Friday, February 25, 2011


(Note that this is not an official Basic Draft article. It does not meet Basic Draft standards and will not be listed. If you've arrived through a search engine, please use the search function (top-left) to find a more suitable article).
Load balancing is the act of balancing a stream of packets for a network over different paths. The paths can be derived statically or using a dynamic protocol.

Here's the decision in pseudocode:
if (sameNetwork())
if (sameAD())
if (sameMetric())

Load balancing can be done per-packet or per-host. Per-packet splits the stream of packets towards the same host over different paths. Per-host allocates each stream of packet to different hosts a different path.

Routing Loop problems such as the Count To Infinity occurs when neighbors are not immediately updated when a route goes down. The neighbors may then re-advertise the downed down back to the originator, resulting in the originator believing that there exists an alternate route through the neighbor. The neighbor still thinks that the route exists through the original router, therefore resulting in a loop. The original router then re-advertises back to the neighbor at a higher metric. The higher metric path replaces the previous one. The neighbor then re-advertises back to the originator, and it replaces the path. This will cause the hop-count to count up.

The crude solution to fixing Count to Infinity is the restriction of the hop-count to a maximum of 15. That way, if there really is a loop, it would only count to 15 and be dropped.

According to the seminar, these are the three conditions that may result in a routing loop:
-Alternate Routes
-Slow Convergence
-Inconsistent Routing Tables

Another way to solve the Count to Infinity is to prevent updates about a network from being sent to interfaces where you learned that network from. This means, if you learned about Network A from F0/0, you will not send information about Network A out of F0/0.

Split horizon would reduce incorrect information as well as bandwidth overhead.

Route Poisoning is the poisoning of routes by setting an originated route's Hop Count to 16 if the connected interface goes down. (This is done by the router originating the network).

Poison Reverse is another form of Split Horizon, but it does not reduce bandwidth overhead. This mechanism simply causes information going back to the originator to be marked with the hop count of 16. (e.g. If you learn of Network A through F0/0, then you would advertise Network A as Hop 16 out of F0/0).

Triggered update, as used in LS, can also be used in DV. This causes any information to be sent immediately after a topology change. These triggered updates are sent with poisoning (i.e. They are sent with a hop-count of 16) to cause the routes to be dropped immediately without waiting for the hold-down timer to expire.

Hold-down timers can also be a route-prevention mechanism. The hold-down timer is always activated and reset whenever an update is received. While the timer is counting down, an update from another neighbor with a poorer metric is ignored. An update from another neighbor with a better metric would cause the update to be installed in the routing table and the hold-down timer would reset.

RIP uses the following mechanisms to reduce/avoid Count-down to Infinity:
-Hold-down timers
-Triggered Updates
-Poison Reverse and Route Poisoning
-Split Horizon
-Maximum Hop-count

RIP performs round-robin load balancing only. Which means to say, RIP does per-packet load balancing for a particular destination.

Routing Protocols can be used to advertise and receive default routes as well. As like all Routing Protocols, dynamic routing can be overridden with static routes by adjusting the AD.

The Metric of IGRP is 10,000,000Kbps/(Smallest Bandwidth in Kbps)+(Sum of all Delays in microseconds/10).

IGRP uses Hold-down, Split Horizon and Poison-Reverse.

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