Bridges. Bridge Functions. Example of No-frills Bridge. No-frills Bridges. Example of Learning Bridge. Learning Bridges

Transcription

1 ridge Functions To extend size of LNs either geographically or in terms number of users. Protocols that include collisions can be performed in a collision domain of limited size. In ring networks the number of stations is limited. To connect LNs that use different technologies To avoid using more costly routers ridges Various types of bridges No-frills bridges Learning bridges omplete (Spanning Tree) bridges omplete bridges Topology changes Timeout procedure Settable parameters Source routing bridges VLNs No-frills ridges Example of No-frills ridge Serve to extend the size of a single LN segment, i.e. the size of a collision domain. ridge receives packets from all LNs attached to its ports. ridge receives a packet, stores it, and broadcast it to all of its ports when they become idle, except to the port that received the packet. The total network capacity cannot exceed the capacity of a single LN. P PORT R Learning ridges ridge receives packets from all LNs attached to its ports. Whenever a learning bridge receives a packet from some LN, it reads the packet source address and stores the source and the corresponding port into the cache memory. Whenever a bridge receives a packet, it reads the packet destination address, and the port address to which the destination is attached from the cache memory, if the address is available. ridge transmits the packet to the read port, or to all ports except to the receiving one, if the port address is not available. ache entries are deleted after a specified timeout period. Example of Learning ridge Station sends to station PORT

2 Example of Learning ridge Station sends to station PORT Example of Learning ridge Station sends to station PORT Example of Learning ridge Station sends to station PORT Example of Learning ridge Station sends to station PORT Example of Learning ridge Example of ultiple Learning ridges Station sends to station PORT K T PORT T K PORT LN LN LN3 K T

3 Topology with Loops Learning ridges with Loops 3 LN LN ll three bridges receive a packet, note that station is on LN, and queue the packet for transmission. Say bridge 3 is the first to transmit the packet onto LN. ridges and view the packet as it is transmitted on LN, note that is now on LN, and queue the packet. Say bridge now transmit the first received packet onto LN. ridge 3 note that the packet is on LN and queue the packet. The number of packets transmitted on the network exponentially increases. omplete ridges Spanning Tree (ST) lgorithm omplete bridges are defined by IEEE 80. standard. They run spanning tree algorithm to exclude loops. tree comprising bridges is calculated, and these bridges send messages toward the tree root. Tree is formed in a distributed way, each bridge sends configuration messages, and each bridge forwards only the best configuration message. The procedure stops when all bridges forward the same configuration message. ased on the information from the configuration messages, bridges calculate the spanning tree. ridges choose the bridge to be the tree root. ridges calculate the number of hops to the tree root. For each LN, the designated bridge is determined, which forwards packets to the root. esignated bridge determines the root port through which it forwads packets to the root. onfiguration essage onfiguration message format estination Source SP SSP configuration message SP=SSP= onfiguration message comprises tree root I, cost of forwarding (the number of hops from the tree root), transmitting bridge I, port I at the transmitting bridge, settable parameters. est onfiguration essage The best configuration message has the lowest root I. If multiple messages have the same root I, the best message has the lowest cost. If multiple messages have the same root I, and the same cost, the best message has the lowest transmitting bridge I. If multiple messages have these three values the same, the best one has the lowest port I on the transmitting bridge. Port Port Port 3 Root 5 ost ridge Port becomes a root port, and forwards messages to ports and 3 3

4 est onfiguration essage Port Port Port 3 Root 5 ost ridge Root bridge is, given bridge becomes designated bridge for LNs attached to its ports and 3, the bridge port becomes a root port, and forwards configuration messages to ports and 3, cost (the number of hops) is incremented by becoming 86 and updated in the configuration message which is then forwarded Example of ST lgorithm ridge 9 receives the configuration messages PORT PORT primljene poruke Example of ST lgorithm ridge 9 receives the configuration messages 9 PORT PORT Refinements of ST lgorithm hanges of the topology because of failures or new equipment are announced with the special messages. Upstream bridges acknowledge those notifications. hanges of topology should not introduce loops. For this reason preforwarding time is introduced. Failures of the links or bridges must be detected by the downstream bridges. Root bridge sends configuration messages reapetedly. onfiguration messages have age, and maximum age. ache values with the positions of the stations should be regularly updated. So, cache is deleted after timeout period. Notification of Topology hange Topology changes when a bridge or a link fails, or a new bridge or a new link is added to the network. ridge that notices the topology change sends the topology change notification message on its root port to the upstream bridge, once per hello time, until the upstream bridge acknowledges the receipt of the topology change notification message. Topology change notification messages are propagated in this way bridges in the tree to the root bridge. Root then sets topology change flag in the configuration messages that it sends downstream. voiding Loops as Topology hanges Loops can be formed in transient intervals when there are topology changes. When topology changes a new tree is calculated. Some bridges might turn on before the others turn off, and loop can be formed. efore some bridge start forwarding, it waits during the time interval sufficient for all bridges to get the information about new spanning tree. Waiting time is divided into listening and forwarding intervals. uring the listening interval, the bridge only forwards configuration messages. uring the learning interval, the bridge receives messages only to learn about the positions of the stations, but does not forward them. 4

