+ cost: a configurable value based by default on the bandwidth of the interface
+ load: the amount of activity on a network resource
+ bandwidth: the data capacity of a link
+ hop count: the number of point-to-point links in a transmission path
+ reliability: usually refers to the bit error rate of each network link
+ delay: the amount of time required to move a packet from source to destination
+ poison reverse: A router learns from its neighbor that a route is down and the router sends an update back to the neighbor with an infinite metric to that route
+ LSA: The packets flooded when a topology change occurs, causing network routers to update their topological databases and recalculate routes
+ split horizon: This prevents sending information about a routeback out the same interface that originally learned about the route
+ holddown timer: For a given period, this causes the router to ignore any updates with poorer metrics to a lost network
Dynamic routing protocols
RIP was one of the first routing protocols to be used. RIP was released in 1982 and continues today as RIPv2. RIP has a network size limitation of 15, which means it is only suitable for networks of 15 routers or less. Due to this limitation there are other protocols such as IGRP, EIGRP, and OSPF that can handle the large networks that RIP cannot.
Here are a summary of protocols, their features, and limitations.
“Routing protocols determine the best path to each network which is then added to the routing table. One of the primary benefits to using a dynamic routing protocol is that outers exchange routing information whenever there is a topology change. This exchange allows routers to automatically learn about new networks and also to find alternate paths when there is a link failure to a current network.” (3.1.1)
Purpose of dynamic routing protocols:
• Discovery of remote networks
• Maintaining up-to-date routing information
• Choosing the best path to destination networks
• Ability to find a new best path if the current path is no longer available
Routing Protocol Components
• Data Structures – tables or databases
• Protocol Messages
Advantages and Disadvantages for Static and Dynamic Routing
Static Routing Advantages
• Minimal CPU processing
• Easier for administrator to understand
• Easy to configure
Static Routing Disadvantages
• Configuration and maintenance is time-consuming.
• Configuration is error-prone, especially in large networks.
• Administrator intervention is required to maintain changing route information.
• Does not scale well with growing networks; maintenance becomes cumbersome.
• Requires complete knowledge of the whole network for proper implementation.
Dynamic Routing Advantages
• Administrator has less work maintaining the configuration when adding or deleting networks.
• Protocols automatically react to the topology changes.
• Configuration is less error-prone.
• More scalable, growing the network usually does not present a problem.
Dynamic Routing Disadvantages
• Router resources are used (CPU cycles, memory and link bandwidth).
• More administrator knowledge is required for configuration, verification, and troubleshooting.
Dynamic routing protocols fall into two main categories: Interior Gateway Protocols (IGPs) and Exterior Gateway Protocols (EGPs).
IGPs can be further broken down into Distance Vector or Link State Protocols. The main difference between distance vector and link state protocols are how information is stored, either in table form (DV) or database form (LS).
Distance vector protocols use distance and direction as the method to advertise routes. Distance is defined by a metric value (sometimes hop count) and direction is defined by next-hop router.
Next-hop router is the router that is directly connected to the current router. Hop count is the numerical value of how many router connections are between source Router A and destination Router B. Let Router A = A and Router B = B, then hopcount(A,B) = n, where n is some Natural number.
Routers running a distance vector protocol never have a full understanding of a network. Instead they operate as road signs pointing the way to get to the correct destination yet not knowing how one gets to the destination once it has left it’s interface.
Distance vector protocols work best in situations where:
• The network is simple and flat and does not require a special hierarchical design.
• The administrators do not have enough knowledge to configure and troubleshoot link-state protocols.
• Specific types of networks, such as hub-and-spoke networks, are being implemented.
• Worst-case convergence times in a network are not a concern.
Link State Protocols
Link state protocols use a full network topology to make decisions. Every router has the same information and fully understands how to get from Router A to Router B. Having the complete set of information allows for quick route changes if a topology change occurs. It also requires more memory and processing time because the set of information is complete and therefore larger.
Link-state protocols work best in situations where:
• The network design is hierarchical, usually occurring in large networks.
• The administrators have a good knowledge of the implemented link-state routing protocol.
• Fast convergence of the network is crucial.
Classful vs. Classless Routing
Classful routing is where routing protocols started, during a time when IP shortages were not occurring. Classful routing requires that the properly defined subnets be used according to IPv4 class guidelines. VLSM is not allowed in classful routing. If you are running RIP for example, you will not be able to subnet your Class C network into both /27 and /30. You need to choose either /27 or /30.
Classless routing ignores standard rules and allows the administrator to use VLSM. The following protocols allow classless routing: RIPv2, EIGRP, OSPF, IS-IS, BGP.
A network is converged when all routers have correct routing information and are in a state of consistency.
Convergence time is the amount of time it takes routers to share information, calculate best paths, and update their routing tables.
A metric is a numerical value assigned to a routing path. Metrics are used when multiple paths to a destination are available. Based on the routing protocol used to learn about the path a metric is created (therefore metrics are unique to the routing protocol used).
Examples of routing protocols use of metrics:
• RIP uses hop count to determine it’s metric.
• EIGRP uses bandwidth and delay.
• OSPF uses bandwidth.
Metrics used in IP routing protocols include:
• Hop count
• Cost - A value determined either by IOS or administrator.
To view the metric of a specific route use the following command, show ip route, this reveals the routing table of the router. Identify a route, specifically one learned from a dynamic routing protocol. For example,
R 192.168.8.0/24 [120, 2] via 192.168.4.1, 00:00:26, Serial 0/0/1The metric in this example is 2. Because this route was learned via the RIP routing protocol (R) then the 2 represents hop count, which implies there is one router between you and the destination network.
The value 120 is called the administrative distance.
Administrative distance (AD) is a value between 0 and 255. It defines the preference of a routing source. Due to the fact a router can learn about routes from multiple sources (static, dynamic routing protocol) the AD is used to define the preferred method of learning a route.
If there is a time when two routes are available one from one learning method and another from another learning method, the route with the lowest AD will be chosen.
If there are two or more routes to the same destination with the same AD then the lowest metric value will be chosen.
Basic Administrative Distances
• Directly connected = 0
• Static = 1
• Internal EIGRP = 90
• IGRP = 100
• OSPF = 110
• IS-IS = 115
• RIP = 120
• External EIGRP = 170
• Internal BGP = 200
You do have the ability to change the value of an AD except for directly connected networks.
To view the dynamic routing protocols running issue the following command,
show ip protocolsnote the information that becomes available, such as AD, networks using this protocol, and other features.