Thomas Chipman. “The network is slow”… these four words are a nightmare for network administrators. No matter how well a network design meets the organization’s current and projected needs, those four words will undoubtedly find their way into a trouble ticket. If you believe the anecdotal evidence, you will hear those four words 12 minutes before you leave for your weekend getaway. They are often uttered by your CEO while she is standing in front of your door with her laptop. Today is that day, and she won’t leave your office until your work has been completed. That is why she made you her senior vice-architect of network engineering.
You are already prioritizing, shaping and working your usual QoS voodoo in order to squeeze every packet from your existing uplinks. But the day is coming when even that will not be enough. A famous network engineer once said, “Cap’n, I’m giving her everything she has!” The network will fail if I push it any harder. However, increasing network bandwidth is the obvious solution. However, this solution can sometimes be costly or complicated. If your CEO is constantly breathing down your neck, expensive and time-consuming solutions won’t work.
What if there were a cost-effective and simple solution to this problem? Let’s take an overview of the network to see what we can discover. Figure 1 shows us an overhead view of the network.
Figure 1 shows that SwitchA and SwitchB share the same redundant link. Excellent! Excellent! Unfortunately, no. Spanning Tree Protocol (STP), is probably the most likely to be used in this scenario. STP is great at preventing accidental loops within the network. But what if you put a loop in your network? What if you really wanted two paths between two switches? And you wanted both of them to be available at all times? STP is not going to be your friend in this case. STP actually makes the network look more like Figure 2.
You might be thinking, “Inconceivable!” “Why would a switch ignore an uplink that is perfectly good?” That’s the point of STP. A redundant link can cause all manner of network problems, including broadcast storms. STP will shut down redundant ports to save your network. You can see the partial output of the show spanning tree command.
SwitchA#show Spanning Tree VLAN0001 Spanning Tree Enabled Protocol pvst RootID Priority 24576 Address 000C.1380.3538 Hello Time 2 sec Max age 20 sec Forward Delayed 15 sec BridgeID Priority 32768 address 000C.1289.3959 Hi Time 2 sec Max age 20 sec Forward Declay 15 sec Aging Times 15
Interface Role Sts Cost Prio.Nbr Typ ——— —- — —- — — —- Fa0/1 Root 19 128.1 P2p Desg BLK 19 128.2 P2p
The command output below confirms that FastEthernet0/2 interface is not forwarding packets. This interface will forward packets simultaneously with the FastEthernet 1 interface. This will double the bandwidth between the switches. STP can be convinced by EtherChannel that both ports should forward packets simultaneously.
EtherChannel can be used up to eight ports.