Routing

Static vs. Dynamic Routing -

There are two basic methods of building a routing table:

• Static Routing
• Dynamic Routing

A static routing table is created, maintained, and updated by a network

administrator, manually. A static route to every network must be configured

on every router for full connectivity. This provides a granular level of

control over routing, but quickly becomes impractical on large networks.

Routers will not share static routes with each other, thus reducing

CPU/RAM overhead and saving bandwidth. However, static routing is not

fault-tolerant, as any change to the routing infrastructure (such as a link

going down, or a new network added) requires manual intervention. Routers

operating in a purely static environment cannot seamlessly choose a better

route if a link becomes unavailable.

Static routes have an Administrative Distance (AD) of 1, and thus are always

preferred over dynamic routes, unless the default AD is changed. A static

route with an adjusted AD is called a floating static route, and is covered in

greater detail in another guide.

A dynamic routing table is created, maintained, and updated by a routing

protocol running on the router. Examples of routing protocols include RIP

(Routing Information Protocol), EIGRP (Enhanced Interior Gateway

Routing Protocol), and OSPF (Open Shortest Path First). Specific dynamic

routing protocols are covered in great detail in other guides.

Routers do share dynamic routing information with each other, which

increases CPU, RAM, and bandwidth usage. However, routing protocols are

capable of dynamically choosing a different (or better) path when there is a

change to the routing infrastructure.

Do not confuse routing protocols with routed protocols:

• A routed protocol is a Layer 3 protocol that applies logical

addresses to devices and routes data between networks (such as IP)

• A routing protocol dynamically builds the network, topology, and

next hop information in routing tables (such as RIP, EIGRP, etc.)

The following briefly outlines the advantages and disadvantages of static

routing:

Advantages of Static Routing

• Minimal CPU/Memory overhead
• No bandwidth overhead (updates are not shared
• between routers)
• Granular control on how traffic is routed
• Infrastructure changes must be manually adjusted
• No “dynamic” fault tolerance if a link goes down
• Impractical on large network

Advantages of Dynamic Routing

• Simpler to configure on larger networks
• Will dynamically choose a different (or better) route if a link goes down
• Ability to load balance between multiple links
• Updates are shared between routers, thus consuming bandwidth
• Routing protocols put additional load on router CPU/RAM
• The choice of the “best route” is in the hands of the routing protocol, and not the network administrator

Distance-vector Routing Protocols

All distance-vector routing protocols share several key characteristics:

• Periodic updates of the full routing table are sent to routing

neighbors.

• Distance-vector protocols suffer from slow convergence, and are

highly susceptible to loops.

• Some form of distance is used to calculate a route’s metric.

• The Bellman-Ford algorithm is used to determine the shortest path.

A distance-vector routing protocol begins by advertising directly-connected

networks to its neighbors. These updates are sent regularly (RIP – every 30

seconds; IGRP – every 90 seconds).

Neighbors will add the routes from these updates to their own routing tables.

Each neighbor trusts this information completely, and will forward their full

routing table (connected and learned routes) to every other neighbor. Thus,

routers fully (and blindly) rely on neighbors for route information, a concept

known as routing by rumor.

There are several disadvantages to this behavior. Because routing

information is propagated from neighbor to neighbor via periodic updates,

distance-vector protocols suffer from slow convergence. This, in addition to

blind faith of neighbor updates, results in distance-vector protocols being

highly susceptible to routing loops.

Distance-vector protocols utilize some form of distance to calculate a

route’s metric. RIP uses hopcount as its distance metric, and IGRP uses a

composite of bandwidth and delay.

Dynamic Routing Categories

Link-state routing protocols were developed to alleviate the convergence

and loop issues of distance-vector protocols. Link-state protocols maintain

three separate tables:

• Neighbor table – contains a list of all neighbors, and the interface

each neighbor is connected off of. Neighbors are formed by sending

Hello packets.

• Topology table – otherwise known as the “link-state” table, contains

a map of all links within an area, including each link’s status.

• Shortest-Path table – contains the best routes to each particular

destination (otherwise known as the “routing” table”)

Link-state protocols do not “route by rumor.” Instead, routers send updates

advertising the state of their links (a link is a directly-connected network).

All routers know the state of all existing links within their area, and store

this information in a topology table. All routers within an area have identical

topology tables.

The best route to each link (network) is stored in the routing (or shortestpath)

table. If the state of a link changes, such as a router interface failing,

an advertisement containing only this link-state change will be sent to all

routers within that area. Each router will adjust its topology table

accordingly, and will calculate a new best route if required.

By maintaining a consistent topology table among all routers within an area,

link-state protocols can converge very quickly and are immune to routing

loops.

Additionally, because updates are sent only during a link-state change, and

contain only the change (and not the full table), link-state protocols are less

bandwidth intensive than distance-vector protocols. However, the three

link-state tables utilize more RAM and CPU on the router itself.

Link-state protocols utilize some form of cost, usually based on bandwidth,

to calculate a route’s metric. The Dijkstra formula is used to determine the

shortest path.