classless routing protocol
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Classless Inter-Domain Routing
Classless Inter-Domain Routing (CIDR /ˈsaɪdər, ˈsɪ-/) is a method for allocating IP addresses and for IP routing. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous classful network addressing architecture on the Internet. Its goal was to slow the growth of routing tables on routers across the Internet, and to help slow the rapid exhaustion of IPv4 addresses.[1][2]
IP addresses are described as consisting of two groups of bits in the address: the most significant bits are the network prefix, which identifies a whole network or subnet, and the least significant set forms the host identifier, which specifies a particular interface of a host on that network. This division is used as the basis of traffic routing between IP networks and for address allocation policies.
IPv4
Whereas classful network design for IPv4 sized the network prefix as one or more 8-bit groups, resulting in the blocks of Class A, B, or C addresses, under CIDR address space is allocated to Internet service providers and end users on any address-bit boundary. In IPv6, however, the interface identifier has a fixed size of 64 bits by convention, and smaller subnets are never allocated to end users.
CIDR encompasses several concepts. It is based on variable-length subnet masking (VLSM) which allows the specification of arbitrary-length prefixes. CIDR introduced a new method of representation for IP addresses, now commonly known as CIDR notation, in which an address or routing prefix is written with a suffix indicating the number of bits of the prefix, such as 192.0.2.0/24 for IPv4, and 2001:db8::/32 for IPv6.
OSPF
CIDR introduced an administrative process of allocating address blocks to organizations based on their actual and short-term projected needs. The aggregation of multiple contiguous prefixes resulted in supernets in the larger Internet, which whenever possible are advertised as aggregates, thus reducing the number of entries in the global routing table.
Packets take different routes when sent in large and complex networks. Some routes can be longer while others are shorter.
The fastest available routes are always the best hence a mechanism is needed to identify them.
This article will cover how we can apply classless routing protocols on networks to identify fast routes using the Open Shortest Path First (OSPF) protocol.
Terminologies
Classless routing – It refers to a type of routing where the router uses the default route to forward traffic if no other specific routes are found. It includes subnet mask information in the routing update and is supported by RIPv2, OSPF, and EIGRP protocols.
Its Protocol
Classless routing protocols are those protocols that include the subnet mask information when the routing tables or updates are exchanged. Classless routing protocols allow using the networks with the different length of subnet masks. Hence, these routing protocols support the CIDR (Classless Inter-Domain Routing) and VLSM (Variable Length Subnet Mask). Classless routing protocols do not send the periodic updates (which means sending the routing updates at the regular intervals) like classful routing protocols.
Classless routing protocols exchange the entire routing table with the neighbor routers only at the very first time. Once the network is fully converged, routing updates are sent only when there is any changes occur in the network topology. It helps to reduce the bandwidth consumption. However, classless routing protocols send Hello messages at the regular intervals to their neighbor routers (directly connected routers), regardless when changes occur in the network topology.
IS-IS
They use the classless routing protocols exchange the entire routing table with the neighbor routers only at the very first time. Once the network is fully converged, routing updates are sent only when there is any changes occur in the network topology. It helps to reduce the bandwidth consumption. However, classless routing protocols send Hello messages at the regular intervals to their neighbor routers (directly connected routers), regardless when changes occur in the network topology. They use the Hello packets to verify whether the neighbor routers are alive or not. The examples of classless routing protocols include OSPF and IS-IS.
Administrative Distance (AD) – This feature is used by routers to select the best path possible.
Metric – It refers to the ways used by each classless routing protocol to determine the best path to a network. OSPF uses the Sum of Inverse of Bandwidth while RIP uses Hop Count. EIGRP relies on Min Bandwidth + Delay.
32-bit
Wildcard Mask – It refers to the inverses of subnet masks configured on interfaces and used by OSPF to specify the range of IP addresses to examine for a match. It is usually 32-bit long.
A logical grouping of OSPF networks, routers, and links with the same area identification is referred to as an OSPF area. A router within an area is required to have a topological database for that particular region.
OSPF neighbors – It is the relationship that exists between two OSPF-enabled routers in the same area that is connected by a common network.
Difference between Classful Routing and Classless Routing
Classless Routing imports subnet mask and in this, triggered updates used. In classless routing, VLMS (Variable Length Subnet Mask) supported and also CIDR (Classless Inter-Domain Routing). Hello messages used for checking status in classless routing. In classless routing, subnet mask is not same throughout, it may vary for all devices, we can see it in the given picture.
Let’s take a look at an example to emphasize this:
We have three routers and a bunch of networks. Take a close look at the networks that we have here:
- 172.16.1.0 /24
- 172.16.2.0 /24
- 192.168.12.0 /24
- 192.168.23.0 /24
Remember the class A,B and C ranges?
- 172.16.1.0 and 172.16.2.0 fall within the class B range.
- 192.168.12.0 and 192.168.23.0 fall within the class C range.
What subnet mask do class B and class C networks have by default?
- Class B: 255.255.0.0
- Class C: 255.255.255.0
A classful routing protocol will not send the subnet mask along with the routing update so this is what will happen:
R1 and R3 don’t send the subnet mask along with the routing update so it will advertise the classful network which is 172.16.0.0 in this case. So what happens with R2? It thinks it can reach the 172.16.0.0 network by sending packets either left or right and if the metric is equal it will try to load-balance. Obviously this is going to cause problems.
Classless routing protocols advertise the subnet mask along with their updates:
As you can see R1 is now advertising its 172.16.1.0 subnet with a subnet mask. R3 is advertising its 172.16.2.0 subnet with a subnet mask as well.
Classful Versus Classless Routing Protocols
Classful routing protocols do not carry subnet masks; classless routing protocols do. Older routing protocols, including RIP and IGRP, are classful. Newer protocols, including RIP-2, EIGRP, and OSPF, are classless. What are the implications of using classful versus classless routing protocols in your networks?
Let’s say that a router R received a RIP-1 update with the IP address 172.0.0.0. R would assume that the route being advertised was for the Class B network 172.0.0.0/16. In other words, since the subnet mask is lacking in the routing update, R assumes a natural mask of /8, /16, and /24 for Class A, B, and C addresses, respectively.
The only time a classful routing protocol can associate a mask other than the natural mask with an update is if R has a directly connected network with an IP address belonging to the same class as the IP address received in the update. For example, when Ames receives an update of 172.16.1.0 from NewYork, Ames associates a mask of /24 with the update because Ames is able to deduce the mask from its own interface.
IP address
RIP-2 updates carry a subnet mask in each route entry. A routing protocol that carries subnet masks in its updates earns the label “classless routing protocol.” The term “classless” implies that routing decisions not tied to the class of the IP address — A, B, or C — but may be based on any portion of the 32-bit IP address as specified by the mask. Router R could receive an update with the …
Which routing algorithm or protocol should used for a specific network?
There is no straightforward answer to use a specific routing algorithm or protocol for a specific network. The answer to the preceding question depends upon the various factors and may also vary on the case-by-case scenarios. However, if you have a clear idea about all the different routing algorithms and protocols, you can take a decision that best suited for your network.
In this post, we have discussed the differences between classful and classless routing protocols. Hope, it helped you.
Thank you for staying with this post “classless routing protocol” until the end.
