-consuming configuration, error-prone setup, and poor scaling.
• IPv4 and IPv6 static routes can be configured by specifying a next-hop address.
• IPv4 and IPv6 default routes can be configured.
• CIDR replaces classful addressing.
• Hierarchical addressing schemes can be designed and implemented.
• IPv4 and IPv6 summary network addresses reduce routing table updates.
• A floating static route provides a backup connection.
• When a static route is configured, routers process packets in a certain way.
• Static and default route configuration issues that are common need to be troubleshooted
• Dynamic routing involves the automation of remote routes using a dynamic routing protocol.

Why Use Static Routing?

• Static routes are not advertised over the network, increasing security.
• Less bandwidth is used by static routes compared to dynamic routing protocols. Static routes do not require CPU cycles to calculate and communicate routes.
• The path that is used by a static route to send data is well-known.
• Initial configuration and maintenance can be time-consuming.
• Configuration is error prone, especially in large networks.
• Requires admin intervention to maintain changing route information.
• Does not scale well with growing networks.
• Complete knowledge of the whole network is required for proper implementation.

When to Use Static Routes

• Routing table maintenance in smaller networks improves with static routes. Especially for those networks not expected to grow significantly.
• Routing to and from stub networks makes use of static routes, where a router has no other neighbors.
• A single default route represents a path to any network, and are used to send traffic to any destination beyond the next upstream router.

Static Route Applications

• Static routes connect to specific networks.
• A Gateway of Last Resort can be implemented for a stub network.
• The number of routes advertised can be reduced by summarizing contiguous networks as one static route.
• Backups can also be created in case primary routes fail.

Types of Static Routes

• Standard static routes connect to a stub network.
• Default static routes match all packets and identify the gateway to which a router sends all IP packets for which it don't have learned routes. They use an IPv4 address of 0.0.0.0/0.
• Summary static routes use one static route, such as representing the networks 172.20.0.0/16, 172.21.0.0/16, 172.22.0.0/16, and 172.23.0.0/16 with the single summary route of 172.20.0.0/14.
• Floating static routes are used to provide a backup path to a primary route. They are only used if the primary route cannot be found. Floating routes have a higher administrative distance, so the primary route is used in preference.

IPv4 Static Routes

• The ip route command configures IPv4 static routes.

ip route Command Syntax

• Router(config)#ip route network-address subnet-mask {ip-address | exit-intf} configures IPv4 static routes.
• network-address is the destination network address of the remote network to be added to the routing table.
• subnet-mask is the subnet mask of the remote network to be added to the routing table. It can also be modified to summarize a group of networks.
• ip-address is the next-hop router's IP address and it's typically used when connecting to a broadcast media (Ethernet). It also commonly creates a recursive lookup.
• exit-intf uses an outgoing interface to forward packets to the destination network. It is also referred to as a directly attached static route, and is typically used when connecting in a point-to-point configuration.

Next-Hop Options

• The next hop is identified by an IP address, exit interface, or both.
• The destination specified determines the following route types:

IPv4 Route Types

• Next-hop route: Only the next-hop IP address is specified.
• Directly connected static route: Only the router exit interface is specified.
• Fully specified static route: Both the next-hop IP address and exit interface are specified.

Route Verification

• show ip route, show ip route static, and show ip route network are commands used to verify static routes. ping and traceroute are also useful.

Default Static Route

• A default static route matches any packets
• Identifies the gateway IP address to which the router sends all IP packets.
• Is a static route with 0.0.0.0/0 as the destination IPv4 address

Classful Addressing

• Class A addresses have a range from 0.0.0.0 to 127.255.255.255 and start with 0xxxxxxx.
• Class B addresses have a range from 128.0.0.0 to 191.255.255.255 and start with 10xxxxxx.
• Class C addresses have a range from 192.0.0.0 to 223.255.255.255 and start with 110xxxxx.
• Multicast addresses have a range from 224.0.0.0 to 239.255.255.255 and start with 1110xxxx.
• Reserved addresses have a range from 240.0.0.0 to 255.255.255.255 and start with 1111xxxx.

Classful Subnet Masks

• Class A uses a subnet mask of /8, or 255.0.0.0
• Class B uses a subnet mask of /16, or 255.255.0.0
• Class C uses a subnet mask of /24, or 255.255.255.0

Classless Inter-Domain Routing (CIDR)

• CIDR is also known as efficient.

Route Summarization

• It summarizes a supernet route. Route aggregation advertises a contiguous set of addresses as a single address with a less-specific subnet mask.
• CIDR is a form of route summarization and is synonymous with the term supernetting.
• Summarization help to reduce the number of entries in routing updates and lowers the number of entries in local routing tables.

Fixed Length Subnet Masking (FLSM)

• Basic subnet schemes are supported.

Variable Length Subnet Masking (VLSM)

• Uses subnets of varying sizes.
• VLSM allows different masks for each subnet and are further subnetted.
• VLSM is subnetting a subnet, and can be thought of as the addresses of "sub-subnets"

IPv4 Summary Routes

• Steps to calculate a route: listing networks in binary format, counting the number of far left matching bits to determine the mask, and copying the matching bits and add zero bits to determine the network address.

Floating Static Routes

• Floating static routes have an administrative distance greater than other routes.
• The administrative distance of a static route is increased to make the route less desirable than another.
• These static routes "float" and are not used, but take over if the preferred route is lost.
• Can be tested using various commands and methods such as show ip route, traceroute, or disconnecting primary links

Troubleshooting Routes

• Common IOS troubleshooting commands include ping, traceroute, show ip route, show ip interface brief, and show cdp neighbors detail.
• Connectivity problems can be solved in a methodical process that confirms a destination unreachable, reveals the closest router for unexpected response, looks at configuration, or incorrect routes.