TNMK09 Computer Networks Copyright © 2005 Di Yuan, ITN, LiTH 1  Non-hierarchical routing, static or dynamic, won’t work in the Internet  None of the two approaches scales  Networks belong to different operators, each having its own routing preference  The Internet is divided into many Autonomous Systems (AS)  The routing solution within an AS is handled by its administration  Then, a common routing protocol distributes routes between AS Hierarchical Routing in the Internet (Distance vector)(Link State)

TNMK09 Computer Networks Copyright © 2005 Di Yuan, ITN, LiTH 2  Every router sends periodically its knowledge of the entire network to its neighbors  Typically, every link has a cost of one  A router uses information from its neighbor(s) to update the table Distance Vector Routing (1)

TNMK09 Computer Networks Copyright © 2005 Di Yuan, ITN, LiTH 3  Eventually every router builds up a table that can be used to reach all networks Distance Vector Routing (2)

TNMK09 Computer Networks Copyright © 2005 Di Yuan, ITN, LiTH 4  Another example showing the idea of distance vector routing Distance Vector Routing (3) B C

TNMK09 Computer Networks Copyright © 2005 Di Yuan, ITN, LiTH 5  A router talks to its neighbors to get link state information  Which neighbors are up and working?  Every router sends periodically its link state database to all the other routers  Eventually all routers have identical link state information of the entire network Link State Routing (1)

TNMK09 Computer Networks Copyright © 2005 Di Yuan, ITN, LiTH 6  Every router uses the Dijkstra algorithm to compute the shortest path tree Link State Routing (2)

TNMK09 Computer Networks Copyright © 2005 Di Yuan, ITN, LiTH 7 Link State Routing (3)