Internetworking Different networks –Different bit rates –Frame lengths –Protocols

Internetworking Interconnect at –Physical Layer - Repeaters e.g. connecting segments –Layer 2 - Bridges LANs with same upper layer 2 LLC Different or same MAC Layer –Layer 3 - Routers Typically LAN-WAN, WAN-WAN –Layer 4 or above - Gateways

Internetworking Emphasis is on Bridges –Can be local or remote Remote - –bridge split in half –Network in between

Internetworking Reason for Interconnecting –To communicate across legacy networks –To interconnect several dispersed LANs –To improve performance –Reliability –Security

Internetworking Problems –Different frame formats processing delays –Different Speeds queueing delays –Different frame lengths segmentation not possible –Protocol Differences automatic ACKs –Priorities

Internetworking Types of Bridges –Transparent Bridges –Source Routeing (802.5) Principle is simple

Internetworking Transparent Bridges –Main Decision for Bridge When should frame be forwarded? –Forward if destination address is NOT on the LAN on which it arrived –(Otherwise it is already on the correct LAN for delivery)

Internetworking The decision on whether to Forward raises the following questions –How does the bridge know which port to forward on? Maintains an address table which associates destination addresses with ports (Much the same as a routeing table in a WAN)

Internetworking What if there is no entry for the destination address in the table? Flood the frame on every port (Clearly not the port it arrived on) –How is address table compiled and maintained? By learning (transparently) where everything is

Internetworking Learning –Frames arriving at bridge Note source address and port number New table entries may be created if entry doesn’t already exist Existing entries are confirmed –Bridge is continually learning and maintaining network topology information

Frame received OK at some port, X Destination Known? Destination on LAN X? Forward frame on all LANs except X No Yes Forward frame to next (known) LAN Discard frame Yes No Forwarding Phase

Source Add. known? Add Source Add. to database. Note direction and time Update direction and time End No Yes Learning Phase

Internetworking Looping –Because of topology, captured in routeing information, frames may circle (loop) around the network many times. –This problem also exists in WANs (Time-to-live field in IP) –Topology will almost certainly have included loops for reliability

Internetworking Result - Spanning Tree –Logical tree which overlays the actual network –Achieved by Allowing some ports to forward (those in tree) Blocking remainder

Internetworking Spanning tree construction –Identify root bridge Bridge with lowest address Initially all assume they are root Actual root identified after information exchange –Identify root port Root bridge doesn’t do this Bridge port with ‘cheapest’ route to root bridge –Identify root bridge for each LAN Bridge with ‘cheapest’ route to root bridge –Choose designated port for each LAN

Internetworking Source Routeing Bridges –Typically used with LANs (IBM) –Source determines the complete route (Other approach is link-by-link) –Complete route is carried in header Series of Bridge number / LAN (port) number Carried after address information e.g. B4/L3, B2/L6 etc

Internetworking Route Discovery –By source –Broadcast indication frame sent to destinationalong single route (Where are you?) –Destination responds by sending broadcast frame along multiple routes –Using selective flooding, multiple copies arrive back at source –Contain details of route they followed –Source selects most appropriate route –This is carried in every frame.

Internetworking Spanning tree is a solution to looping problem –Produces tree which links all LANs –Exactly one (enabled) route between any two interconnected LANs –802.1 Spanning Tree Algorithm is distributed –Similar to tree constructing algorithms used in routeing / topology design –Bridges exchange information to construct tree

Internetworking Transparent versus Source –Transparent - topology independent –Source - needs to know about topology –No information frame overhead with transparent –Route discovery frames are extra traffic –Source can give current optimal route –Transparent gives current spanning tree route