On Sunday, September 05, 2010 22:51:16 you wrote: >Thanks for the informative discussion! > >On Sep 5, 2010, at 3:56 PM, Boyd Stephen Smith Jr. wrote: >> Switches are smart devices, ... They do not use out-of-band >> information to make >> switching decisions. > >Does Spanning Tree Protocol and/or V-lan tagging count as "out-of-band >information"?
Vlan mapping is configuration information. Single ports are tagged as either trunked (expect vlan header and process it) or vlan $n (expect packets with no vlan header and "add" it; remove vlan header when sending). The headers themselves are in-band, since they are directly attached to the packet they apply to. I suppose there might be some protocol I'm not familiar with that dynamically rearranges vlan mappings much the same way BGP plays with routing tables. I don't believe it part of Ethernet standard, but it certainly could exist. I believe a device that spoke this hypothetical protocol would be more appropriately called a router (or a "layer 2" router). Whether STP is on is a configuration information. Keeping track of which MACs are on each port, and which ports are part of the same tree, also just requires "memorizing" information about the existing packet flow, and can be read from the packets themselves. I referred to BGP as "out-of-band" since it contains information about how to route "unrelated" packets. Seeing that a certain MAC/IP was visible on a certain port and sending packets destined to that address down that port is remembering information about related packets, which is what switches do. STP is an extension to this process. Some switches are less feature-complete then others and have very little (or nothing) in the way of configuration information. They are barely more than a hub, and don't support vlans or STP, but only the base Ethernet specification. When they first came out, they were layer 2 devices, now they don't even support all our layer 2 features, but as long as they understand a bit more than the physical layer, they are a switch and not a hub. -- Boyd Stephen Smith Jr. ,= ,-_-. =. b...@iguanasuicide.net ((_/)o o(\_)) ICQ: 514984 YM/AIM: DaTwinkDaddy `-'(. .)`-' http://iguanasuicide.net/ \_/
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