Chapter 3: Dynamic Routing

To propagate a default route in RIP, the edge router must be configured with:

A default static route using the ip route 0.0.0.0 0.0.0.0 command. The default-information originate router configuration command. This instructs R1 to originate default information, by propagating the static default route in RIP updates. The example in Figure 2 configures a fully-specified default static route to the service provider and then the route is propagated by RIP. Notice that R1 now has a Gateway of Last Resort and default route installed in its routing table.

Level 1 Route

A level 1 route is a route with a subnet mask equal to or less than the classful mask of the network address. Therefore, a level 1 route can be a: Network route - A network route that has a subnet mask equal to that of the classful mask. Supernet route - A supernet route is a network address with a mask less than the classful mask, for example, a summary address. Default route - A default route is a static route with the address 0.0.0.0/0.

Ultimate Route

An ultimate route is a routing table entry that contains either a next-hop IPv4 address or an exit interface. Directly connected, dynamically learned, and local routes are ultimate routes. In the figure, the highlighted areas are examples of ultimate routes. Notice that all of these routes specify either a next-hop IPv4 address or an exit interface.

The entry identifies the following information:

Route source - Identifies how the route was learned. Destination network - Identifies the address of the remote network. Administrative distance (AD) - Identifies the trustworthiness of the route source. The AD for static routes is 1 and the AD for connected routes is 0. Dynamic routing protocols have an AD higher than 1 depending upon the protocol. Metric - Identifies the value assigned to reach the remote network. Lower values indicate preferred routes. The metric for static and connected routes is 0. Next hop - Identifies the IPv4 address of the next router to forward the packet to. Route timestamp - Identifies from when the route was last heard. Outgoing interface - Identifies the exit interface to use to forward a packet toward the final destination.

Figure 2 displays a routing table entry on R1 for the route to remote network 2001:DB8:CAFE:3::/64 on R3. The entry identifies the following information:

Route source - Identifies how the route was learned. Common codes include O (OSPF), D (EIGRP), R (RIP), and S (Static route). Destination network - Identifies the address of the remote IPv6 network. Administrative distance - Identifies how trustworthiness of the route source. IPv6 uses the same distances as IPv4. Metric - Identifies the value assigned to reach the remote network. Lower values indicate preferred routes. Next hop - Identifies the IPv6 address of the next router to forward the packet to. Outgoing interface - Identifies the exit interface to use to forward a packet toward the final destination.

As shown in Figure 3, directly connected route entries display the following information:

Route source - Identifies how the route was learned. Directly connected interfaces have two route source codes (C identifies a directly connected network while L identifies that this is a local route.) Directly connected network - The IPv6 address of the directly connected network. Administrative distance - Identifies the trustworthiness of the route source. IPv6 uses the same distances as IPv4. A value of 0 indicates the best, most trustworthy source. Metric - Identifies the value assigned to reach the remote network. Lower values indicate preferred routes. Outgoing interface - Identifies the exit interface to use when forwarding packets to the destination network.

The entries contain the following information:

Route source - Identifies how the route was learned. Directly connected interfaces have two route source codes. C identifies a directly connected network. Directly connected networks are automatically created whenever an interface is configured with an IP address and activated. L identifies that this is a local route. Local routes are automatically created whenever an interface is configured with an IP address and activated. Destination network - The address of the remote network and how that network is connected. Outgoing interface - Identifies the exit interface to use when forwarding packets to the destination network.

Sending out unneeded updates on a LAN impacts the network in three ways:

Wasted Bandwidth - Bandwidth is used to transport unnecessary updates. Because RIP updates are either broadcasted or multicasted, switches also forward the updates out all ports. Wasted Resources - All devices on the LAN must process the update up to the transport layers, at which point the devices will discard the update. Security Risk - Advertising updates on a broadcast network is a security risk. RIP updates can be intercepted with packet sniffing software. Routing updates can be modified and sent back to the router, corrupting the routing table with false metrics that misdirect traffic.

automatic summarization

enabled technology that puts a summary address in the routing table with Null 0 as the exit interface

router rip

enables Routing Information Protocol (RIP) routing

Verify RIP Routing

1. RIP routing is configured and running on router R1. 2. The values of various timers; for example, the next routing update, is sent by R1 in 16 seconds. 3. The version of RIP configured is currently RIPv1. 4. R1 is currently summarizing at the classful network boundary. 5. The classful networks are advertised by R1. These are the networks that R1 includes in its RIP updates. 6. The RIP neighbours are listed, including their next-hop IP address, the associated AD that R2 uses for updates sent by this neighbour, and when the last update was received from this neighbour.

The main components of dynamic routing protocols include:

Data structures - Routing protocols typically use tables or databases for its operations. This information is kept in RAM. Routing protocol messages - Routing protocols use various types of messages to discover neighboring routers, exchange routing information, and other tasks to learn and maintain accurate information about the network. Algorithm - An algorithm is a finite list of steps used to accomplish a task. Routing protocols use algorithms for facilitating routing information and for best path determination.

The purpose of dynamic routing protocols includes:

Discovery of remote networks Maintaining up-to-date routing information Choosing the best path to destination networks Ability to find a new best path if the current path is no longer available

Static routing has several primary uses, including:

Providing ease of routing table maintenance in smaller networks that are not expected to grow significantly. Routing to and from a stub network, which is a network with only one default route out and no knowledge of any remote networks. Accessing a single default route (which is used to represent a path to any network that does not have a more specific match with another route in the routing table).

For instance, common codes for remote networks include:

S - Identifies that the route was manually created by an administrator to reach a specific network. This is known as a static route. D - Identifies that the route was learned dynamically from another router using the EIGRP routing protocol. O - Identifies that the route was learned dynamically from another router using the OSPF routing protocol. R - Identifies that the route was learned dynamically from another router using the RIP routing protocol.

Routes are discussed in terms of:

Ultimate route Level 1 route Level 1 parent route Level 2 child routes

no router rip

disable and eliminate rip

Level 1 Parent Route

a level 1 parent route is a level 1 network route that is subnetted. A parent route can never be an ultimate route.

Level 2 Child Route

a route with a subnet mask greater than the classful mask of the network address