Routing Protocol Selection Criteria - dummies

Routing Protocol Selection Criteria

By Edward Tetz

People use two basic methods to classify routing protocols — by where they are used and by how they calculate routing. You can see how to choose protocols based on where you plan to use them, the way the protocol manages data, and how your router chooses which protocol to use when more than one protocol is installed.

Classifying by where protocols are used

When classifying protocols by where they are used, you are talking about interior (IGP) versus exterior (EGP). In most cases, people are most concerned with the protocols that are running on the inside of their networks and maintaining their routing data because this is where most of their traffic is concentrated.

In making your decisions about which protocol class you want to apply first to your network, it is likely the interior protocol because the exterior protocols typically pass information that was generated by the interior protocols. The two main breakdowns for protocols are

  • Interior protocols include RIP, EIGRP, OSPF, and ISIS.

  • Exterior protocols include BGP.

Classifying by how protocols calculate routing

In addition to classifying protocols by where they are used, you can also choose to classify protocols by how they calculate routing. When classifying them this way, you are talking about distance-vector protocols versus link-state protocols:

  • Distance-vector protocols include RIP, BGP, and EIGRP.

  • *Link-state protocols include OSPF and IS-IS.

Distance-vector protocols

RIP, BGP, and EIGRP are in the distance-vector category. Distance-vector protocols base their routing choices on two things: the direction or vector they need to send the data, and the distance of the target network, which is calculated as hops or routers that the data needs to pass through.

Link-state protocols

Link-state protocols include OSPF and ISIS. Link-state protocols gather information about network connections known by all the routers in their group and build topology maps identifying how they see all the connections across the entire network or area in which they function.

They then use this information to build their own routing table. In link-state routing, routing tables are not passed between routers; only the connection information is transferred between routers.

Other types of protocols

The exception to this grouping is EIGRP, which Cisco developed and calls an Advanced Distance Vector or hybrid protocol. EIGRP shares its routing table with its neighbors like a distance-vector protocol does. However, it sends the entire table only at startup; then it only sends updates like a link-state protocol does, so it has a bit of both sets of features.

It is not uncommon to find that EIGRP has been added to either classification list, though in most cases you see it on the distance-vector protocol list when people have not separated EIGRP by itself.

Administrative distance

You can enable several routing protocols on your router at the same time, so you could be using EIGRP, OSPF, and RIP on your network simultaneously. If this is the case, your router will learn about routes to the same network through each of the available protocols, so it must choose the route to take to any given network.

Because routers are not good at making these judgment calls, administrative distances are used. The administrative distance for a routing protocol is a numeric representation of how accurate the routing protocol is expected to be, where the lowest number is given to the most accurate protocol. This administrative distance is sometimes referred to as how believable a routing protocol or routing table entry is.

Each routing protocol is assigned a default administrative distance. Although these are default distances, you can change them on a per-router, per-protocol, or per-route basis using IOS commands. Typically, you do not need to change these default distances because they have been ordered so that the most believable routes are given the shortest distance.

Given a route to a network, such as, if two routes in my routing table are networking, one is directly connected and one is retrieved by RIP. The route that says the router is directly connected to the network is likely to be the most believable or accurate route.

Default Administrative Distances for Protocols
Route Source Default Distance
Connected Network 0
Static Route 1
OSPF 110
RIPv2 120
External EIGRP 170
Unknown or unbelievable 255 (never used)

The routes that are expected to be the most reliable routes are given preference over other options for a network. The networks that your router is directly connected to are the most reliable choices, and static routes that you have put in are only slightly less reliable.

If you have put in a static route, the router determines that you must want to use it. If you have a choice, the shortest distance is always preferable.