52 lines
2.6 KiB
Markdown
52 lines
2.6 KiB
Markdown
<!--# set var="title" value="Spanning Tree Protocol Introduction" -->
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<!--# set var="date" value="March 16, 2006" -->
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<!--# include file="include/top.html" -->
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I’ve been considering increased network redundancy for awhile. After trying a few more things live than I should have, I decided to set up some gear in the lab and do some testing. First up is spanning tree.
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STP is, simply, an ethernet-level protocol for disabling redundant links until they’re needed to avoid loops (which effectively kill ethernet networks). It works by electing a root “bridge” (meaning switch in this case). Every other bridge checks to see if it has more than one link to the root; if it does, it puts every link but one in blocking state. This lets you build neat redundant networks:
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<img src="data:image/webp;base64,<!--# include file="images/stp.webp.base64" -->" alt="">
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(__A__, __B__ and __C__ are switches; __1__ and __2__ are servers)
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I just plugged the above arrangement in (three Catalyst 2950s) and it worked. STP is enabled on Cisco switches by default.
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A little closer examination showed more details. On __B__:
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#show spanning-tree
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….
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This bridge is the root
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….
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Fa0/2 Desg FWD
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Fa0/3 Desg FWD
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__B__ is the root, and both links out of it are “desg” — designated links to those switch segments and “FWD” — forwarding packets.
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On __A__:
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#show spanning-tree
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….
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Fa0/1 Desg FWD
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Fa0/2 Root FWD
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This is the first oddity: Fa0/1 links to __C__, and should be blocking. However, apparently only one end of each link blocks. The switch knows that Fa0/2 is a link to the root bridge.
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On __C__:
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#show spanning-tree
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….
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Fa0/1 Altn BLK
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Fa0/3 Root FWD
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This switch has realized that there’s a loop, and blocked the port that provides the longest route to the root bridge, as expected.
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Now, I start a ping of __2__ from __1__. Then I unplug the link between __A__ and __B__ that is currently carrying the traffic. The ping stops. 49 seconds later, it starts again. The Fa0/1 interface on __C__ went from BLK to LIS to LRN to FWD.
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Now, I plug the link back in. The ping stops again. Fa0/1 on __C__ sees the loop immediately and goes to BLK. The restored interface on __B__ goes to LIS, then LRN, and finally FWD. The ping resumes in 31 seconds.
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Spanning tree isn’t particularly fast, and it doesn’t really “route”. Ethernet packets often traverse less-than-optimal paths through the switch fabric since STP doesn’t care where they came from or where they’re going; it only shuts down interfaces that might loop. Nevertheless, it’s simple and effective at building switch redundancy.
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<!--# include file="include/bottom.html" -->
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