This chapter is from the book
This chapter is from the book
This chapter is from the book
Routing Protocol Examples
Now that you understand some of the characteristics that distinguish one routing protocol from another, this section contrasts some of the most popular routing protocols used in modern networks:
Routing Information Protocol (RIP): RIP is a distance-vector routing protocol that uses the metric hop count. The maximum number of hops between two routers in an RIP-based network is 15. Therefore, a hop count of 16 is considered to be infinite. Also, RIP is an IGP.
Open Shortest Path First (OSPF): OSPF is a link-state routing protocol that uses the metric cost, which is based on the link speed between two routers. OSPF is a popular IGP because of its scalability, fast convergence, and vendor interoperability.
Intermediate System-to-Intermediate System (IS-IS): This link-state routing protocol is similar in operation to OSPF. It uses a configurable, yet dimensionless, metric associated with an interface and runs Dijkstra’s shortest path first algorithm. Although IS-IS is an IGP that offers the scalability, fast convergence, and vendor-interoperability benefits of OSPF, it has not been as widely deployed as OSPF.
Enhanced Interior Gateway Routing Protocol (EIGRP): EIGRP is a Cisco-proprietary protocol that is popular in Cisco-only networks but less popular in mixed-vendor environments. Like OSPF, EIGRP is an IGP that offers fast convergence and scalability. EIGRP is more challenging to classify as a distance-vector or a link-state routing protocol.
By default, EIGRP uses bandwidth and delay in its metric calculation; however, other parameters can be considered, including reliability, load, and maximum transmission unit (MTU) size. Using delay as part of the metric, EIGRP can take into consideration the latency caused by the slowest links in the path.
Some literature calls EIGRP an advanced distance-vector routing protocol, and some literature calls it a hybrid routing protocol (mixing characteristics of both distance-vector and link-state routing protocols). EIGRP uses information from its neighbors to help select an optimal route (like distance-vector routing protocols). However, EIGRP also maintains a database of topological information (like a link-state routing protocol). The algorithm EIGRP uses for its route selection is not Dijkstra’s shortest path first algorithm. Instead, EIGRP uses Diffusing Update Algorithm (DUAL).
Border Gateway Protocol (BGP): BGP is the only EGP in widespread use today. In fact, BGP is considered to be the routing protocol that runs the Internet, which is an interconnection of multiple autonomous systems. Although some literature classifies BGP as a distance-vector routing protocol, it can more accurately be described as a path-vector routing protocol, meaning that it can use as its metric the number of autonomous system hops that must be transited to reach a destination network, as opposed to a number of required router hops. BGP’s path selection is not solely based on autonomous system hops, however. BGP can consider a variety of other parameters. Interestingly, none of those parameters are based on link speed. In addition, although BGP is incredibly scalable, it does not quickly converge in the event of a topological change.
Table 10-2 compares the key characteristics of dynamic routing protocols.
Table 10-2 Comparing Dynamic Routing Protocols
Routing Protocol |
IGP or EGP |
Type |
Metric |
|---|---|---|---|
RIP |
IGP |
Distance vector |
Hop count |
OSPF |
IGP |
Link state |
Cost (based on bandwidth) |
EIGRP |
IGP |
Hybrid |
Composite (bandwidth and delay by default) |
BGP |
EGP |
Path vector |
Path attributes |
A network can simultaneously support more than one routing protocol through the process of route redistribution. For example, a router could have one of its interfaces participating in an OSPF area of the network and have another interface participating in an EIGRP area of the network. This router could then take routes learned via OSPF and inject those routes into the EIGRP routing process. Similarly, EIGRP-learned routes could be redistributed into the OSPF routing process.