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Routers can forward packets over static routes or dynamic routes, based on the router
configuration. There are two ways to tell the router how to forward packets to networks that are
not directly connected:
■ Static: The router learns routes when an administrator manually configures the static route.
The administrator must manually update this static route entry whenever an internetwork
topology change requires an update. Static routes are user-defined routes that specify the path
that packets take when moving between a source and a destination. These administratordefined
routes enable precise control over the routing behavior of the IP internetwork.
■ Dynamic: The router dynamically learns routes after an administrator configures a routing
protocol that helps determine routes. Unlike the situation with static routes, after the network
administrator enables dynamic routing, the routing process automatically updates route
knowledge whenever new topology information is received. The router learns and maintains routes
to the remote destinations by exchanging routing updates with other routers in the internetwork.
Dynamic routing relies on a routing protocol to disseminate knowledge. A routing protocol defines
the rules that a router uses when it communicates with neighboring routers to determine paths to
remote networks and maintains those networks in the routing tables. Figure 3-1 illustrates that a
router in the network can have knowledge of networks that are not directly connected to an interface
on that device. These routes must be configured statically or learned via routing protocols.
Figure 3-1 Routing Protocols
Network
10.1.1.0
Network
172.16.0.0
Network
Protocol
EIGRP
OSPF
Destination
Network
10.1.1.0
172.16.0.0
Exit
Interface
FA0/1
FA0/2
Reviewing Dynamic Routing 99
The following are the differences between a routed protocol and a routing protocol:
■ Routed protocol: Any network protocol that provides enough information in its network
layer address to enable a packet to be forwarded from one host to another host based on the
addressing scheme, without knowing the entire path from source to destination. Packets
generally are conveyed from end system to end system. IP is an example of a routed protocol.
■ Routing protocol: Facilitates the exchange of routing information between networks,
enabling routers to build routing tables dynamically. Traditional IP routing stays simple
because it uses next-hop (next-router) routing, in which the router needs to consider only
where it sends the packet and does not need to consider the subsequent path of the packet on
the remaining hops (routers). Routing Information Protocol (RIP) is an example of a routing
protocol.
Routing protocols describe the following information:
■ How updates are conveyed
■ What knowledge is conveyed
■ When to convey the knowledge
■ How to locate recipients of the updates
You can classify dynamic routing protocols using many different methods. One method to classify
a routing protocol is to determine if it is used to route in an autonomous system or between
autonomous systems. An autonomous system is a collection of networks under a common
administration that share a common routing strategy.
There are two types of routing protocols:
■ Interior Gateway Protocols (IGP): These routing protocols exchange routing information
within an autonomous system. Routing Information Protocol version 2 (RIPv2), Enhanced
Interior Gateway Routing (EIGRP), and Open Shortest Path First (OSPF) are examples of
IGPs.
■ Exterior Gateway Protocols (EGP): These routing protocols are used to route between
autonomous systems. Border Gateway Protocol (BGP) is the EGP of choice in networks
today.
100 Chapter 3: Medium-Sized Routed Network Construction
Figure 3-2 shows the logical separation of where an IGP operates and where an EGP operates.
Figure 3-2 IGP Versus EGP
Within an autonomous system, most IGP routing can be further classified as conforming to one of
the following algorithms:
■ Distance vector: The distance vector routing approach determines the direction (vector) and
distance (such as hops) to any link in the internetwork.
■ Link-state: The link-state approach, which utilizes the shortest path first (SPF) algorithm,
creates an abstraction of the exact topology of the entire internetwork, or at least of the
partition in which the router is situated.
■ Advanced distance vector: The advanced distance vector approach combines aspects of the
link-state and distance vector algorithms. This is also sometimes referred to as a hybrid
routing protocol.
There is no single best routing algorithm for all internetworks. All routing protocols provide the
information differently.
NOTE The Internet Assigned Numbers Authority (IANA) assigns autonomous system
numbers for many jurisdictions. Use of IANA numbering is required if your organization plans
to use BGP. However, it is good practice to be aware of private versus public autonomous system
numbering schema.
IGPs:
(RIPv2, EIGRP, OSPF)
Autonomous System 50100 Autonomous System 50200
EGPs
(BGP)
Reviewing Dynamic Routing 101
Multiple routes to a destination can exist. When a routing protocol algorithm updates the routing
table, the primary objective of the algorithm is to determine the best route to include in the table.
Each distance vector routing protocol uses a different routing metric to determine the best route.
The algorithm generates a number called the metric value for each path through the network. With
the exception of BGP, the smaller the metric, the better the path.
Metrics can be calculated based on a single characteristic of a path. More complex metrics can be
calculated by combining several path characteristics. The metrics that routing protocols most
commonly use are as follows:
■ Hop count: The number of times that a packet passes through the output port of one router
■ Bandwidth: The data capacity of a link; for instance, normally, a 10-Mbps Ethernet link is
preferable to a 64-kbps leased line
■ Delay: The length of time that is required to move a packet from source to destination
■ Load: The amount of activity on a network resource, such as a router or link
■ Reliability: Usually refers to the bit error rate of each network link
■ Cost: A configurable value that on Cisco routers is based by default on the bandwidth of the
interface
Figure 3-3 shows an example of multiple routes between two hosts and the way different routing
protocols compute metrics.
Multiple routing protocols and static routes can be used at the same time. If there are several
sources for routing information, an administrative distance value is used to rate the trustworthiness
of each routing information source. By specifying administrative distance values, Cisco IOS
Software can discriminate between sources of routing information.
An administrative distance is an integer from 0 to 255. A routing protocol with a lower
administrative distance is more trustworthy than one with a higher administrative distance. As
shown in Figure 3-4, if Router A receives a route to network 172.16.0.0 advertised by EIGRP and
by OSPF at the same time, Router A would use the administrative distance to determine that
EIGRP is more trustworthy.
102 Chapter 3: Medium-Sized Routed Network Construction
Figure 3-3 Routing Protocol Metrics
Figure 3-4 Administrative Distance
Router A would then add the EIGRP route to the routing table because EIGRP is a more trusted
routing source than OSPF. The administrative distance is an arbitrary value that Cisco IOS sets to
handle cases when multiple routing protocols send information about the same routes. Table 3-1
shows the default administrative distance for selected routing information sources.
256k
T1
T1 256k
RIP
Hop Count
OSPF
COST
Bandwidth
Delay
Load
Reliability
EIGRP
A
B
OSPF Advertises
172.16.0.0
EIGRP Advertises
172.16.0.0
A
Network
172.16.0.0
Reviewing Dynamic Routing 103
If nondefault values are necessary, you can use Cisco IOS Software to configure administrative
distance values on a per-router, per-protocol, and per-route basis.
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