|
|
Irfan
Hello Miss Anderson, I'll be continuing the interview about LAN switching. Gillian I'm so sorry to hear about Michael. Irfan You're sorry? He owed me twenty bucks! Gillian Oh dear, that's dreadful. So his imminent demise is especially painful for you. They say that time heals all heartache, so someday you won't feel so sad about him. Irfan About who? Gillian That guy, Michael something, anyway, it doesn't matter, let's talk about switching! Irfan Great! You were talking about vlans earlier and how a router is required to move packets between vlans. What would you like to talk about now? Gillian I'd like to talk about a serious problem with switches. Switches by default forward broadcasts. If your switch topology contains redundant paths or bridge loops, a broadcast storm of Biblical proportions could result. Routers on the other hand, do not have this problem because they do not forward broadcasts. Switches use something called the Spanning Tree Protocol to ensure a loop free topology. The world of Spanning Tree has two warring factions, each bent on the destruction of the other. The two factions are the IEEE and DEC. These two spanning tree protocols are not compatible and should never be enabled on the same network simultaneously. Use the IEEE version of Spanning Tree unless you work at DEC. Irfan I've heard about the spanning tree protocol. I've never really thought it was that important. Gillian I know you just said that to get me fired up, problems relating to spanning tree can easily create broadcast storms the lock up routers so bad you can't even get a console connection! You must understand spanning tree or STP as it's called if you are going to be a big city Cisco engineer! Irfan I knew I'd get you going! Tell us about STP. Let's hear the gory details! Gillian OK, you asked for it. Right off the bat, a switch is really a group of bridges that are wired up in a star pattern. Every port on a switch is a bridge and every behind every port they are wired together. So you can say switch port or bridge but you are talking about the same thing. Spanning tree is a bridging protocol and as such is applied to individual ports on a switch. Irfan Do switches use STP to talk to other switches? Gillian Don't think "switch", the switch is a box that houses the bridge ports. It's the bridge ports that use Spanning Tree to talk to other bridge ports. When Spanning Tree is running on your switch, what is going on behind the scenes is that every active port on your switch is sending out little packets called Bridge Protocol Data Units or BDPUs. Sometimes BDPUs are called "hello messages". These BDPUs are beaconed out every 2 seconds by default. When you introduce a port into a spanning tree environment, it listens for these BDPUs to learn about the spanning tree topology. Irfan How is the topology created by spanning tree? Gillian The entire topology must resemble a tree to work properly and avoid loops. A tree has small branches that flow downward into larger branches to a common, single root. If you have your switches wired together that contains multiple paths or that is "meshy", spanning tree will run what's called the Spanning Tree Algorithm and choose which ports offer the best path to the root of the tree. Those ports which spanning tree selects as the way out of the local network will be put in a forwarding state and the other, non optimal ports will be put in a blocking state. Irfan Is there a central SPT server that does this calculation, then tells all the ports in the enterprise what state to become? Gillian No, as long as each switch is using the same version of spanning tree, IEEE for example, they all know the rules and can read the BDPUs and determin which ports are blocked and which ports can forward. Irfan What other states are there that a port can be in? Gillian Well at first when a port comes up it goes int the Listening State. It listens for BDPUs and by examing the contents of the BDPUs and figures out the topology of the existing network. After the listening state, the port may decide that it should go into a Blocking State. In the blocking state, no data can pass through the port but it is still examining BDPUs. The port remains in the Listening State for however long a duration is specified by the Forward Delay Timer. The Forward Delay Timer is set to 15 seconds by default. Irfan So it could take a port up to 15 seconds to begine forwarding data? Gillian Not so fast, Buckaroo! After the 15 second listening state, if the port does not go into blocking state, it then spends another leisurely 15 seconds in the Learning State. In the Learning State the port is learning MAC addresses and adding those entries into it's CAM table. After the Learning State is complete, then the port goes into Forwarding State and can transmit data. Irfan 30 seconds before the port will send data?!?!? That just won't do! If I have a single workstation plugged into a port on a switch and it takes 30 seconds, Windows will not wait that long, my workstation will never contact the DHCP server in time! My workstation will not get an IP address! All is lost! We are doomed! Gillian Relax, Irfan! There's a work around! When you have a single workstation or server plugged into a port, you can selectively disable the spanning tree protocol on that port. After all, spanning tree's only job is to prevent loops by shutting off redundant paths in the network. But your workstation only has one path to the network, through that port! Irfan So we can turn off spanning tree? Gillian Well, were not going to turn off spanning tree, we'll just disable it on this one port. But now that you mention it, it very possible that you are the administrator of a network that by physical design, does not have any loops in the topology. Nearly all small networks are like that. If that's the case, yes, you can disable spanning tree network wide and save the bandwidth and CPU cycles that STP would use to do a job you don't need. Irfan My network has redundant paths, so I'll use spanning tree, but can you tell me exactly how to shut it off on a single port, so my workstation can access the network immediately? Gillian Certainly, Cisco calls it PortFast and you enter the following command: set spantree portfast <module>/<port> enable So if you plug your workstation into the 12th port on the 2nd slot in your Catalyst 5505, you would enter the command: set spantree portfast 2/12 enable To disable PortFast on this port: set spantree portfast 2/12 disable As you can see, those engineers at Cisco made this easy. Irfan You call that easy? Cisco should provide a mouse and a graphical interface like Microsoft does, now that's what I'd call easy... Gillian But if it was that easy, everyone could do it. Think about it. Irfan (smiling knowingly) Ah yes, very good point! There's a wonderful page at Cisco that explains how to Configure Spanning Tree . Watch for part 3 of this
|
