What are some examples of routing metrics that can be used to determine the best path for a network?

If a routers learns about multiple different paths to the same network from the same routing protocol, a measure called metric is used to decide which route will be placed in the routing table. Just like with the administrative distance, the lower number represents the better route.

Table of Contents Show

INtroduction
Functional Examples
PROTOCOL EXAMPLES
RIP Metrics
OSPF Metrics
BGP Metrics
Types of Routing
Static Routing
Advantages Of Static Routing
Disadvantages of Static Routing:
Default Routing
Dynamic Routing
Advantages of Dynamic Routing:
Disadvantages of Dynamic Routing:

Each routing protocol has its own way to calculate the metric; Routing Information Protocol (RIP) uses hop counts, OSPF uses a parameter called cost, EIGRP uses bandwidth and the delay to compute the metrics, etc. Note that metrics of different routing protocols can not be directly compared – an EIGRP route might have a metric of 4.042.334, while a RIP route can have a metric of 3.

If two routes have the same AD as well as the same metrics, the routing protocol will load-balance to the remote network, meaning that data will be sent down each link.

To understand the importance of routing metrics, consider the following example:

Let’s say that all routers are running RIP. R1 receives two possible routes to the 10.0.0.0/24 network; one going through R2, and one going through R3 and R4. Both routes are RIP routes and have the same administrative distance, so the metric is used to determine the best route. The RIP metric is hop count, which is simply a number of routers between the source and destination. In this case, the route going through R2 will have a metric of one, because only one router is in the path to the 10.0.0.0/24 network. The route going through R3 and R4 will have a metric of two. The first route will be placed in the routing table and used for packets sent to the 10.0.0.0/24 subnet.

In the example above you can see the problem of using hop counts as a routing metric. Imagine if the first path through R2 was the 56 kbps modem link, while the other path is a high speed WAN link. Router R1 would still chose the path going through R2 as the best route, because RIP uses only the hop count as its metric.

Introduction
Functional Examples
Protocol Examples

INtroduction

Routers use the lowest cost routes to reach a destination. Put another way, routers use the routes that are the easiest and/or fastest way to get somewhere. Cost doesn't have anything to do with the amount of money required to use a connection, but an administrator can configure the metrics to be worse on communications paths that cost more money and thus make communications links that cost less money to be more preferred. This was part of why metrics were originally developed, however high speed communications links have dropped in price dramatically since the early days of the Internet.

The cost of a route is calculated using what are called routing metrics. Routing metrics are assigned to routes by routing protocols to provide measurable values that can be used to judge how useful (how low cost) a route will be. The most useful routes are inserted into the IP routing table on the router. Routes may have more than one metric and the metrics used may be exchanged between routers, or it may be entirely local to that one router. Metrics provide a quantitative value to indicate the specific characteristics of the route.

Functional Examples

The simplest example of a metric is the weight value. Many different routers can use static routes and static routes can be assigned a weight. Higher weight means higher cost. This allows the administrator to create several routes to a destination and then set them up so that one will be preferred over the others, or so that two or more will be used equally. Assigning a route a higher weight causes that route to be less preferred and will not be used until all the more preferred routes with lower weights are no longer available. Equal weights make the static routes equally preferred.

PROTOCOL EXAMPLES

RIP Metrics

RIP is a distance vector protocol which uses hop count to determine the best path through the network. The path with the fewest number of routed hops is considered the shortest path.

OSPF Metrics

The primary OSPF metric is cost, which Cisco and other manufacturers configure to be "inversely proportional to the bandwidth of that interface". Lower cost means a faster interface and shorter end-to-end transmission times and thus the shortest path. The bandwidth of an interface is indirectly passed on with the OSPF route in the form of an additive 'cost' metric to indicate the speed of the entire path to the destination via the local interface link. Because Open Shortest Path First (OSPF) is a dynamic link state routing protocol, higher speed links have a lower cost than low speed links.

BGP Metrics

Border Gateway Protocol (BGP) is a path-vector routing protocol which uses attributes such as weight, local preference, aggregate, AS-path length, origin type, origin, and multi-exit discriminators to determine the best path.

