" alt="">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.
+ +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:
+ +" alt="">
(A, B and C are switches; 1 and 2 are servers)
+ +I just plugged the above arrangement in (three Catalyst 2950s) and it worked. STP is enabled on Cisco switches by default.
+ +A little closer examination showed more details. On B:
+ +#show spanning-tree
+….
+This bridge is the root
+….
+Fa0/2 Desg FWD
+Fa0/3 Desg FWD
+B is the root, and both links out of it are “desg” — designated links to those switch segments and “FWD” — forwarding packets.
+ +On A:
+ +#show spanning-tree
+….
+Fa0/1 Desg FWD
+Fa0/2 Root FWD
+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.
+ +On C:
+ +#show spanning-tree
+….
+Fa0/1 Altn BLK
+Fa0/3 Root FWD
+This switch has realized that there’s a loop, and blocked the port that provides the longest route to the root bridge, as expected.
+ +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.
+ +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.
+ +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.
+ + diff --git a/images/stp.png.base64 b/images/stp.png.base64 new file mode 100644 index 0000000..27f0007 --- /dev/null +++ b/images/stp.png.base64 @@ -0,0 +1,82 @@ +iVBORw0KGgoAAAANSUhEUgAAAMQAAAEQCAIAAABgHEyxAAAACXBIWXMAAAsTAAALEwEAmpwYAAAA +B3RJTUUH1gMQFhUtdcsdfgAAAB10RVh0Q29tbWVudABDcmVhdGVkIHdpdGggVGhlIEdJTVDvZCVu +AAARgklEQVR42u2de3RU1b3H93nNeyYhIRESEJVAIKA1cJGHQCm0UF4VUJEk1FQKXS5aFrB6l7iu +LYj31nVxXV/Vy1VBJQgJPggiSIstxfLyhYApREKARcAEeYRk3jPnef/IkBkCJpPkzMyZM9/PymKd +CTPn/PbsT/b+nb33OYdSFIUAoAY0vgIAmQBkApAJAMgEIBOATAAyAQCZAGQCuoRN1IHPSLVfCIeq +pCO1Us138oVG+aqf+AghZmLJpHv2ofsOYPLvYYaN5Mb0ZwagnpICKs5zc2ek2q3Bil3CjhqpOsqP +5DMF07iZDxqLYBVkCvGV+Pm6wKs7+G1d3sNMw+xFpt+NYEeh2lJXpga5fo1v9Xv8ZlX2NtdQssKy +KofOReWlnEzlgbKVvie8xKPiPq3E9ozluWJTKeovhWR60ru0LLg+RjsvNS78b+vLqEL9y8ST4AJ3 +0R5hd0yjn8RNecteYSBGVKRuZeJJsMj1wCFxfxwKMIYdV+HYDp+0QEwGLRe4i+JjEiHkkLh/gbsI +FalPmZ70Lo1179aGPcLuJ71LUZd6k6k8UBa7jLsdyoLrywNlqE795EwNcv345mHqjgJ0arxgX/oR 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+
+(__A__, __B__ and __C__ are switches; __1__ and __2__ are servers)
+
+I just plugged the above arrangement in (three Catalyst 2950s) and it worked. STP is enabled on Cisco switches by default.
+
+A little closer examination showed more details. On __B__:
+
+ #show spanning-tree
+ ….
+ This bridge is the root
+ ….
+ Fa0/2 Desg FWD
+ Fa0/3 Desg FWD
+
+__B__ is the root, and both links out of it are “desg” — designated links to those switch segments and “FWD” — forwarding packets.
+
+On __A__:
+
+ #show spanning-tree
+ ….
+ Fa0/1 Desg FWD
+ Fa0/2 Root FWD
+
+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.
+
+On __C__:
+
+ #show spanning-tree
+ ….
+ Fa0/1 Altn BLK
+ Fa0/3 Root FWD
+
+This switch has realized that there’s a loop, and blocked the port that provides the longest route to the root bridge, as expected.
+
+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.
+
+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.
+
+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.
+
+
diff --git a/markdown/index.md b/markdown/index.md
index 90f8b67..3d7aa63 100644
--- a/markdown/index.md
+++ b/markdown/index.md
@@ -39,6 +39,7 @@
1. 2009-Sep-11: [Confusing BIND with CNAMEs](2009-09-11-confusing-bind-with-cnames.html)
1. 2009-Feb-19: [The odd case of my mugging](2019-02-19-the-odd-case-of-my-mugging.html)
1. 2009-Feb-03: [5-packet TCP connection?](2009-02-03-5-packet-tcp-connection.html)
+1. 2006-Mar-16: [Spanning Tree Protocol Introduction](2006-03-16-spanning-tree-protocol-introduction.html)
1. 2006-Mar-15: [M-Audio Fast Track Pro in Linux](2006-03-15-maudio-fast-track-pro-in-linux.html)
1. 2006-Mar-15: [MySQL base64 functions](2006-03-15-mysql-base64-functions.html)
1. 2006-Mar-03: [Mail relaying with NetSuite](2006-03-03-mail-relaying-with-netsuite.html)