CCNA Expl 2. Chapter 4. Distance vector routing protocols

CCNA Exploration 2.Routing protocols and Concepts.
RESUME. 

Chapter 4.  Distance vector routing protocols.

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. 

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.

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.

Distance vector routing protocols share certain characteristics.

• Periodic Updates are sent at regular intervals (30 seconds for RIP and 90 seconds for IGRP). Even if the topology has not changed in several days, periodic updates continue to be sent to all neighbors.
• Neighbors are routers that share a link and are configured to use the same routing protocol.      Routers using distance vector routing are not aware of the network topology.
• Broadcast Updates are sent to 255.255.255.255.
• Entire Routing Table Updates are sent, with some exceptions  periodically to all neighbors.

The algorithm used for the routing protocols defines the following processes:

• 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.

When a router cold starts or powers up, it knows nothing about the network topology. The only information that a router has is from its own saved configuration file stored in NVRAM.

If a routing protocol is configured, the routers begin exchanging routing updates. Initially, these updates only include information about their directly connected networks. Upon receiving an update, the router checks it for new information. Any routes that are not currently in its routing table are added.

The speed of achieving convergence consists of:

• 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.

A network is not completely operable until it has converged, therefore, network administrators prefer routing protocols with shorter convergence times.

RIP Timers

1. Update timer

2. Invalid timer

     If an update has not been received to refresh an existing route after 180 seconds (the default),
     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.

3. Flush timer

    By default, 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.

4. Holddown timer

    Once a route is marked as unreachable, it must stay in holddown long enough for all routers in the topology

     to learn about the unreachable network. By default, the holddown timer is set for 180 seconds. 

Чтобы посмотреть таймеры используем команды : "show ip route", "show ip protocols".

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EIGRP sends bounded updates about a route when a path changes or the metric for that route changes.

            EIGRP uses updates that are:
                 ~ 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.


To speed up the convergence when there is a topology change, RIP uses triggered updates.
Triggered updates are sent when one of the following occurs:
              ~An interface changes state (up or down)
              ~A route has entered (or exited) the "unreachable" state
              ~A route is installed in the routing table.

There are two problems with triggered updates:
1. Packets containing the update message can be dropped or corrupted by some link in the network.
2. 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.

Sending updates at the same time is known as the synchronization of updates. Synchronization can become a problem with distance vector routing protocols due to their usage of periodic updates.

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. 

The loop may be a result of:

• 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 routes 

To eventually stop the incrementing of the metric, "infinity" is defined by setting a maximum metric value. For example, RIP defines infinity as 16 hops - an "unreachable" metric.
Чтобы  остановить увеличивающиеся метрики, "бесконечность"определяется путем установления максимального значения метрики.RIP - 16 прыжков.

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

Holddown timers work in the following way:
          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.

1. The split horizon rule says that a router should not advertise a network through the interface from which the update came.
2. Route poisoning is used to mark the route as unreachable in a routing update that is sent to other routers. 
3. 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.

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. 

Purpose of the TTL field is to avoid a situation in which an undeliverable packet keeps circulating on the network endlessly. 

For distance vector routing protocols, there really are only two choices: RIP or EIGRP. 

Factors of choice: 

          1. Size of the network

          2. Compatibility between models of routers

          3. Administrative knowledge required.