1 00:00:00,000 --> 00:00:05,000 In the previous videos, we've configured these interfaces 2 00:00:05,000 --> 00:00:07,000 between the switches as trunk ports 3 00:00:07,000 --> 00:00:13,000 and configured the links to the routers and switches as access ports. 4 00:00:13,000 --> 00:00:16,000 This port as an example is in VLAN 10 5 00:00:16,000 --> 00:00:20,000 this port is in VLAN 20, this port is in VLAN 1 6 00:00:20,000 --> 00:00:27,000 and this port is in VLAN 1 and these core ports are trunk ports permitting all VLANs. 7 00:00:27,000 --> 00:00:32,000 Now, this is a layer 2 infrastructure running between our switches. 8 00:00:32,000 --> 00:00:36,000 In a layer 2 network when you have redundant links as we do here 9 00:00:36,000 --> 00:00:41,000 we need to run Spanning Tree to prevent loops in your topology. 10 00:00:41,000 --> 00:00:44,000 Spanning Tree is enabled by default on Cisco switches 11 00:00:44,000 --> 00:00:49,000 so when I type sh run | include span 12 00:00:49,000 --> 00:00:55,000 you’ll notice that the switch is configured to run Rapid PVST+ 13 00:00:55,000 --> 00:00:58,000 and is using an extended system IDs. 14 00:00:58,000 --> 00:01:02,000 so when I use the command sh spanning-tree vlan 1 as an example 15 00:01:02,000 --> 00:01:05,000 you can see the output for VLAN 1 16 00:01:05,000 --> 00:01:10,000 the default priority for this switch is 32768 in decimal 17 00:01:10,000 --> 00:01:12,000 that’s 8000 in hexa decimal. 18 00:01:12,000 --> 00:01:16,000 So the switch is using the default Spanning Tree priority 19 00:01:16,000 --> 00:01:19,000 but because its using extended systems IDs 20 00:01:19,000 --> 00:01:21,000 and this is VLAN 1 21 00:01:21,000 --> 00:01:26,000 the priority of the switch is 32769 in other words 32768 + 22 00:01:26,000 --> 00:01:30,000 the VLAN number, we could see that as an example for VLAN 10 23 00:01:30,000 --> 00:01:34,000 priority 32768 + extended system ID 24 00:01:34,000 --> 00:01:38,000 so the priority of this switch is 32778. 25 00:01:38,000 --> 00:01:43,000 For VLAN 20 we can see that the priority is 32788. 26 00:01:43,000 --> 00:01:48,000 So looking at VLAN 10 as an example, switch has this priority. 27 00:01:48,000 --> 00:01:52,000 The roots of the Spanning Tree also has that priority. 28 00:01:52,000 --> 00:01:57,000 The Spanning Tree that we're using is Rapid PVST 29 00:01:57,000 --> 00:01:59,000 this is the address of the root bridge 30 00:01:59,000 --> 00:02:02,000 this our local address or local MAC address. 31 00:02:02,000 --> 00:02:06,000 So the bridge ID is this number + this number 32 00:02:06,000 --> 00:02:10,000 but the local switch is not the root bridge. 33 00:02:10,000 --> 00:02:12,000 Another switch is the root bridge 34 00:02:12,000 --> 00:02:17,000 and the port used to get to the root bridge is port 0/0. 35 00:02:17,000 --> 00:02:21,000 so from switch 1 points of view, the root bridge is somewhere here 36 00:02:21,000 --> 00:02:24,000 so we could have a look as an example 37 00:02:24,000 --> 00:02:27,000 as switch 2 to see if that switch is the root. 38 00:02:27,000 --> 00:02:33,000 So on switch 2, sh spanning-tree vlan 10 39 00:02:33,000 --> 00:02:37,000 shows us that this switch is the root bridge. 40 00:02:37,000 --> 00:02:39,000 In other words, this bridge is the root 41 00:02:39,000 --> 00:02:43,000 switch 2 is the Spanning Tree root for VLAN 10. 42 00:02:43,000 --> 00:02:48,000 Notice it doesn’t show a path cost to get to the root 43 00:02:48,000 --> 00:02:53,000 it shows us that switch is the root and we can also see that by looking at the address. 44 00:02:53,000 --> 00:02:56,000 notice the MAC address of the root is the same 45 00:02:56,000 --> 00:03:00,000 as the MAC address of the local bridge or local switch. 46 00:03:00,000 --> 00:03:06,000 All interfaces on this switch are designated ports and all ports are forwarding 47 00:03:06,000 --> 00:03:12,000 whereas if we look at switch 1 once again, the switch has a root port. 48 00:03:12,000 --> 00:03:17,000 the root port is forwarding and the switches root port is gigabit 0/0 49 00:03:17,000 --> 00:03:20,000 we can see that once again by looking at the output here 50 00:03:20,000 --> 00:03:24,000 but hopefully, at this point, you can see a problem in this topology. 51 00:03:24,000 --> 00:03:31,000 This core switch is blocking on all ports except gigabit 0/0. 52 00:03:31,000 --> 00:03:36,000 So if we draw this topology and mark which port are forwarding and blocking 53 00:03:36,000 --> 00:03:39,000 we'll be able to see what’s actually happening in the network. 54 00:03:39,000 --> 00:03:44,000 Looking at topology as follows, it looks like you have a lot of redundancy 55 00:03:44,000 --> 00:03:53,000 and traffic from this host as an example may take an optimal path to get to destination 56 00:03:53,000 --> 00:03:56,000 but that may not actually be true because of the ports that Spanning Tree are blocking 57 00:03:56,000 --> 00:04:01,000 by default, so let’s have a look at what’s going on in this topology.