CCNA2 LS CH6

Static routing 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. Notice that the advantages of one method are the disadvantages of the other.

IPv6 How the destination is specified creates one of three route types:

- Next-hop static IPv6 route - Only the next-hop IPv6 address is specified. - Directly connected static IPv6 route - Only the router exit interface is specified. - Fully specified static IPv6 route - The next-hop IPv6 address and exit interface are specified.

Types of IPv4 and IPv6 static routes

- Standard static route - Default static route - Summary static route - Floating static route

Static routing advantages

- 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.

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.

Verify a Static Route

- show ip route - show ip route static - show ip route network - ping - traceroute

Verify IPv6 Static Routes

- show ipv6 route - show ipv6 route static - show ipv6 route network - ping - tracerotue

Configure a Next-Hop Static Route steps

1. Looks for a match in the routing table and finds that it has to forward the packets to the next-hop IPv4 address. 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. Routers look up routes to determine where to forward to. 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. 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

Default Static Route

A default route is a static route that matches all packets. Rather than storing all routes to all networks in the routing table, a router can store a single default route to represent any network that is not in the routing table. A default route is used when no other routes in the routing table match the destination IP address of the packet. In other words, if a more specific match does not exist, then the default route is used as the Gateway of Last Resort. The default static route matches all packets for which a more specific match does not exist.

distance

A parameter is used to create a floating static route by setting an administrative distance that is higher than a dynamically learned route.

default static route

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. 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.

Classful Subnet Masks

As specified in RFC 790, each network class has a default subnet mask associated with it. An advantage of assigning specific default subnet masks to each class is that it made routing update messages smaller. Classful routing protocols do not include the subnet mask information in their updates. The receiving router applies the default mask based on the value of the first octet which identifies the class.

Configure a Next-Hop Static Route

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. The route resolvability process will vary depending upon the type of forwarding mechanism being used by the router. CEF (Cisco Express Forwarding) is the default behavior on most platforms running IOS 12.0 or later.

#show ip route static

Command output displays the contents of the routing table. Note the asterisk (*)next to the route with code 'S'. As displayed in the Codes table in the figure, the asterisk indicates that this static route is a candidate default route, which is why it is selected as the Gateway of Last Resort.

Configure a Directly Connected Static Route

Configuring a directly connected static route with an exit interface allows the routing table to resolve the exit interface in a single search, instead of two searches. Although the routing table entry indicates "directly connected", the administrative distance of the static route is still 1. Only a directly connected interface can have an administrative distance of 0. For point-to-point interfaces, you can use static routes that point to the exit interface or to the next-hop address. For multipoint/broadcast interfaces, it is more suitable to use static routes that point to a next-hop address. Although static routes that use only an exit interface on point-to-point networks are common, the use of the default CEF forwarding mechanism makes this practice unnecessary.

#ipv6 unicast-routing

Global configuration command must be configured to enable the router to forward IPv6 packets.

stub network

Host, switch and router. nothing more.

Fully Specified Static Route

In a fully specified static route, both the output 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 output 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. 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.

Configure a Fully Specified Static IPv6 Route

In a fully specified static route, both the output interface and the next-hop IPv6 address are specified. Similar to fully specified static routes used with IPv4, this would be used if CEF were not enabled on the router and the exit interface was on a multi-access network. With CEF, a static route using only a next-hop IPv6 address would be the preferred method even when the exit interface is a multi-access network.

Class B addresses begin with 10

Intended for medium-to-large organizations; includes all addresses from 128.0.0.0 (10000000) to 191.255.255.255 (10111111).

Class C addresses begin with 110

Intended for small-to-medium organizations; includes all addresses from 192.0.0.0 (11000000) to 223.255.255.255 (11011111).

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. A static route does not change until the administrator manually reconfigures it. Static routes are useful for smaller networks with only one path to an outside network. The administrative distance (AD) of a static route is 1 Dynamically - Remote routes are automatically learned using a dynamic routing protocol.

Class D Multicast addresses begin with 1110

Multicast addresses are used to identify a group of hosts that are part of a multicast group. This helps reduce the amount of packet processing that is done by hosts, particularly on broadcast media (i.e., Ethernet LANs). Routing protocols, such as RIPv2, EIGRP, and OSPF use designated multicast addresses (RIP = 224.0.0.9, EIGRP = 224.0.0.10, OSPF 224.0.0.5, and 224.0.0.6).

