Cisco®CCDA Exam Cram Notes : Routing Protocol Scalability Considerations


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2. Addressing and Routing Protocols in an Existing Network

2.2 Routing protocol scalability considerations

There are two basic categories of routing protocols: distance vector and link-state.

1. Distance Vector (Number of hops): Distance vector routing determines the direction (vector) and distance to any link in the internetwork. Typically, the smaller the metric, the better the path. EX: Examples of distance vector protocols are RIP and IGRP. Distance vector routing is useful for smaller networks. The limitation is that any route which is greater than 15 hops is considered unreachable. One important thing that differentiates distance vector with Link state is that distance vector listens to second hand information to learn routing tables whereas, Link state builds its routing tables from first hand information. Distance vector algorithms call for each router to send its entire routing table to each of its adjacent neighbors.

2. Link State Routing: Link State algorithms are also known as Shortest Path First (SPF) algorithms. SPF recreates the exact topology of the entire network for route computation by listening at the first hand information. Link State takes bandwidth into account using a cost metric. Link State protocols only send updates when a change occurs, which makes them more attractive for larger networks. Bandwidth and delay are the most heavily weighed parts of the metric when using Link-State protocols. EX: OSPF and NLSP.

Benefits of Link State protocols:

  • Allows for a larger scalable network
  • Reduces convergence time
  • Allows "supernetting"

3. Balanced Hybrid: Balanced Hybrid combines some aspects of Link State and Distance Vector routing protocols. Balanced Hybrid uses distance vectors with more accurate metrics to determine the best paths to destination networks. EX: EIGRP.

Holddown timers: The purpose is to provide the routers enough time to propogate the routes and to ensure that no routing loops occur while propagation occurs. It helps inpreventing routing loops during periods when the topology is converging. Holddowns tell routers to hold any changes that might affect routes for some period of time. The holddown period is usually calculated to be just greater than the period of time necessary to update the entire network with a routing change.

Poison Reverse-When a router advertises a poisoned route to its neighbors,its neighbors break the rule of split horizon and send back to the originator the same poisoned route, with an infinite metric.

Split Horizon- If a neighboring router sends a route to a router, the receiving router will not propagate this route back to the advertising router on the same interface. Blocks the information about routes from being advertised by any router to the interface from which the information originated.

LSA's- The packets flooded when a topology change occurs, causing network routers to update their topological databases and recalculate routes prevent regular update messages from reinstating a route that has gone bad. Here, if a route fails, the router waits a certain amount of time before accepting any other routing information about that route.

Defining a maximum count- Used for preventing Updates from looping the network indefinitely.

Route Poisoning- Advertises an infinite metric for a failed route to all its neighbors

Triggered update Allows a RIP router to announce route changes almost immediately rather than waiting for the next periodic announcement.

Flow control is the process of managing the rate of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with data from transmitting node. Flow control should be distinguished from congestion control, which is used for controlling the flow of data when congestion has actually occurred. Flow control mechanisms can be classified by whether or not the receiving node sends feedback to the sending node.

Topology: Hub and spoke topology has the advantage of better scalability and faster convergence. Full mesh topology has the advantage of complete link redundancy, but the convergence takes some time. Implementing security policies is also cumbersome when compared to Hub and spoke topology. In Hub and spoke, if a link is broken, the communication is broken with the node on that particular link. A partial mesh topology is a mid-way between the two, as it provides some amount of redundancy and also not as expensive as full mesh topology.

Impact of routing table of performance:

You generally find the convergence times in this order, from shortest to longest:

1. EIGRP with feasible successors.

2. Intrarea OSPF or IS-IS with fast or tuned timers.

EIGRP without feasible successors.

Intrarea OSPF or IS-IS with standard timers.

Interarea OSPF or IS-IS.

The last three are highly variable, in reality. In any particular network, OSPF, IS-IS, and EIGRP without feasible successors might swap positions on the list. The network design, configuration, and several other factors impact the convergence time more than the routing protocol does. Needless to say that network design plays a significant role in arriving at fast convergence times.

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