5 Topology hange ue to Failures Root transmits configuration messages with age equal to 0 once per each hello time. Root also specifies the maximum age. Each bridge increments message age field in each slot of a specified duration. It sends this message every hello time. When the message age exceeds the maximum age, the bridge discards the configuration message in question, and recalculates the spanning tree. Example of Failure onfiguration message at root port 4 expires, and port 3 becomes a root port PORT PORT Example of Failure onfiguration message at root port 3 expires, and port 5 becomes a root port. 9 PORT PORT 4 Example of Failure onfiguration message at root port 5 expires, and bridge 9 becomes a root bridge. 9 PORT PORT ache uration ecause placement of stations changes, the cache entries linking stations and ports should be deleted occassionaly, after the cache timeout period. ache timeout period should be as long as several minutes. ut, when the bridges get the configuration messages with the topology change flag set, they set the cache timeout period to the forwarding delay. Settable Parameters ridge and the port priorities: two and one octet respectively. Hello time: the time that elapses between two consecutive configuration messages, or between consecutive topology change notification messages. Recommended s. ax age: the configuration message age value for which it is discarded as too old. Recommended value 0s, s per hop. 5

6 Settable Parameters Forward delay: the duration of the listening modes, and the learning mode before a bridge starts forwarding data. It is half the time needed for the topology information to spread. Recommended value 30s. Long cache timer: recommended 5min. Path cost: the cost to be added to the cost field at some bridge. onfiguration essage Format broj okteta 8 Topology hange 4 ck 8 Flag protocol identifier version message type T reserved T root I cost of path to root bridge I port I message age max age hello time forward delay Topology hange Flag Topology hange Notification essage Format broj okteta protocol identifier version message type Problems of ridging The probability of packet loss increases. The delay increases. Error rate increases when R is changed. Packet reordering when the tree is reconfigured. Packet duplication because of temporary loops. Stations cannot use the maximum packet size. LN specific information such as priority may be lost. Unexpected packet format conversion may occur. Remote ridges One bridge must inform the other if it ignores its packets. Packet format has to be agreed upon. Each bridge has to be connected to all bridges on WN separately. They have to have individual network addresses because multicasting is not supported. Source Routing ridges It is optional in IEEE 80.. Packet header carries the information about the path that it should follow. ontrol packets, all path explorers or spanning tree explorers, discover a path from the source to the destination that want to communicate. Routes are deleted from the station cache after a timeout period. 6

7 Packet Format estination Source RI ata Route information (RI): Type (3bits): specifically routed, all paths explorer, spanning tree explorer Length (5bits) of RI in bytes irection (bit) Largest frame (3bits): 56bytes-65535bytes Route: a sequence of bytes fields, bits for LN number and 4bits for bridge number. Finding a Route ll path explorer packets are sent like in learning bridges, while spanning tree explorers are sent like in transparent bridges. There are several possible ways to find a route. Source sends all path explorer, and destination gets multiple copies of it, it sends each of the copies back along its route. They collect the route and the maximum packet length on the paths. The packet that is the fastest to return back determines the route. lternatively, destination may choose a path, and reply with the specifically routed packet. Source may first send spanning tree explorer to check the availability of the destination. estination may respond with the reply to the spanning tree explorer, and/or all path explorer. Virtual LN (VLN) VLN is equivalent to the broadcast domain. otivations for VLNs are: separation of broadcast domains, moving stations without changing their IP addresses, security. ultiple VLNs can be attached to one packet switch. Stations attached to one port may belong to one or more VLNs. Packet travelling between switches have VLN tag comprising bytes. References Radia Perlman, Interconnections: ridges, Routers, Switches and Internetworking Protocols, ddison-wesley January 000. HOEWORK: Problems 5,7, in Section 3 Transparent ridges. 7