Routing algorithm

Each protocol uses a slightly different algorithm for choosing the best route between two addresses on the network. The algorithm is the “intelligent” part of a dynamic protocol because the algorithm is responsible for deciding which route is best and should be added to the local routing table. RIP and BGP use distance vector algorithms, where OSPF and IS-IS use link-state or a shortest path first algorithm.

Vector algorithms are essentially based on the number of hops between the originator and the destination in a route, possibly weighting hops based on how reliable, fast, and error-free they are.

The link-state algorithm used by OSPF and IS-IS is called the Dijkstra algorithm. Link-state treats each interface as a link, and records information about the state of the interface. The Dijkstra algorithm creates trees to find the shortest paths to the routes it needs based on the total cost of the parts of the routes in the tree.

For more information on the routing algorithm used, see Comparison of dynamic routing protocols on page 287.

 

Authentication

If an attacker gains access to your network, they can masquerade as a router on your network to either gain information about your network or disrupt network traffic. If you have a high quality firewall configured, it will help your network security and stop many of this type of threat. However, the main method for protecting your routing information is to use authentication in your routing protocol. Using authentication on your FortiGate unit and other routers prevents access by attackers — all routers must authenticate with passwords, such as MD5 hash passwords, to ensure they are legitimate routers.

When configuring authentication on your network, ensure you configure it the same on all devices on the network. Failure to do so will create errors and outages as those forgotten devices fail to connect to the rest of the network.

For example, to configure an MD5 key of 123 on an OSPF interface called ospf_test, enter the following CLI

command:

config router ospf config ospf-interface

edit ospf_test

set authentication md5 set md5-key 123

end end

 

Convergence

Convergence is the ability of a networking protocol to re-route around network outages. Static routing cannot do this. Dynamic routing protocols can all converge, but take various amounts of time to do this. Slow convergence can cause problems such as network loops which degrade network performance.

You may also hear robustness and redundancy used to describe networking protocols. In many ways they are the same thing as convergence. Robustness is the ability to keep working even though there are problems, including configuration problems as well as network outages. Redundancy involves having duplicate parts that can continue to function in the event of some malfunction, error, or outage. It is relatively easy to configure dynamic routing protocols to have backup routers and configurations that will continue to function no matter the network problem short of a total network failure.

 

IPv6 Support

IPv4 addressing is in common use everywhere around the world. IPv6 has much larger addresses and it is used by many large companies and government departments. IPv6 is not as common as IPv4 yet, but more companies are adopting it.

If your network uses IPv6, your dynamic routing protocol must support it. None of the dynamic routing protocols originally supported IPv6, but they all have additions, expansions, or new versions that do support IPv6. For more information, see Routing Information Protocol (RIP) on page 300, Border Gateway Protocol (BGP) on page 338, Open Shortest Path First (OSPF) on page 377, or Intermediate System to Intermediate System Protocol(IS-IS) on page 419.

 

When to adopt dynamic routing

Static routing is more than enough to meet your networking needs when you have a small network. However, as your network grows, the question you need to answer is at what point do you adopt dynamic routing in your networking plan and start using it in your network? The main factors in this decision are typically:

• Budget
• Current network size and topology
• Expected network growth
• Available resources for ongoing maintenance

 

Budget

When making any business decision, the budget must always be considered. Static routing does not involve special hardware, fancy software, or expensive training courses.

Dynamic routing can include all of these extra expenses. Any new hardware, such as routers and switches, will need to support your chosen routing protocols. Network management software and routing protocol drivers may be necessary as well to help configure and maintain your more complex network. If the network administrators are not well versed in dynamic routing, either a training course or some hands-on learning time must be budgeted so they can administer the new network with confidence. Together, these factors can impact your budget.

Additionally, people will always account for network starting costs in the budgets, but usually leave out the ongoing cost of network maintenance. Any budget must provide for the hours that will be spent on updating the network routing equipment, and fixing any problems. Without that money in the budget, you may end up back at static routing before you know it.