Administrative distance allows one to select the best path for reaching a destination when there are two or more routes defined by two different routing protocols for reaching the same destination node.
An autonomous system is a group of more than one associated internet protocol (IP) prefixes along with a clearly described policy about how the exchange of routing information takes place between the other autonomous system.
Scope
- In this article we will study the administrative distance with its example.
- And we will also look at the best path selection process by dynamic protocol.
- Then we will study the introduction of autonomous systems, public and private Autonomous System numbers.
- At last we will see the topic assigning AS numbers.
Administrative Distance (AD)
Administrative distance provides the facility to choose the best path for reaching its destination when there are two or more routes defined by two different routing protocols for reaching the same destination node. The reliability of a routing protocol is defined by the administrative distance.
With the help of the value of administrative distance, every routing protocol is prioritized in order of its reliability more to least.
Example
As small as the AD value, the routing protocol is more reliable.
Let us understand it with an example. Suppose that the advertised route is received by the router to a destination remote network from EIGRP and OSPF, then the best route will be considered as the EIGRP advertised route. And this route will be put in the routing table because this route has the lower AD.
Process of Choosing the Best Path by Dynamic Protocol
First of all, the value of AD is checked when the same advised route is received by the router for the remote network from more than one source. Preference will be given to the AD that has the least AD value. If the advertised routes have the same AD values, then the advertised route’s metrics will be checked. And in this situation, the advertised route having the least metric is placed in the routing table. And if there is a situation where both metric and AD of the advertised route are the same, then load balancing is used to deice which route will be placed in the routing table.
In load balancing traffic will travel from both routes. Two load balancing are equal or unequal. In equal load balancing, equal amounts of traffic are travelled from both routes but at different times. And in unequal load balancing, different amounts of traffic are travelled from both routes.
Autonomous System (AS)
An autonomous system is a group of more than one associated internet protocol (IP) prefixes along with a clearly described policy about how the exchange of routing information takes place between the other autonomous system. In other words, we can also consider an autonomous system as a collection of IP networks that are connected. And need a single administrative entity for management purposes. University, commercial organizations, governments or other types of internet service providers (ISP) are some examples of a single administrative entity.
Generally, an autonomous system is defined as a group of routers. A single administrative entity is used for the management of these routers.
The autonomous system applies to class networks A, B and C. And the autonomous system is designed around the IP prefixes. When the term prefix is used in conjunction with AS then, it is considered a Classless Inter-Domain Routing (CIDR) block and this block is a combination of various IP addresses that have a similar number of bits and also share the same prefixes. That’s why an autonomous system is also considered a routing domain.
This is a criterion through which a device in a network of one AS exchanges packets with the device present in the network of another AS.
Refer to the below image for the example of the CIDR block:
See the example given below:
The AS1 autonomous system has network NET1 and the AS2 autonomous system has network NET2. When the procedure of exchanging information protocols and creating a connection between two autonomous systems is completed then, the computer system available at network NET1 will be able to transfer or exchange messages with the computer system present at network NET2.
Public Autonomous System Number
The size of the Public Autonomous System Number is 16-bits and range of its value is from 1 to 64511. When the customer is connected to multiple ISPs (like multihoming) then the public AS is provided by the service provider. And the value of the global autonomous number is unique and it is provided when the customer requires to propagate its BGP routes by the help of 2 ISPs.
Private Autonomous System Number
The size of the Private Autonomous System Number is 16 bit and range of its value is from 64512 to 65535. Private autonomous system numbers are provided by the service provider to the customer when the requirement of the customer is the multi-connection to the single ISP (dual home or single home network) but not provided in the case of more than one ISPs. This is done so that the autonomous system numbers can be conserved.
Assigning of AS Numbers
Internet Assign Number Authority abbreviated as IANA and firstly autonomous number is assigned to respective regional registries by IANA. Then the autonomous numbers(i.e. Provided by the IANA in blocks) are distributed by the regional registry to the entities that are in their designated area.
Conclusion
- Administrative distance provides the facility to choose the best path for reaching its destination when there are two or more routes defined by two different routing protocols for reaching the same destination node.
- Generally an autonomous system is defined as a group of routers. A single administrative entity is used for the management of these routers.
- The size of the Public Autonomous System Number is
16-bitsand its value ranges from1 to 64511. - The size of the Private Autonomous System Number is
16bit and values of this system number range from64512 to 65535. - Internet Assign Number Authority abbreviated as IANA and firstly autonomous numbers are assigned by it to the respective regional registries.
