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There are two basic methods of building a routing table:
- Static Routing
- Dynamic Routing
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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.
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
Disadvantages of Static Routing
- Infrastructure changes must be manually adjusted
- No “dynamic” fault tolerance if a link goes down
- Impractical on large network
The following briefly outlines the advantages and disadvantages of dynamic routing:
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
Disadvantages of Dynamic Routing
- 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
Dynamic Routing Categories
There are two distinct categories of dynamic routing protocols:
- Distance-vector protocols
- Link-state protocols
Examples of distance-vector protocols include RIP and IGRP. Examples of link-state protocols include OSPF and IS-IS.
EIGRP exhibits both distance-vector and link-state characteristics, and is considered a hybrid protocol.
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.
Link-State Routing Protocols
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.