This section discusses the different types and characteristics of routing protocols.
Routing protocols have the following characteristics:
- Distance-vector, link-state, or hybrid: These types have to do with how routes are learned.
- Interior or exterior: Some protocols are used in private networks and others on the public Internet.
- Classless (classless interdomain routing [CIDR] support) or classful: CIDR enables aggregation of network advertisements (supernetting) between routers.
- Fixed-length masks or variable-length subnet masking (VLSM): Masking can be used to conserve addresses in a network.
- Flat or hierarchical: These factors are related to scalability in large internetworks.
- IPv4 or IPv6: Newer routing protocols are used for IPv6 networks.
This section covers the default administrative distances assigned to routes learned from each routing protocol and from static assignment. Routes are categorized as statically (manually) configured or dynamically learned from a routing protocol. The following sections cover all these characteristics.
Static Versus Dynamic Route Assignment
Static routes are manually configured on a router. When configured manually and not learned from a neighbor, static routes do not react to network outages. The one exception is when a static route specifies the outbound interface or the IP address of the next hop is not resolved in the routing table. In this situation, if the interface goes down or the next-hop IP address is not resolved in the routing table, the static route is removed from the routing table. Because static routes are unidirectional, they must be configured for each outgoing interface the router will use. The size of today’s networks makes it impossible to manually configure and maintain all the routes in all the routers in a timely manner. Human configuration can involve many mistakes. Dynamic routing protocols were created to address these shortcomings. They use algorithms to advertise, learn about, and react to changes in the network topology.
The main benefit of static routing is that a router generates no routing protocol overhead. Because no routing protocol is enabled, no bandwidth is consumed by route advertisements between network devices. Another benefit of static routing is that static routing protocols are easier to configure and troubleshoot than dynamic routing protocols. Static routing is recommended for hub-and-spoke topologies with low-speed remote connections and where only a single path to the network exists. A default static route is configured at each remote site because the hub is the only route used to reach all other sites. Static routes are also used at network boundaries (the Internet or partners) where routing information is not exchanged. These static routes are then redistributed into the internal dynamic routing protocol used.
Figure 3-1 shows a hub-and-spoke WAN with static routes defined in the remote WAN routers because no routing protocols are configured. This setup eliminates routing protocol traffic on the low-bandwidth WAN circuits.
Figure 3-1 Static Routes in a Hub-and-Spoke Network
Routing protocols dynamically determine the best route to a destination. When the network topology changes, the routing protocol adjusts the routes without administrative intervention. Routing protocols use a metric to determine the best path toward a destination network. Some use a single measured value such as hop count. Others compute a metric value using one or more parameters. Routing metrics are discussed later in this chapter. The following is a list of dynamic routing protocols:
- RIPv1
- RIPv2
- EIGRP
- OSPF
- IS-IS
- RIPng
- OSPFv3
- EIGRP for IPv6
- Border Gateway Protocol (BGP)