# 4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 1 Internet Protocol - Continued.

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4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 1 Internet Protocol - Continued

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 2 Networks with Loops Learning strategies can fail, causing packets to circulate until maximum hop count Need method to generate a spanning tree –touches every node only once –has no closed loops

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 3 Perlman Spanning Tree Algorithm Developed for LAN bridges Each bridge blocks redundant ports Deleted ports unblocked upon link failure Algorithm –node with lowest identifier is “root” –root node forwards through all ports –each LAN has designated port “nearest” root –smallest identifier breaks ties

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 4 Example Network

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 5 Example Network The network has two loops –one through the “60” network –one through the “80” network Root distances to networks are as follows: –“10” (d=0)– “60” (d=2, 3) –“20” (d=1) – “70” (d=3) –“30” (d=2)– “80” (d=1, 3) –“50” (d=2)

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 6 Procedure The longer route to the “60” network through B4 is “pruned” by designating Port 3 as the route from B3. The longer route to the “80” network through B4 is “pruned” by designating Port 3 as the route from B1. Ports 3 and 4 of B4 are blocked

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 7 Spanning Tree of Network

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 8 Routing Table for B1

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 9 Distance Vector Routing Typically used with RIP protocol Bellman-Ford algorithm

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 10 Bellman-Ford Algorithm Find the shortest paths to the source vertex, given metric = 1 Assign cost to node(s) contacted Find next shortest path, to unassigned node and assign cost to it Continue until all nodes have costs assigned

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 11 Example Network

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 12 Initial Routing Table for R1

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 13 New Routing Information

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 14 Path of Length =1

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 15 Path of Length =2

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 16 Path of Length =3

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 17 Path of Length =4

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 18 Link State Routing Typically used with OSPF Dijkstra algorithm

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 19 Dijkstra’s Algorithm Computes shortest path from “root” node to all other nodes (routers) Method: –maintain list U of nodes with shortest paths assigned –maintain list V of neighbors of U list (1-hop) –choose shortest path from V and move to U

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 20 Example Network

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 21 Example Network

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 22 Dijkstra Algorithm - Step 1

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 23 Example Network

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 24 Dijkstra Algorithm - Step 2

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 25 Example Network

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 26 Dijkstra Algorithm - Step 3

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 27 Example Network

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 28 Dijkstra Algorithm - Step 4

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 29 Example Network

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 30 Dijkstra Algorithm - Step 4

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 31 Reverse Path Forwarding Assumptions –each router stores shortest paths to all nodes –each router stores next-hop to all nodes Algorithm –source broadcasts multicast packet –each router retransmits packets on all outgoing ports except incoming only if incoming packet was from next-hop node to source

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 32 Example

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 33 Example

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 34 Example

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 35 Example

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 36 Example

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 37 Example

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 38 Example

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 39 Example

4/12/2015© 2009 Raymond P. Jefferis IIILect 08 - 40 RPF Tree from R1

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