Download Understanding Layer 3 Redundancy with HSRP: A Network's Backbone and more Lecture notes Mechanics in PDF only on Docsity! Sunset Learning Institute www.sunsetlearning.com | 888.888.5251 Authorized Cisco Learning Partner Specialized Layer 3 Redundancy with HSRP By Sunset Learning Instructor Andrew Stibbards Hot Standby Router Protocol (HSRP) is a Cisco proprietary protocol which allows several routers or multilayer switches to appear as a single gateway IP address. It provides redundancy for the layer 3 functions in our networks. Other protocols that provide the same redundancy include Virtual Router Redundancy Protocol (VRRP) and Gateway Load Balancing Protocol (GLBP). VRRP is a standard protocol, but is very similar to HSRP in operation. GLBP becomes advantageous when you start load balancing, which I will discuss later. But why do we need these? The figure below is a given network, the user’s machines are assigned a default gateway. They use ARP to acquire the layer 2 address of the default gateway. As far as the user’s machine is concerned, this is all they know. They have no backup or alternate default gateway. When the router that is the default gateway goes down, or the physical path to the router, your machine no longer has a default gateway. Even if there is another router present on that network segment, its information does not match what the user’s machine is assigned. This is where layer 3 redundancy comes in. Figure 1: Network With No Layer 3 Redundancy. Sunset Learning Institute www.sunsetlearning.com | 888.888.5251 Authorized Cisco Learning Partner Specialized HSRP Function Overview HSRP groups multiple physical routers or multilayer switches into a single virtual router. All of the routers participating in the HSRP group share a single virtual IP address and a single virtual MAC address. This is what is advertised into our networks. So when a router goes down or the link into the router fails, there is a second physical device ready to respond to the same default gateway address information. From the user’s perspective, there is no loss in connectivity. From an administrator’s perspective, this means they can focus on fixing the problem without fielding tickets from users complaining about network loss. Figure 2: Network With Layer 3 Redundancy. Internal Mechanics Within HSRP there is a concept of an active router and a standby router. The active router is responsible for responding to ARP requests and handling packet forwarding. It is also sending hello messages via multicast every 3 seconds to the standby router. HSRP uses multicast address 224.0.0.2 or 224.0.0.102, for versions one and two respectively. The standby router does not take any action except to listen for the hello messages from the active router. The default hold timer (how long it will wait after missing hello packets) is 10 seconds. So if a standby router misses 3 hello intervals, it will assume the active status, and begin using the virtual IP address and MAC address. Because these are the exact same addresses that the previous active router was using, there is no need to reconfigure the user’s machines. The virtual IP address is chosen by the administrator, and the MAC address is auto generated. For version 1, a MAC address of 0000.0c07.acXX is used, where XX is the group number in hex format. Version 2 uses MAC address 0000.0c9f.fXXX, with the last 3 digits again representing group number in hex format.
