Chapter 32 Configuring Hsrp - Cisco Catalyst 3550 series Software Configuration Manual

Chapter 32 Configuring HSRP
Understanding HSRP
Note Routers in an HSRP group can be any router interface that supports HSRP, including Catalyst 3550
routed ports and switch virtual interfaces (SVIs).
HSRP provides high network availability by providing redundancy for IP traffic from hosts on networks.
In a group of router interfaces, the active router is the router of choice for routing packets. The standby
router is the router that takes over the routing duties when an active router fails or when preset conditions
are met.
The standby ip interface configuration command activates HSRP on a Layer 3 interface. If an IP address
is specified, that address is used as the designated address for the Hot Standby group. If no IP address is
specified, the address is learned through the standby function.
HSRP is useful for hosts that do not support a router discovery protocol and that cannot switch to a new
router when the selected router reloads or loses power. When HSRP is configured on a network segment,
it provides a virtual MAC address and IP address that is shared among grouped router interfaces that are
running HSRP. The router selected by the protocol to be the active router receives and routes packets
destined for the group MAC address. For n routers running HSRP, there are n +1 IP and MAC addresses
assigned.
HSRP detects when the designated active router fails, and a selected standby router assumes control of
the Hot Standby group MAC and IP addresses. A new standby router is also selected at that time. Devices
running HSRP send and receive multicast UDP-based hello packets to detect router failure and to
designate active and standby routers. When HSRP is configured on an interface, Internet Control
Message Protocol (ICMP) redirect messages are disabled by default for the interface.
The switch supports HSRP MAC addresses for up to 16 unique HSRP groups. Because each group
address can be used on up to 16 Layer 3 interfaces, the maximum number of HSRP interfaces is 256.
However, the relationship between the number of HSRP interfaces and the number of active IP routing
protocols and other configured features might have an impact on CPU utilization. Because of other
switch feature configurations, we recommend that you do not assign more than 64 HSRP interfaces. The
switch returns an error message after a period of up to 1 minute if you exceed the HSRP MAC address
limitation of 256.
Each of the 16 HSRP MAC addresses can be used by 16 consecutive Layer 3 interfaces because each
address is associated with a group of VLANs by using a 4-bit mask. The mask requires that all Layer 3
interfaces be the same multiple of 16. When you create an HSRP group, you can use the same HSRP
MAC address on a single Layer 3 interface, several Layer 3 interfaces that are all the same multiple of
16, or a consecutive range of 16 Layer 3 interfaces that are all the same multiple of 16.
For example, HSRP Group 1 might be assigned to interface VLANs 16 to 31, which equals the group
maximum of 16 VLANs. VLAN IDs between 16 and 31 are all the same multiple of 16 (1, or 1 plus some
small amount.) Therefore, if Group 1 is assigned to interface VLANs 16 to 31, only one HSRP MAC
address entry is used in hardware. If Group 1 is also assigned to interface VLAN 32, an additional MAC
address entry is used in hardware because it is not in the same multiple of 16 as VLANs 16 to 31.
Instead, if Group 1 is assigned to interface VLAN 16, and Group 2 is assigned to interface VLAN 17,
two HSRP MAC address entries are used in hardware. Group 1 uses one MAC address entry, and Group 2
uses the other MAC address entry. If Group 1 or Group 2 are later configured on interfaces VLAN 18
through VLAN 31, an additional HSRP MAC address entry is not used because the MAC address entries
for these two groups have already been created and can be used for all VLAN interfaces between 16 and
31.
The SVI VLAN ID number is the same as the interface VLAN ID number (for example, interface Vlan
16 uses VLAN 16). For routed ports, the switch automatically assigns a VLAN ID to the interface.
Assigned numbers begin at the first available VLAN above 1024. These assigned numbers are also
limited to the range of 16 consecutive VLANs per group.
Catalyst 3550 Multilayer Switch Software Configuration Guide
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