Next-Hop Options

The next hop can be identified by an IP address, exit interface, or both. How the destination is specified creates one of the three following 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 - The next-hop IP address and exit interface are specified.

exit-intf

The outgoing interface to use to forward the packet to the next hop.

#show ip route <IP>

The output reflects the use of static routes using the next-hop address.

ip route 192.168.2.0 255.255.255.0 GigabitEthernet 0/1

To eliminate the recursive lookup, a directly connected static route can be implementing using the following command.

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.

Classful Routing Protocol Example

Using classful IP addresses meant that the subnet mask of a network address could be determined by the value of the first octet, or more accurately, the first three bits of the address. Routing protocols, such as RIPv1, only need to propagate the network address of known routes and do not need to include the subnet mask in the routing update. This is due to the router receiving the routing update determining the subnet mask simply by examining the value of the first octet in the network address, or by applying its ingress interface mask for subnetted routes. The subnet mask was directly related to the network address.

Configure a Directly Connected Static IPv6 Route

When configuring a static route on point-to-point networks, an alternative to using the next-hop IPv6 address is to specify the exit interface. This is an alternative used in older IOSs or whenever CEF is disabled, to avoid the recursive lookup problem. Configuring a directly connected static route with an exit interface allows the routing table to resolve the exit interface in a single search instead of two searches. Recall that with the use of the CEF forwarding mechanism, static routes with an exit interface are considered unnecessary. A single lookup is performed using a combination of the FIB and adjacency table stored in the data plane.

CEF

provides optimized lookup for efficient packet forwarding by using two main data structures stored in the data plane: a FIB (Forwarding Information Base), which is a copy of the routing table and an adjacency table that includes Layer 2 addressing information. The information combined in both of these tables work together so there is no recursive lookup needed for next-hop IP address lookups. In other words, a static route using a next-hop IP requires only a single lookup when CEF is enabled on the router.

Static Route Applications

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

Useful Links:

"Internet Protocol," http://www.ietf.org/rfc/rfc791.txt "Internet Multicast Addresses," http://www.iana.org/assignments/multicast-addresses

IPv6 Default static routes are commonly used when connecting:

- A company's edge router to a service provider network. - A router with only an upstream neighbor router. The router has no other neighbors and is, therefore, referred to as a stub router.

Default static routes are commonly used when connecting:

- An edge router to a service provider network - A stub router (a router with only one upstream neighbor router)

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. - 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 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.

IPv6 static routes can also be implemented as:

- Standard IPv6 static route - Default IPv6 static route - Summary IPv6 static route - Floating IPv6 static route

network-address

Destination network address of the remote network to be added to the routing table, often this is referred to as the prefix.

Class A addresses begin with 0

Intended for large organizations; includes all addresses from 0.0.0.0 (00000000) to 127.255.255.255 (01111111). The 0.0.0.0 address is reserved for default routing and the 127.0.0.0 address is reserved for loopback testing.

difference between an Ethernet multi-access network and a point-to-point serial network

Is that a point-to-point 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.

Router(config)# ip route network-address subnet-mask { ip-address | interface-type interface-number [ ip-address ]} [ distance ] [ name name ] [ permanent ] [ tag tag ]

Static routes are configured using the ip route global configuration command.

Router(config)# ipv6 route ipv6-prefix/prefix-length { ipv6-address | exit-intf }

Static routes for IPv6 are configured using the ipv6 route global configuration command. Figure 1 shows the simplified version of the command syntax.

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.

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.

#ipv6 route ::/0 { ipv6-address | exit-intf }

The basic command syntax of a default static route.

#ip route 0.0.0.0 0.0.0.0 { ip-address | exit-intf }

The command syntax for a default static route is similar to any other static route, except that the network address is 0.0.0.0 and the subnet mask is 0.0.0.0. An IPv4 default static route is commonly referred to as a quad-zero route.

Class E Reserved IP addresses begin with 1111

These addresses were reserved for experimental and future use.

#show ipv6 route static

command output displays the contents of the routing table.