1 00:00:00,000 --> 00:00:05,000 In the previous video, we made switch 1 2 00:00:05,000 --> 00:00:09,000 the Spanning Tree root for VLAN 10 and VLAN 1 3 00:00:09,000 --> 00:00:14,000 as well as making switch 2 the Spanning Tree root for VLAN 20 4 00:00:14,000 --> 00:00:18,000 we can see that as an example by going into switch 1 5 00:00:18,000 --> 00:00:21,000 and typing show spanning-tree vlan 10 6 00:00:21,000 --> 00:00:27,000 Notice this switch or this bridge is the Spanning Tree root for VLAN 10 7 00:00:27,000 --> 00:00:33,000 as well as the Spanning Tree root for VLAN 1 8 00:00:33,000 --> 00:00:35,000 here's switch 2 9 00:00:35,000 --> 00:00:40,000 so show spanning-tree vlan 20 10 00:00:40,000 --> 00:00:43,000 this switch or bridge is the root for VLAN 20. 11 00:00:43,000 --> 00:00:46,000 Once again, the reason why we use terms bridge 12 00:00:46,000 --> 00:00:48,000 rather than switch in Spanning Tree 13 00:00:48,000 --> 00:00:51,000 is that Spanning Tree has been around for a long time 14 00:00:51,000 --> 00:00:54,000 so we talk about bridges rather than switches 15 00:00:54,000 --> 00:00:56,000 in some of the Spanning Tree terminology. 16 00:00:56,000 --> 00:01:01,000 Spanning Tree was developed when bridges were used instead of switches. 17 00:01:01,000 --> 00:01:05,000 So we’ve got the roots configured in Spanning Tree. 18 00:01:05,000 --> 00:01:11,000 We have enabled in previous videos redundancy between these switches. 19 00:01:11,000 --> 00:01:16,000 So as an example, switch 3 has 2 uplinks to the core and so the switch 4 20 00:01:16,000 --> 00:01:19,000 so we have redundancy at layer 1 and layer 2 21 00:01:19,000 --> 00:01:23,000 but now we need to implement redundancy at layer 3. 22 00:01:23,000 --> 00:01:25,000 Here’s the problem 23 00:01:25,000 --> 00:01:30,000 root 1 which is acting as PC 1 in this topology is in VLAN 10. 24 00:01:30,000 --> 00:01:34,000 It needs to be configured with the default gateway 25 00:01:34,000 --> 00:01:37,000 so which switch will become its default gateway 26 00:01:37,000 --> 00:01:42,000 switch 1 or switch 2? Both switches have IP addresses in VLAN 10. 27 00:01:42,000 --> 00:01:47,000 Switch 1 is 10.1.10.1 and switch 2 is 10.1.10.2 28 00:01:47,000 --> 00:01:51,000 so which switch will be configured as the default gateway 29 00:01:51,000 --> 00:01:55,000 and what will happen when that switch goes down 30 00:01:55,000 --> 00:02:00,000 as an example, you probably wanna make a switch 1 the default gateway 31 00:02:00,000 --> 00:02:03,000 for router 1 because switch 1 is the Spanning Tree root 32 00:02:03,000 --> 00:02:07,000 which should mean that traffic at both layer 2 and layer 3 33 00:02:07,000 --> 00:02:09,000 will traverse this link and get to switch 1 34 00:02:09,000 --> 00:02:15,000 the problem is, if you make switch 1 the default gateway for PCs 35 00:02:15,000 --> 00:02:17,000 in VLAN 10 and switch 1 goes down 36 00:02:17,000 --> 00:02:21,000 where will those PCs send their traffic? 37 00:02:21,000 --> 00:02:23,000 they won’t be able to reach their default gateway 38 00:02:23,000 --> 00:02:28,000 which means that they won’t be able to send traffic off VLAN 10. 39 00:02:28,000 --> 00:02:30,000 in other words, they won’t be able to access the Internet 40 00:02:30,000 --> 00:02:33,000 or other service and devices in a separate VLAN 41 00:02:33,000 --> 00:02:36,000 the same is true for devices in VLAN 20 42 00:02:36,000 --> 00:02:40,000 if VLAN 20 devices have switch 2 as the default gateway 43 00:02:40,000 --> 00:02:45,000 in other words, we configured the default gateway as 10.1.20.2 44 00:02:45,000 --> 00:02:47,000 and switch 2 goes down, what happens then? 45 00:02:47,000 --> 00:02:49,000 the default gateway is down 46 00:02:49,000 --> 00:02:54,000 which means that they won’t be able to for example ping devices in VLAN 10 47 00:02:54,000 --> 00:02:57,000 or access devices on the Internet. 48 00:02:57,000 --> 00:03:00,000 So this is where first hop redundancy protocols 49 00:03:00,000 --> 00:03:05,000 such as Hot Standby router Protocol or HSRP are used. 50 00:03:05,000 --> 00:03:08,000 HSRP is a Cisco propriety protocol 51 00:03:08,000 --> 00:03:11,000 that allows you to implement first hop redundancy. 52 00:03:11,000 --> 00:03:14,000 The industry standard version of the protocol 53 00:03:14,000 --> 00:03:17,000 is VRRP or Virtual router Redundancy Protocol. 