3.3.c [i] Distance vector
The name distance vector is derived from the fact that routes are advertised as vectors of (distance, direction), where distance is defined in terms of a metric and direction is defined in terms of the next-hop router. For example, “Destination A is a distance of 7 hops away, in the direction of next-hop router Y.” As that statement implies, each router learns routes from its neighboring routers’ perspectives and then advertises the routes from its own perspective . Because each router depends on its neighbors for information, which the neighbors in turn may have learned from their neighbors, and so on, distance vector routing is sometimes also referred to as “routing by rumor.”
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2650-2655). . Kindle Edition.
3.3.c [ii] Link state
Link state routing protocols are like a roadmap. They have a complete picture of the network. The reason is that unlike the routing-by-rumor approach of distance vector, link state routers have firsthand information from all their peer routers. Each router originates information about itself, its directly connected links, and the state of those links. This information is passed around from router to router, each router making a copy of it, but never changing it. The ultimate objective is that every router has identical information about the internetwork, and each router independently calculates its own best paths.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2659-2663). . Kindle Edition.
3.3.c [iii] Path vector
A path vector protocol is a routing protocol which maintains the path information that gets updated dynamically. Updates which have looped through the network and returned to the same node are easily detected and discarded. This algorithm is sometimes used in Bellman– Ford routing algorithms to avoid “Count to Infinity” problems. BGP is an example of a path vector protocol.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2668-2670). . Kindle Edition.
3.3.c (i) Distance vector
3.3.c (ii) Link state
3.3.c (iii) Path vector
Distance – hop or metric Vector – direction
periodic transmission of entire routing table
mathematical comparison using measurement of distance
limited by hop count
ex. OSPF IS-IS
sends local connection information to all nodes on the internetwork
forms adjacencies with and sends link state information to same protocol speaking connected neighbors
about the state of it’s own links
constructs a view of the entire topology with this information
DUAL (Diffusing Update Algrithm)
exhibits characteristics of both link state and DV
subset of DV
constructs lists of passed AS paths to establish metrics and remain loop free
attributes are employed to affect sending and reception of traffic
3.3.c [iii] Path vector
Like Interior Gateway Protocols (IGP), BGP exchanges topology information in order for routers to eventually learn the best routes to a set of IP prefixes. Unlike IGPs, BGP does not use a metric to select the best route among alternate routes to the same destination. Instead, BGP uses several BGP path attributes (PA) and an involved decision process when choosing between multiple possible routes to the same subnet.
why? bgp is path vector not distance vector… big deal, right…
in the distance part of distance vector you can read metric… a loose definition of vector means direction… metric direction versus AS direction… AS’s versus calculations…