RIP Routing Fundamentals

RIP Routing Fundamentals
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Posted: 08 Oct 2010 04:40 PM PDT

RIP stands for Routing Information Protocol.
RIP is a dynamic, distance vector routing protocol and was developed for smaller IP based networks. As mentioned earlier, RIP calculates the best route based on hop count.
There are currently two versions of RIP protocol.

RIPv1, and
RIPv2

RIPv1: RIP version 1 is among the oldest protocols.

Limitations of RIPv1:
1. Hop Count Limit: Destination that is more than 15 hops away is considered unreachable by RIPv1.
2. Classful Routing Only: RIP is a classful routing protocol. RIPv1 doesn't support classless routing. RIP v1 advertises all networks it knows as classful networks, so it is not possible to subnet a network using RIP v1.
3. Metric limitation: The best route in RIP is determined by counting the number of hops required to reach the destination. A lower hop count route is always preferred over a higher hop count route. One disadvantage of using hop count as metric is that if there is a route with one additional hop, but with significantly higher bandwidth, the route with smaller bandwidth is taken. This is illustrated in the figure below:

The RIP routed packets take the path through 56KBPS link since the destination can be reached in one hop. Though, the alternative provides a minimum bandwidth of 1MBPS (though using two links of 1MBPS, and 2MBPS each), it represents 2 hops and not preferred by the RIP protocol.

Features of RIP v2:

RIP v2 is a revised version of its predecessor RIP v1. The following are the important feature enhancements provided in RIPv2:
1. RIPv2 packets carry the subnet mask in each route entry, making RIPv2 a classless routing protocol. It provides support for variable-length subnet masking (VLSM) and classless addressing (CIDR).
2. Next Hop Specification: In RIPv2, each RIP entry includes a space where an explicit IP address can be entered as the next hop router for datagrams intended for the network in that entry.
For example, this field can be used when the most efficient route to a network is through a router that is not running RIP. Since, that a router will not exchange RIP messages, explicit Next Hop field allows the router to be selected as the next hop router.

3. Authentication: RIPv1 does not support authentication. This loophole may be used maliciously by hackers, that may resulting in delivering the data packets to a fictitious destination as determined by the hacker. RIPv2 provides a basic authentication scheme, so that a router can accept RIP messages from a neighboring router only after ascertaining its authenticity.
4. Route Tag: Each RIPv2 entry includes a Route Tag field, where additional information about a route can be stored. It provides a method for distinguishing between internal routes (learned by RIP) and external routes (learned from other protocols).

Limitations of RIP v2:

One of the biggest limitations of RIPv1 still remains with RIPv2. It is hop count limitation, and metric. The hop count of 16 still remains as unreachable, and the metric still remains hop count. A smaller hop count limits the network diameter, that is the number of routers that can participate in the RIP network.
Example Question:
While the packet travels from source to destination through an Internetwork, which of the following statements are true? (Choose 2 best answers).
A. The source and destination hardware (interface) addresses change
B. The source and destination hardware (interface) addresses remain constant.
C. The source and destination IP addresses change
D. The source and destination IP addresses remain constant.
Ans. A, D
Explanation: While a packet travels through an Internetwork, it usually involves multiple hops. It is important to know that the logical address (IP address) of the source (that created the packet) and destination (final intended destination) remain constant, whereas the hardware (interface) addresses change with each hop.

The Cisco Three-Layered Hierarchical Model

Posted: 08 Oct 2010 04:44 PM PDT

Cisco has defined a hierarchical model known as the hierarchical internetworking model. This model simplifies the task of building a reliable, scalable, and less expensive hierarchical internetwork because rather than focusing on packet construction, it focuses on the three functional areas, or layers, of your network:

Core layer: This layer is considered the backbone of the network and includes the high-end switches and high-speed cables such as fiber cables. This layer of the network does not route traffic at the LAN. In addition, no packet manipulation is done by devices in this layer. Rather, this layer is concerned with speed and ensures reliable delivery of packets.