54 00:03:17,000 --> 00:03:21,000 The idea here is that you configure your 2 switches 55 00:03:21,000 --> 00:03:24,000 with a virtual IP address 56 00:03:24,000 --> 00:03:27,000 these 2 switches will have an election 57 00:03:27,000 --> 00:03:31,000 and choose who is in charge of forwarding traffic 58 00:03:31,000 --> 00:03:34,000 on behalf of a virtual router 59 00:03:34,000 --> 00:03:39,000 In this topology, we'll have 2 physical switches configured 60 00:03:39,000 --> 00:03:43,000 with IP addresses in say VLAN 10 but a virtual switch 61 00:03:43,000 --> 00:03:47,000 or a virtual router is created through the configuration 62 00:03:47,000 --> 00:03:50,000 and that virtual router or virtual switch 63 00:03:50,000 --> 00:03:53,000 becomes the default gateway for your devices. 64 00:03:53,000 --> 00:03:56,000 I’ll talk about virtualrouters rather than virtual switches 65 00:03:56,000 --> 00:03:59,000 or layer 3 switches for most of this discussion 66 00:03:59,000 --> 00:04:02,000 because that’s how HSRP is written and describe 67 00:04:02,000 --> 00:04:06,000 but essentially what you do on your PC is you configure 68 00:04:06,000 --> 00:04:09,000 a default gateway of a virtual switch. 69 00:04:09,000 --> 00:04:19,000 Logically a router is created through HSRP 70 00:04:19,000 --> 00:04:24,000 now this router is not a physical router or real router 71 00:04:24,000 --> 00:04:26,000 that’s going to exist in our topology. 72 00:04:26,000 --> 00:04:31,000 It’s simply created through the HSRP commands configured on the switches. 73 00:04:31,000 --> 00:04:38,000 PCs such as the PC in VLAN 10 will be configured 74 00:04:38,000 --> 00:04:43,000 with a default gateway in our example of 10.0.254 75 00:04:43,000 --> 00:04:47,000 so rather than the PC being configured 76 00:04:47,000 --> 00:04:51,000 with the default gateway of switch 1 or switch 2 77 00:04:51,000 --> 00:04:56,000 the default gateway is this virtual HSRP router 78 00:04:56,000 --> 00:05:01,000 this HSRP router will have its own IP address as shown here for VLAN 10 79 00:05:01,000 --> 00:05:08,000 we'll also configure another virtual router for VLAN 20. 80 00:05:08,000 --> 00:05:12,000 In addition, this router has its own MAC address 81 00:05:12,000 --> 00:05:16,000 based on a group number configured in the HSRP. 82 00:05:16,000 --> 00:05:21,000 The PC's are unaware that they are talking to a virtual device 83 00:05:21,000 --> 00:05:24,000 they think they’re talking to a physical router 84 00:05:24,000 --> 00:05:27,000 but in actual fact, they're talking to the switches 85 00:05:27,000 --> 00:05:30,000 which are pretending to be this virtual router 86 00:05:30,000 --> 00:05:33,000 We can manipulate which physical switch 87 00:05:33,000 --> 00:05:37,000 is going to be forwarding traffic on behalf of the virtual router 88 00:05:37,000 --> 00:05:41,000 by changing a priority, the default priority in HSRP is 100 89 00:05:41,000 --> 00:05:44,000 and the highest priority 1's 90 00:05:44,000 --> 00:05:48,000 so we'll influence HSRP so that switch 1 91 00:05:48,000 --> 00:05:52,000 becomes what’s called the active router for VLAN 10 92 00:05:52,000 --> 00:05:56,000 and switch 2 will be the standby router for VLAN 20 93 00:05:56,000 --> 00:05:58,000 switch 2 will be the active router 94 00:05:58,000 --> 00:06:01,000 and switch 1 will be the standby router 95 00:06:01,000 --> 00:06:05,000 and that’s because switch 1 is the root in Spanning Tree for VLAN 10 96 00:06:05,000 --> 00:06:10,000 and switch 2 is the root in Spanning Tree for VLAN 20. 97 00:06:10,000 --> 00:06:14,000 We want to ensure that traffic from this host in VLAN 20 is forwarded 98 00:06:14,000 --> 00:06:17,000 to its default gateway here 99 00:06:17,000 --> 00:06:20,000 which is the same device that’s a Spanning Tree root. 100 00:06:20,000 --> 00:06:22,000 In other words, traffic will take this path 101 00:06:22,000 --> 00:06:26,000 rather than the traffic having to go across multiple links 102 00:06:26,000 --> 00:06:28,000 to get to the default gateway by the same token 103 00:06:28,000 --> 00:06:35,000 this switch will be the active router or active forwarder for VLAN 10 104 00:06:35,000 --> 00:06:39,000 so that VLAN 10 traffic uses this uplink 105 00:06:39,000 --> 00:06:44,000 to get to the Spanning Tree root as well as the default gateway.