SEMESTER 2 Chapter 4 Distance Vector Routing Protocols V 4.0

4.1.1 What are the three distance vector routing protocols?

Rip, IGRP, EIGRP

What are the key characteristics of RIP?

Hop count is used as the metric for path selection. If the hop count for a network is greater than 15, RIP cannot supply a route to that network. Routing updates are broadcast or multicast every 30 seconds, by default.

What are the key characteristics of IGRP?

Bandwidth, delay, load and reliability are used to create a composite metric. Routing updates are broadcast every 90 seconds, by default. IGRP is the predecessor of EIGRP and is now obsolete.

What are the key characteristics of EIGRP?

It can perform unequal cost load balancing. It uses Diffusing Update Algorithm (DUAL) to calculate the shortest path. There are no periodic updates as with RIP and IGRP. Routing updates are sent only when there is a change in the topology

4.1.2 How are distance vector routes advertised?

As the name implies, distance vector means that routes are advertised as vectors of distance and direction

Does a router in a distance vector network have knowledge of the entire path to a destination network?

A router using a distance vector routing protocol does not have the knowledge of the entire path to a destination network

What are the 2 things a distance vector router knows?

The direction or interface in which packets should be forwarded and The distance or how far it is to the destination network

4.1.2.2 What are the two reasons sending an entire routing table in an update inefficient?

Some distance vector routing protocols call for the router to periodically broadcast the entire routing table to each of its neighbors. This method is inefficient because the updates not only consume bandwidth but also consume router CPU resources to process the updates.

What is the periodic update time for RIP?

30 seconds for RIP

What is the periodic update time for IGRP?

90 seconds for IGRP

What is the address for broadcast updates?

Broadcast Updates are sent to 255.255.255.255

4.1.3 What is the algorithm used for? The algorithm is used to calculate the best paths and then send that information to the neighbors.

What processes does the routing protocol define?

Mechanism for sending and receiving routing information. Mechanism for calculating the best paths and installing routes in the routing table. Mechanism for detecting and reacting to topology changes.

4.1.4 Define Time to Convergence. Time to convergence defines how quickly the routers in the network topology share routing information and reach a state of consistent knowledge. The faster the convergence, the more preferable the protocol. Routing loops can occur when inconsistent routing tables are not updated due to slow convergence in a changing network.

Define Scalability. Scalability defines how large a network can become based on the routing protocol that is deployed. The larger the network is, the more scalable the routing protocol needs to be.

Define Classless (Use of VLSM) or Classful.

Classless routing protocols include the subnet mask in the updates. This feature supports the use of Variable Length Subnet Masking (VLSM) and better route summarization. Classful routing protocols do not include the subnet mask and cannot support VLSM.

Define Resource Usage. Resource usage includes the requirements of a routing protocol such as memory space, CPU utilization, and link bandwidth utilization. Higher resource requirements necessitate more powerful hardware to support the routing protocol operation in addition to the packet forwarding processes.

Define Implementation and Maintenance

Implementation and maintenance describes the level of knowledge that is required for a network administrator to implement and maintain the network based on the routing protocol deployed.

List the advantages for distance vector protocols.

Simple implementation and maintenance, Low resource requirements

List the disadvantages for distance vector protocols.

Slow convergence, Limited scalability, Routing loops

4.1.4.2 Definitely practice the exercise at the right. It will help you remember each protocol.

4.2.1 What are the routes that router will initially discover after booting?

When a router cold starts or powers up, it knows nothing about the network topology. It does not even know that there are devices on the other end of its links. The only information that a router has is from its own saved configuration file stored in NVRAM.

After completing the initial discovery what does the router start to do?

Once a router boots successfully, it applies the saved configuration. As described in Chapter 1 and Chapter 2, if the IP addressing is configured correctly, then the router will initially discover its own directly connected networks

4.2.2.1 What does the initial exchange Once a router boots successfully, it applies the saved

of routing information contain? configuration. As described in Chapter 1 and Chapter 2, if the IP addressing is configured correctly, then the router will initially discover its own directly connected networks

What does the router do with the information that is not contained in its routing table?

Upon receiving an update, the router checks it for new information. Any routes that are not currently in its routing table are added.

After the first exchange of routing updates what has been added to each routing table?

After this first round of update exchanges, each router knows about the connected networks of their directly connected neighbors

4.2.3.1 What is convergence? Speed and ability of a group of internetworking devices running a specific routing protocol to agree on the topology of an internetwork after a change in that topology

What is slit horizon? Routing Technique in which information about routes is prevented from exiting the router interface through which that information was received. Split-horizon updates are useful in preventing routing loops.

4.2.4 The amount of time it takes for a network to converge is directly proportional to ____________________

The amount of time it takes for a network to converge is directly proportional to the size of that network

The speed of achieving convergence consists of what two things?

How quickly the routers propagate a change in the topology in a routing update to its neighbors. The speed of calculating best path routes using the new routing information collected.

4.3.1 What are the two reasons routers exchange routing updates?

What is the period update time for RIP?

For RIP, these updates are sent every 30 seconds as a broadcast (255.255.255.255) whether or not there has been a topology change

What are the four reasons listed for topology changes?

Failure of a link Introduction of a new link Failure of a router Change of link parameters

4.3.1.2 What are the three additional timers?

Invalid Flush Holddown

If an update has not been received to refresh an existing route after 180 seconds, what happens to that route in the routing table?

the route is marked as invalid by setting the metric to 16. The route is retained in the routing table until the flush timer expires

What happens to the route after 240 seconds have elapsed?

the flush timer is set for 240 seconds, which is 60 seconds longer than the invalid timer. When the flush timer expires, the route is removed from the routing table.

What does a holddown timer do?

This timer stabilizes routing information and helps prevent routing loops during periods when the topology

is converging on new information

What are the two commands that will show the timers?