Distribution layer: This layer includes LAN-based routers and layer 3 switches. This layer ensures that packets are properly routed between subnets and VLANs in your enterprise. This layer is also called the Workgroup layer.

Access layer: This layer includes hubs and switches. This layer is also called the desktop layer because it focuses on connecting client nodes, such as workstations to the network. This layer ensures that packets are delivered to end user computers.

Figure INT.2.1 displays the three layers of the Cisco hierarchical model.

When you implement these layers, each layer might comprise more than two devices or a single device might function across multiple layers.The benefits of the Cisco hierarchical model include:

High Performance: You can design high performance networks, where only certain layers are susceptible to congestion.
Efficient management & troubleshooting: Allows you to efficiently organize network management and isolate causes of network trouble.
Policy creation: You can easily create policies and specify filters and rules.
Scalability: You can grow the network easily by dividing your network into functional areas.
Behavior prediction: When planning or managing a network, the model allows you determine what will happen to the network when new stresses are placed on it.

Core Layer

The core layer is responsible for fast and reliable transportation of data across a network. The core layer is often known as the backbone or foundation network because all other layers rely upon it. Its purpose is to reduce the latency time in the delivery of packets. The factors to be considered while designing devices to be used in the core layer are:

High data transfer rate: Speed is important at the core layer. One way that core networks enable high data transfer rates is through load sharing, where traffic can travel through multiple network connection
Low latency period: The core layer typically uses high-speed low latency circuits which only forward packets and do not enforcing policy.
High reliability: Multiple data paths ensure high network fault tolerance; if one path experiences a problem, then the device can quickly discover a new route.

At the core layer, efficiency is the key term. Fewer and faster systems create a more efficient backbone. There are various equipments available for the core layer. Examples of core layer Cisco equipment include:

Cisco switches such as 7000, 7200, 7500, and 12000 (for WAN use)

Catalyst switches such as 6000, 5000, and 4000 (for LAN use)

T-1 and E-1 lines, Frame relay connections, ATM networks, Switched Multimegabit Data Service (SMDS)

Distribution Layer

The distribution layer is responsible for routing. It also provides policy-based network connectivity, including:

Packet filtering (firewalling): Processes packets and regulates the transmission of packets based on its source and destination information to create network borders.
QoS: The router or layer 3 switches can read packets and prioritize delivery, based on policies you set.
Access Layer Aggregation Point: The layer serves the aggregation point for the desktop layer switches.
Control Broadcast and Multicast: The layer serves as the boundary for broadcast and multicast domains.
Application Gateways: The layer allows you to create protocol gateways to and from different network architectures.
The distribution layer also performs queuing and provides packet manipulation of the network traffic.

It is at this layer where you begin to exert control over network transmissions, including what comes in and what goes out of the network. You will also limit and create broadcast domains, create virtual LANs, if necessary, and conduct various management tasks, including obtaining route summaries. In a route summary, you consolidate traffic from many subnets into a core network connection. In Cisco routers, the command to obtain a routing summary is:

show ip route summary

You can practice viewing routing information using a free CCNA exam router simulator available from SemSim.com. You can also determine how routers update each other's routing tables by choosing specific routing protocols.

Examples of Cisco-specific distribution layer equipment include 2600,4000, 4500 series routers

Access Layer

The access layer contains devices that allow workgroups and users to use the services provided by the distribution and core layers. In the access layer, you have the ability to expand or contract collision domains using a repeater, hub, or standard switch. In regards to the access layer, a switch is not a high-powered device, such as those found at the core layer.

High reliability: Multiple data paths ensure high network fault tolerance; if one path experiences a problem, then the device can quickly discover a new route.

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Posted: 08 Oct 2010 09:00 AM PDT

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Posted: 08 Oct 2010 09:00 AM PDT

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Posted: 08 Oct 2010 09:00 AM PDT

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