Bridges, Routers and Switches Part One Three things are certain: Death, taxes, and lost data. Guess which has occurred. Haiku error message

Bridging Bridges connect two independent LANs to form internetworks

Types of Bridges Transparent Source Routing Transnational

Bridges Each LAN is referred to as a Network Segment

Bridge Ports Bridge Ports connect the Bridge to the Network Consist of a Network Interface (Ethernet, Token Ring, FDDI)

Bridges Form One Logical Network

Bridging and ISO Model Bridges function at at the Media Access (MAC) layer

Bridging and ISO Model Bridges “see” the network at the MAC layer Bridges make forwarding decisions based on MAC (Physical) addresses

Bridging and ISO Model Bridges have no knowledge of paths between address Bridges are transparent to higher level protocol Bridges are protocol independent, the same bridge can pass IP, IPX, DecNet traffic

Transparent Bridges Usually used on Ethernet networks Called Transparent because nodes are not aware of its presence Defined by IEEE standard Forwards frames between networks Self learning

Transparent Bridges Maintains a data base of all known node address on each port

Transparent Bridges Transparent Bridges Transparent Bridges are self learning Examine every frame which passes a port Determines the source address Compares each address to its data base If address is not in data base then the bridge adds the address

Transparent Bridges

Transparent Bridge Frame Forwarding Examines Destination address If address is on the same LAN as the incoming port, ignores the frame If Not Looks up destination address in its Data Base If Address is found Forward out the appropriate Port If Address is not found Forwards out all Ports

Spanning Tree Algorithm

Eliminates all active loops in the network by creating a set of paths with only one path between each node Bridges negotiate which paths are to remain open and which paths will be blocked If a path becomes inaccessible, the bridges will reconfigure the network, creating a new spanning tree and new set of paths Spanning Tree Algorithm

Source Route Bridging Only used in Token Ring environments

Source Route Bridging NOTE: Despite its name Source Routing is Bridging and takes place at the MAC layer

Source Route Bridging Node maintain their own routing table When a frame is transmitted to a node on another ring, the packet must contain the route or path Node “discovers” the route by sending out an Explorer Frame with the address of the destination Node Depending on the protocols used the node may send an All Route Exploder Frame or a Singe Route Explorer Frame

Source Route Bridging The route explorer frame records the Bridge Number and Ring Number of all bridges and networks crossed When it reaches the destination node, the destination node sets a flag and transmits the frame back to the sending node The Route Explorer Frame now has the route to the destination station If two Routes return to the sending node, it usually uses the first route received

Source Route Bridging

Source Route Bridges can connect rings running at different speeds

What’s Wrong with Source Routing? Takes processing resources of end nodes Each Ring and Bridge must be assigned a Ring Number and Bridge Number - Any error in assigning these numbers causes fatal errors Fatal errors happen often

Source Route Bridging

Allows loops How many paths though this network?

Source Route Bridging What else is wrong with Source Routing? What is good about Source Routing?

Source Route and Transparent Bridges in Same Network Usually not very reliable - may work in some simple networks Source Routed traffic can only cross Source Route Bridges

Transnational Bridges Connect two dissimilar topologies

Advantages of Bridging Easy to install and configure Inexpensive Transparent to Network Automatically adapt to Network Changes Bridge unroutable protocols

Disadvantages of Bridges Can not take advantage of redundant paths in the network Can not prevent Broadcast Storms Because bridges form a single logical network the network becomes large and therefore difficult to manage

Routers What makes a Protocol Routable To be routable a protocol must assign addresses so that the address contain Network and Node address Routable Protocols include IP IPX DecNet Vines Non-Routable Protocols NetBios

Routers Function of a Router is to efficiently forward packets between networks Very often more than one path exists, a router must chose the “best” path Routers function at the Network layer of the OSI Model, routing decisions are made using the address assigned by a Network layer protocol such as IP

Routers Since routers function at the Network layer routers are protocol dependent devices Must support IP, IPX, AppleTalk, DecNet, etc.

