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19-4
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
OL-20002-02
Chapter 19 Configuring HSRP
Information About HSRP
HSRP version 2 has a different packet format than HSRP version 1. The packet format uses a
type-length-value (TLV) format. HSRP version 2 packets received by an HSRP version 1 router are
ignored.
HSRP Authentication
HSRP message digest 5 (MD5) algorithm authentication protects against HSRP-spoofing software and
uses the industry-standard MD5 algorithm for improved reliability and security.
HSRP Addressing
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 will use 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.
HSRP version 2 uses the new IP multicast address 224.0.0.102 to send hello packets instead of the
multicast address of 224.0.0.2, which is used by version 1. HSRP version 2 permits an expanded group
number range of 0 to 4095 and uses a new MAC address range of 0000.0C9F.F000 to 0000.0C9F.FFFF.
HSRP Messages
Routers that are configured with HSRP exchange the following three types of multicast messages:
• Hello—The hello message conveys the HSRP priority and state information of the router to other
HSRP routers.
• Coup—When a standby router wants to assume the function of the active router, it sends a coup
message.
• Resign—A router that is the active router sends this message when it is about to shut down or when
a router that has a higher priority sends a hello or coup message.
HSRP Load Sharing
HSRP allows you to configure multiple groups on an interface in. You can configure two overlapping
HSRP groups to load share traffic from the connected hosts while providing the default router
redundancy expected from HSRP. Figure 19-2 shows an example of a load-sharing HSRP configuration.
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