Specifications

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14-3
Cisco Nexus 3000 Series NX-OS Unicast Routing Configuration Guide, Release 5.0(3)U2(2)
OL-25782-02
Chapter 14 Configuring HSRP
Information About HSRP
Figure 14-1 HSRP Topology With Two Enabled Routers
The virtual router does not physically exist but represents the common default router for interfaces that
are configured to provide backup to each other. You do not need to configure the hosts on the LAN with
the IP address of the active router. Instead, you configure them with the IP address (virtual IP address)
of the virtual router as their default router. If the active router fails to send a hello message within the
configurable period of time, the standby router takes over, responds to the virtual addresses, and becomes
the active router, assuming the active router duties. From the host perspective, the virtual router remains
the same.
Note Packets received on a routed port destined for the HSRP virtual IP address will terminate on the local
router, regardless of whether that router is the active HSRP router or the standby HSRP router. This
includes ping and Telnet traffic. Packets received on a Layer 2 (VLAN) interface destined for the HSRP
virtual IP address will terminate on the active router.
HSRP for IPv4
HSRP routers communicate with each other by exchanging HSRP hello packets. These packets are sent
to the destination IP multicast address 224.0.0.2 (reserved multicast address used to communicate to all
routers) on UDP port 1985. The active router sources hello packets from its configured IP address and
the HSRP virtual MAC address while the standby router sources hellos from its configured IP address
and the interface MAC address, which may or may not be the burned-in address (BIA). The BIA is the
last six bytes of the MAC address that is assigned by the manufacturer of the network interface card
(NIC).
Because hosts are configured with their default router as the HSRP virtual IP address, hosts must
communicate with the MAC address associated with the HSRP virtual IP address. This MAC address is
a virtual MAC address, 0000.0C07.ACxy, where xy is the HSRP group number in hexadecimal based on
the respective interface. For example, HSRP group 1 uses the HSRP virtual MAC address of
0000.0C07.AC01. Hosts on the adjoining LAN segment use the normal Address Resolution Protocol
(ARP) process to resolve the associated MAC addresses.
192.0.2.1
192.0.2.2
192.0.2.3
Active
router
Virtual
router
Standby
router
185061
Internet or
ISP backbone
Host A Host B
LAN
Host C Host D