Cisco® CCNP Route Exam Notes : Address Types (unicast, Broadcast, Multicast, And Vlsm)

3. Layer3 Technologies

3.1 Identify IPv4 addressing and subnetting

3.1.a Address types (Unicast, broadcast, multicast, and VLSM)

Unicast addresses are assigned to a single interface on a device. They are used for one-to-one communication.

Broadcast addresses are assigned to all interfaces in a subnet. Broadcast packets are sent from one host to everyone.

Multicast addresses are assigned to a group of devices on various subnets. These are used for one-to-many communications. The multicast address range specified by RFC 1112 is through

VLSM (Variable Length Subnet Mask): VLSM (Variable Length Subnet Masking) allows efficient use of IP addresses. Networks implemented with VLSM can be summarized more efficiently due to manual control. With a distance vector protocol such as RIP , only one subnet mask value can be used on a network, as subnet mask values are not sent in routing updates.

EIGRP supports aggregation and variable length subnet masks (VLSM). Unlike Open Shortest Path First (OSPF), EIGRP allows summarization and aggregation at any point in the network. EIGRP supports aggregation to any bit. This allows properly designed EIGRP networks to scale exceptionally well without the use of areas. EIGRP also supports automatic summarization of network addresses at major network borders.

EIGRP is an advanced routing protocol that combines many of the features of both link-state and distance-vector routing protocols, EIGRP's DUAL algorithm contains many features which make it more of a distance vector routing protocol than a link-state routing protocol.

Given below are some important features of classful and classless routing protocols:

Classfull routing protocols: RIPv1, IGRP are examples of classful routing protocols. It is important to know that classful routing protocols do not exchange subnet information during routing information exchanges. The summarization is always done automatically at major network boundaries.

Classless routing protocols: RIPv2, EIGRP, OSPF, BGPv4, and IS-IS are examples of classless routing protocols. In classless routing protocols, subnet information is exchanged during routing updates. This results in more efficient utilization of IP addresses. The summarization in classless networks is manually controlled.

IP route summarization is used to make networks more flexible and efficient. Although some routing protocols such as RIPv1 and IGRP summarize only at the boundaries of major network numbers, others support route summarization (aggregation) at any bit boundary. Variable-length subnet masks enable routing protocols to summarize on bit boundaries. The following are the advantages to summarizing addresses into a hierarchy:

1. Reduces the amount of information stored in routing tables - Without summarization, a reouter needs to process every single route in the network. With summarization, routers can condense network addresses down to a single link advertisement, resulting in a reduction in both the resource load on the router and the overall network complexity. Route summarization is most effective in large networks.

2. Allocates an existing pool of addresses more economically - The available IP addresses are limited. Route summarization ensures that IP addresses are utilized efficiently.

3. Makes the routing process more efficient - With less overhead, routers are faster and more efficient.

4. Lowers the network convergence time - The network convergence time would reduce with route summarization.

5. Isolates topology changes - If any individual route changes, the change would be localized. The summary address may remain the same, thus saving unnecessary updates over the network.

6. Facilitates monitoring, reporting, and troubleshooting - A hierarchical address space is relatively easy to monitor and troubleshoot.

The reduction in route propagation and routing information overhead can be significant. Take a sample network of /24. Without summarization, each router in a large enterprise network of 250 subnets (28 = 256 subnets with 28 - 2 = 254 hosts each) would need to know about 250 routes. With route summarization, you can quickly reduce the size of the routing tables by almost 75%. If the Class B network used 7 bits of subnet address space (/23) instead of 8 bits (/24), the original 250 subnets could be broken up into two major subnetworks of about 125 each. Each router would still need to know all the routes for each subnet in its network number. However, that number would be reduced to 125 routes plus one additional route for the other major network. This process of collapsing many subnet routes into a single network route is a fundamental goal of route summarization.

The following are features of CIDR:

1. Classless Inter-Domain Routing (CIDR) is a replacement for the traditional process of assigning Class A, B and C addresses with a generalized network "prefix". A CIDR address includes the standard 32-bit IP address and also information on how many bits are used for the network prefix. For example, in the CIDR address, the "/25" indicates the first 25 bits are used to identify the network portion of the address.

2. CIDR makes efficient use of IP address space and also minimizes the routing table entries compared to traditional addressing (Class A, Class B, and Class C) schemes.

3. With CIDR, organizations are recommended to get the IP addresses directly from their ISPs. This is because, it would enable hierarchical IP addressing and hence less number of routing table entries.

Route Summarization : Route summarization means summarizing a group of routes into a single route advertisement. The net result of route summarization, and its most obvious benefit, is a reduction in the size of routing tables on the network. This in turn reduces the latency associated with each router hop since the average speed for routing table lookup will be increased due to the reduced number of entries. The routing protocol overhead can also be significantly reduced since fewer routing entries are being advertised. This can become critical as the overall network (and hence the number of subnets) grows.

Subnetting: is nothing but creating networks within a network. Subnetting allows an organization with a single IP address (Class A /Class B /Class C) to have multiple subnetworks, thus allowing several physical networks with in the organization.

Addresses are written using decimal numbers separated by decimal points. This is called dotted decimal notation of expressing IP addresses.

The different classes of IP addresses is as below:

Class Format Leading Bit Pattern Network address Range Maximum networks Maximum host/nodes
A N.H.H.H 0 0 - 126 127 16,777,214
B N.N.H.H 10 128 - 191 16,384 65,534
C N.N.N.H 110 192 - 223 20,971,52 254
D Not defined 1110 224 -255 Not defined Not defined
E Not defined 1111 240-255 Not defined Not defined

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