Show ip route and show ip protocols

4.3.2 Define bounded update. Updates that are sent only to those routers that need the updated information instead of sending updates to all routers

What are characteristic of EIGRP updates?

Non-periodic because they are not sent out on a regular basis. Partial updates sent only when there is a change in topology that influences routing information. Bounded, meaning the propagation of partial updates are automatically bounded so that only those routers that need the information are updated

4.3.3 What is a triggered update? A triggered update is a routing table update that is sent immediately in response to a routing changez

What are the three reasons to send a triggered update?

An interface changes state (up or down) A route has entered (or exited) the "unreachable" state A route is installed in the routing table

What are the two problems with triggered updates?

Packets containing the update message can be dropped or corrupted by some link in the network. The triggered updates do not happen instantaneously. It is possible that a router that has not yet received the triggered update will issue a regular update at just the wrong time, causing the bad route to be reinserted in a neighbor that had already received the triggered update.

4.3.4 What is it called when all the routers send updates at the same time on a network with a hub at the center?

Synchronized Updates

It this a problem on a switched network?

yes

4.4.1 What is a routing loop? A routing loop is a condition in which a packet is continuously transmitted within a series of routers without ever reaching its intended destination network.

What are some reasons for routing loops?

Incorrectly configured static routes Incorrectly configured route redistribution (redistribution is a process of handing the routing information from one routing protocol to another routing protocol and is discussed in CCNP-level courses) Inconsistent routing tables not being updated due to slow convergence in a changing network Incorrectly configured or installed discard route

What mechanism is built into IP to overcome routing loops?

The IP protocol has its own mechanism to prevent the possibility of a packet traversing the network endlessly. IP has a Time-to-Live (TTL) field and its value is decremented by 1 at each router. If the TTL is zero, the router drops the packet.

What conditions can be created because of routing loops?

Link bandwidth will be used for traffic looping back and forth between the routers in a loop. A router's CPU will be strained due to looping packets. A router's CPU will be burdened with useless packet forwarding that will negatively impact the convergence of the network. Routing updates may get lost or not be processed in a timely manner. These conditions would introduce additional routing loops, making the situation even worse. Packets may get lost in "black holes."

What are some of the mechanisms used to avoid routing loops?

Time-to-live

4.4.2 What is count to infinity? Count to infinity is a condition that exists when inaccurate routing updates increase the metric value to "infinity" for a network that is no longer reachable

4.4.3 What is infinity defined by? is defined by setting a maximum metric value.

What is infinity for RIP? RIP defines infinity as 16 hops - an "unreachable" metric. Once the routers "count to infinity," they mark the route as unreachable.

4.4.4 What is it called when a route goes up, then down, then up etc.?

Flapping

What are holddown timers used for?

Holddown timers are used to prevent regular update messages from inappropriately reinstating a route that may have gone bad.

Describe the process of how a holddown timer works.

1. A router receives an update from a neighbor indicating that a network that previously was accessible is now no longer accessible. 2. The router marks the network as possibly down and starts the holddown timer. 3. If an update with a better metric for that network is received from any neighboring router during the holddown period, the network is reinstated and the holddown timer is removed. 4. If an update from any other neighbor is received during the holddown period with the same or worse

metric for that network, that update is ignored. Thus, more time is allowed for the information about the change to be propagated. 5. Routers still forward packets to destination networks that are marked as possibly down. This allows the router to overcome any issues associated with intermittent connectivity. If the destination network truly is unavailable and the packets are forwarded, black hole routing is created and lasts until the holddown timer expires.

4.4.5 What is the split horizon rule? The split horizon rule says that a router should not advertise a network through the interface from which the update came.

4.4.6 What is route poisoning? Route poisoning is used to mark the route as unreachable in a routing update that is sent to other routers

4.4.6.2 What is split horizon with poison reverse?

The rule for split horizon with poison reverse states when sending updates out a specific interface, designate any networks that were learned on that interface as unreachable.

4.4.7 What is time-to-live (TTL)? Time to Live (TTL) is an 8-bit field in the IP header that limits the number of hops a packet can traverse through the network before it is discarded

What happens when a packet’s TTL reaches 0?

Eventually the TTL value will be decreased to 0 and the packet will be discarded by the router.

4.5.1 What are the factors that affect the distance vector protocol you choose?

Size of the network Compatibility between models of routers Administrative knowledge required

What are the features of RIP? Supports split horizon and split horizon with poison reverse to prevent loops. Is capable of load balancing up to six equal cost paths . The default is four equal cost paths.

What features were introduced with RIPv2?

Includes the subnet mask in the routing updates, making it a classless routing protocol. Has authentication mechanism to secure routing table updates. Supports variable length subnet mask (VLSM). Uses multicast addresses instead of broadcast. Supports manual route summarization.

What are the features of EIGRP?

Triggered updates (EIGRP has no periodic updates). Use of a topology table to maintain all the routes received from neighbors (not only the best paths). Establishment of adjacencies with neighboring routers using the EIGRP hello protocol. Support for VLSM and manual route summarization. These allow EIGRP to create hierarchically structured large networks.

What are the advantages of EIGRP?

Although routes are propagated in a distance vector manner, the metric is based on minimum bandwidth and cumulative delay of the path rather than hop count. Fast convergence due to Diffusing Update Algorithm (DUAL) route calculation. DUAL allows the insertion of backup routes into the EIGRP topology table, which are used in case the primary route fails. Because it is a local procedure, the switchover to the backup route is immediate and does not involve the action in any other routers. Bounded updates mean that EIGRP uses less bandwidth, especially in large networks with many routes. EIGRP supports multiple Network layer protocols through Protocol Dependent Modules, which include support for IP, IPX, and AppleTalk.