Bookmark this page and SHARE:

A Router is a process of selecting path along which the data can be transferred from source to the destination. Routing is performed by a special device known as a router.
A Router works at the network layer in the OSI model and internet layer in TCP/IP model
A router is a networking device that forwards the packet based on the information available in the packet header and forwarding table.
The routing algorithms are used for routing the packets. The routing algorithm is nothing but a software responsible for deciding the optimal path through which packet can be transmitted.
The routing protocols use the metric to determine the best path for the packet delivery. The metric is the standard of measurement such as hop count, bandwidth, delay, current load on the path, etc. used by the routing algorithm to determine the optimal path to the destination.
The routing algorithm initializes and maintains the routing table for the process of path determination.

Routing metrics and costs are used for determining the best route to the destination. The factors used by the protocols to determine the shortest path, these factors are known as a metric.

Metrics are the network variables used to determine the best route to the destination. For some protocols use the static metrics means that their value cannot be changed and for some other routing protocols use the dynamic metrics means that their value can be assigned by the system administrator.

The most common metric values are given below:

Hop count: Hop count is defined as a metric that specifies the number of passes through internetworking devices such as a router, a packet must travel in a route to move from source to the destination. If the routing protocol considers the hop as a primary metric value, then the path with the least hop count will be considered as the best path to move from source to the destination.
Delay: It is a time taken by the router to process, queue and transmit a datagram to an interface. The protocols use this metric to determine the delay values for all the links along the path end-to-end. The path having the lowest delay value will be considered as the best path.
Bandwidth: The capacity of the link is known as a bandwidth of the link. The bandwidth is measured in terms of bits per second. The link that has a higher transfer rate like gigabit is preferred over the link that has the lower capacity like 56 kb. The protocol will determine the bandwidth capacity for all the links along the path, and the overall higher bandwidth will be considered as the best route.
Load: Load refers to the degree to which the network resource such as a router or network link is busy. A Load can be calculated in a variety of ways such as CPU utilization, packets processed per second. If the traffic increases, then the load value will also be increased. The load value changes with respect to the change in the traffic.
Reliability: Reliability is a metric factor may be composed of a fixed value. It depends on the network links, and its value is measured dynamically. Some networks go down more often than others. After network failure, some network links repaired more easily than other network links. Any reliability factor can be considered for the assignment of reliability ratings, which are generally numeric values assigned by the system administrator.

Types of Routing

Routing can be classified into three categories:

Static Routing
Default Routing
Dynamic Routing

Static Routing

Static Routing is also known as Nonadaptive Routing.
It is a technique in which the administrator manually adds the routes in a routing table.
A Router can send the packets for the destination along the route defined by the administrator.
In this technique, routing decisions are not made based on the condition or topology of the networks

Advantages Of Static Routing

Following are the advantages of Static Routing:

No Overhead: It has ho overhead on the CPU usage of the router. Therefore, the cheaper router can be used to obtain static routing.
Bandwidth: It has not bandwidth usage between the routers.
Security: It provides security as the system administrator is allowed only to have control over the routing to a particular network.

Disadvantages of Static Routing:

Following are the disadvantages of Static Routing:

For a large network, it becomes a very difficult task to add each route manually to the routing table.
The system administrator should have a good knowledge of a topology as he has to add each route manually.

Default Routing

Default Routing is a technique in which a router is configured to send all the packets to the same hop device, and it doesn't matter whether it belongs to a particular network or not. A Packet is transmitted to the device for which it is configured in default routing.
Default Routing is used when networks deal with the single exit point.
It is also useful when the bulk of transmission networks have to transmit the data to the same hp device.
When a specific route is mentioned in the routing table, the router will choose the specific route rather than the default route. The default route is chosen only when a specific route is not mentioned in the routing table.

Dynamic Routing

It is also known as Adaptive Routing.
It is a technique in which a router adds a new route in the routing table for each packet in response to the changes in the condition or topology of the network.
Dynamic protocols are used to discover the new routes to reach the destination.
In Dynamic Routing, RIP and OSPF are the protocols used to discover the new routes.
If any route goes down, then the automatic adjustment will be made to reach the destination.

The Dynamic protocol should have the following features:

All the routers must have the same dynamic routing protocol in order to exchange the routes.
If the router discovers any change in the condition or topology, then router broadcast this information to all other routers.