Addison Wesley Interconnections Bridges, Routers, Switches, and Internetworking Protocols (2nd Edition)

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.If no cacheentry exists for the destination, the SR-TB bridge must forward the packet.It has severalchoices.1.Forward the packet as an all paths explorer packet.2.Forward the packet as a spanning tree explorer packet.3.Cache the packet, go through a route discovery procedure, and forward the packet ifor when a route is discovered to the destination.4.Drop the packet (hope that the users will attribute the loss to congested LANs), gothrough some sort of route discovery procedure, and assume that the source willretransmit the packet if it was important.With luck, by the time the packet isretransmitted, a route to the destination may have been found.The choice depends on how routes are being discovered, something that was neverstandardized.Two possible ways that the SR-TB bridge can learn a route to station S are1.Record the route from received specifically routed packets from S2.Choose routes based on received all paths explorer packets launched by SAfter a route is stored, there is the problem of maintaining it.The SR-TB bridge does notnecessarily know whether the packets it forwards along a route to station S actually reach thatstation.There can be no notification by the higher layer that a route is no longer working.Thus, there are only two possibilities for removing cache entries.1.Remove the route to S if no packets from S (with a route identical to the stored route)are received within a specified amount of time.2.Remove the route to S periodically, regardless of received traffic.Neither of these schemes is particularly satisfactory.If routes are deleted quickly, theoverhead increases greatly because of the need to rediscover routes frequently.Also, themore frequently routes change, the greater the probability of out-of-order packets.If thetimers are very slow, a nonfunctional route will persist for a long time.4.2.2 Packets from an SR PortTo deal with packets from the SR side, each portion of the network interconnected withtransparent bridges will be assigned a LAN number.Even if there are hundreds of LANswithin a TB portion, it is still assigned one LAN number and appears to the SR side as asingle LAN (see Figure 4.14).Figure 4.14.Each TB portion assigned its own SR LAN numberThe SR-TB bridge can receive three types of packets:1.Specifically routed2.Spanning tree explorer3.All paths explorerThe first two cases are fairly straightforward.If the SR-TB bridge receives a specifically routedpacket whose LAN number indicates that it should be forwarded onto the TB side, the SR-TBbridge forwards the packet, first removing the source routing header.If the SR-TB bridgereceives a spanning tree explorer packet, it removes the source routing header whenforwarding to ports into the TB portions of the network.The third case is more difficult.If the SR-TB bridge receives an all paths explorer packet andif the launcher of the packet is expecting replies from the packet's target, the SR-TB bridgemust respond on behalf of the target station.If the SR-TB bridge has learned that the targetexists on the TB port, it can respond by adding the LAN number of the TB port and replyingwith a specifically routed packet along the reverse path.If the SR-TB bridge has not learnedthe location of the target station, it could forward the all paths explorer packet, minus thesource routing header, into the TB portion of the network and hope that the target station willtransmit something in response.This would allow the SR-TB bridge to acquire a cache entryfor the target, thereby enabling it to respond to a subsequent all paths explorer packet if oneshould arrive shortly.The protocols for an SR-TB bridge are inherently confusing because the protocols fordiscovering routes have not been standardized.It is in no way obvious that all strategiesinterwork.4.2.3 LoopsAs Figure 4.15 indicates, it is possible for SR-TB bridges to create a loop.It is essential thatone of the following strategies be employed to prevent this.Figure 4.15.Loops of interconnected SR and TB domains1.The entire extended LAN (SR bridges, SR-TB bridges, and TB bridges) canparticipate in one instance of the spanning tree.If that is done, there is the dangerthat the spanning tree would configure itself by breaking up a cloud.In addition,stations within the cloud would have to communicate via SR-TB bridges and clouds ofdifferent types, thereby degrading performance within the cloud.2.The SR-TB bridges can run a separate instance of the spanning tree among onlythemselves.In this case, some SR-TB bridge will break the loop.The clouds willremain intact.4.3 SRT BridgesYou can design a network so that independent sections of the network are tied together withsource routing bridges and independent sections of the network are tied together withtransparent bridges, and then those two types of sections are interconnected with SR-TBbridges.But this strategy proves unsatisfactory.SR-TB bridges have unsolvable problems(they function as "black holes" when their route caches have bad routes; their route findingstrategy must be compatible with all possible source routing end-system implementations).And the restriction that no LAN can contain both types of bridges proved unworkable whenboth the transparent bridge community and the source routing community wanted to bridge toFDDI.The dramatic resolution was that the source routing proponents came to the standardsbody with the SRT proposal, which outlawed pure source routing bridges.Source routing wasremoved from the 802.5 standard and was made an optional "enhancement" to transparentbridges.This means that there are only two standard types of bridges:1 [ Pobierz całość w formacie PDF ]

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