Overview of Different Routing Protocols

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Before you start

Objectives: Learn which different routing protocols can be used and what are their characteristics.

Prerequisites: you must know which different specs can a routing protocol have.

Key terms: protocol, area, rip, routers, routing, areas, different, actually, information, is-is, ospf

Routing Information Protocol (RIP)

RIP is actually one of the oldest routing protocol. RIP is an interior gateway protocol (IGP), which means that it’s used within a private network.The metric which RIP uses is the hop count. RIP has a limitation of 15 hops. That means that if two subnets have more than 15 routers between them, RIP won’t be able to work in that kind of environment. A network with a hop count of 16 indicates a network that is not reachable. RIP is a distance vector routing protocol, which means that it shares it’s entire routing table with the neighbors. RIP actually comes in two versions and there is actually only one difference between those two versions. We have RIP Version 1 which does not support variable length subnet masks (it is classful), and we have RIP Version 2 which supports VLSMs (it is classless).

Enhanced Interior Gateway Routing Protocol (EIGRP)

EIGRP is developed by Cisco and it is also an IGP, as its name implies. The metric which EIGRP uses is based on the bandwidth and a delay. The higher the bandwidth, the lower the metric. EIGRP is actually a hybrid routing protocol which means that it’s a distance vector protocol that includes some improvements (in some cases EIGRP will act as a link state protocol). EIGRP supports variable length subnet masks.

Open Shortest Path First (OSPF)

OSPF is also an IGP. It uses a link cost as its metric. OSPF is a link state protocol. It supports variable length subnet masks. In contrast to RIP, OSPF is designed to support larger. When we use OSPF we actually divide our private network into different areas which are interconnected. Each area can contain multiple subnets so the routers will only keep track of the routes within particular area. When configuring areas we always have to have a special area which we refer to as area 0 (zero), which is connected to all other areas. It serves as a backbone area, so every other area in the network will connect to area zero. This connection can be direct connection or through another area.

Routers which share information within an area are called internal routers. Routers which connect two separate areas are called area border routers. Border routers share information between areas. We also have an autonomous system border router which communicates with routers outside of the autonomous system.

Intermediate System to Intermediate System (IS-IS)

IS-IS is also an IGP. It uses a link cost for the metric. It is a link state protocol and it supports variable length subnet masks. IS-IS also uses areas, similar to OSPF. However, IS-IS doesn’t require an area zero. With IS-IS we also have different “types” of routers. For example, we have an L1 router which shares information about routes within an area, similar to the internal router in OSPF. We also have an L2 router which shares information between areas. In the end we have an L1/L2 router which shares information about internal networks with L2 routers. The thing is, L2 routers can’t communicate with L1 routers directly. Because of that fact we have to use one router as an L1/L2 router, which can then share information with L2 router. L2 router can then share that information with another L2 router in a different area. With IS-IS, the boundary between areas is actually the physical link between areas. IS-IS supports multiple protocols (not just IP), and can route IP Version 6 addresses without any modifications.

Border Gateway Protocol (BGP)

BGP is actually the EGP or Exterior Gateway Protocol. BGP is an advanced distance vector protocol, and it is often refered to as  path vector protocol. It supports VLSM. The BGP is the routing protocol used on the Internet to route data and to share routes between autonomous systems. Typically, as local administrators we won’t actually work with BGP, since we will only use interior gateway protocols. However, very large networks can use BGP internally. It will be used if the autonomous system has two or more connections to the Internet trough different ISPs.

We can use RIP for small private networks. With RIP we have to have in mind that we have a 15 hop count limit. We can use EIGRP or OSPF for larger private networks. IS-IS is typically used on very large networks. BGP is used within the Internet.