Routers Multiple Protocols Most routers can route a verity of protocols The ability to route multiple protocols is implemented in software

Routers

Routers Routers can connect dissimilar LAN topologies Routers can connect LANs to the WAN

Routers - Why use Routers Segment Networks Congestion Control Source Quench technique - Router can request a source to stop transmitting until congestion is relieved

Routers Bridges examine each frame on the network segment Routers only receive packets addressed to them Packets are addressed to routers from workstations or other routers

Routers Node Configuration

Routers Routing Tables Routers maintain a Routing Table The Routing Table instructs the router where (out which interface) to send the packet Packets are routed one hop at a time

Routers Routing Table

Routers - Multiple Routes

Routers Multiple Routes Routing Table Router 1 Target Net Next Hop Router Metric Net4 Attached 0 Net 1 Router 2 1 Net 1 Router 3 2 Net 3 Router 3 1 Net 3 Router 2 2 Net 2 Router 3 1 Net2 Router 2 2

Routers Metric Values Chose route with lowest Metric value Metric value can represent: Hops Transmission Delay Administrative Considerations (Cost) Line Capacity

Routers - Backup Links

Routers - Routing Protocol Routing Tables are dynamically created by the router Router communicate with other routers to learn all paths through the network This communication consists of all or portions of each routers routing table

Routers - Routing Protocol Routers use these Routing Tables to build a map of the network topology The time it takes for all routers in a network to exchange routing tables is called Convergence Router to Router communication is implemented according to Routing Protocols

Routers - Routing Protocol Design Goals Ideally Routing Protocols would meet all these criteria: Be able to find the optimal path Simple enough algorithm to require a minimum of processing power and overhead Robust enough to function when the unexpected happens, such as hardware failures Rapid convergence - failure to converge quickly can cause (temporary) network outages There a numerous Routing Protocol, each has its strengths and weaknesses

Routers - Routing Protocols Two types of Routing Protocols Distance-Vector Algorithm Link-State Algorithm

Routers - Distance-Vector Algorithm Each router calculates the metric value of the links to all its neighbors Each router then sends a broadcast message containing its entire routing table to its neighboring routers The receiving station then uses its neighbor’s routing table to calculate a new routing table The router then broadcasts its new routing table to its neighbors

Routers - Distance-Vector Algorithm Routers use the metric values received to calculate the shortest path through the network The routers do not know how the network segments are connected This algorithm results in slow convergence The exchange of Routing Tables uses a high amount of bandwidth

Routers - Link State Algorithm Protocols Each router broadcasts the portion of its Routing Table which describes its links to its neighboring routers This description includes links and metric values From this information routers can determine the entire network topology - That is they know how all the segments are connected together

Routers - Link State Algorithm Protocols Router use their knowledge of the network topology to calculate the optimum path though the network When a router detects any changes in network topology it broadcasts an update to other routers Converge faster than Distance Vector protocols More CPU/memory intensive than Distance Vector Protocols

Routers Interior Routers route within an Autonomous System Run Interior Routing Protocols Boarder Gateway Routers route between Autonomous Systems Run Boarder Routing Protocols

Routers - Routing Information Protocol - RIP Distance Vector protocol Interior Gateway Protocol One of the original Routing Protocols - Very widely used

Routers - Routing Information Protocol Sends routing updates at regular intervals Uses a Hop Count for metric value Each Hop usually adds one Hop to the metric value Uses IP address as next hop There is limit of 15 hops between source and destination Prevents routing loops

Routers - Interior-Gateway Routing Protocol IGRP Developed by Cisco Systems Distance Vector Interior routing protocol Uses a combination of Inter-network delay, bandwidth, reliability and load to determine the best path A metric value is assigned to each of these factors The weight of each factor is user definable

Routers - Interior-Gateway Routing Protocol Permits Multi-path routing Lines of equal-bandwidth will be used in equal amounts Lines with unequal bandwidth will be used in proportion to its bandwidth, that is, a line with twice the bandwidth will be given twice the amount of traffic Only routes with metrics within a specified range of the best route will be used as multiple routes

Routers - Interior-Gateway Routing Protocol

Routing updates are sent at regular intervals When a router goes down its neighbors are able to detect this by their failure to receive a routing update The router then recalculates it routing table based on this information - all routes though that router are now invalid The router then sends out its routing table at the scheduled interval

Routers - Interior-Gateway Routing Protocol Network Convergence takes time, during this interval a router which is not aware of the invalid routes will broadcast its routing table containing the now invalid routes, this table can find its way to a router which has already received the data about the downed router. This router now replaces its current routing table with an outdated one.

Routers - Interior-Gateway Routing Protocol Hold Down Time Hold down times are used to prevent a router from using a route which may have gone down Routers “hold down” any changes which may affect routes for a specified period of time Hold down times are usually calculated to be greater than the network convergence time

Routers - Interior-Gateway Routing Protocol

Split Horizon Routers never send information about a route back in the same direction it came from There is no reason for Sue to send Harry information about Bob because Harry is closer to Bob

Routers - Boarder Gateway Protocol BGP Routes between autonomous systems Only exchanges information with other BGP routers When a BGP router first joins the network routers exchange their entire routing table after that they exchange incremental information Responsible for exchanging information about the ability to reach other networks