Chapter 8: Configuring High Availability Features 135
Table 8-1 Commands to Display SSO and NSF Information
Display Function Command
Displays redundancy states
(exec) show redundancy states
Verifies Multicast NSF with SSO
(exec) show mls ip sip multicast sso
8-3: Router Redundancy with HSRP
■ Route processors in the same or another chassis can share redundant gateway ad-
dresses on a VLAN by using the Hot Standby Router Protocol (HSRP).
■ Route processors sharing a common HSRP IP address must belong to the same HSRP
group number.
■ The HSRP address appears on the network with a special virtual MAC address—
0000-0C-07-AC-XX, where XX is the HSRP group number (0 to 255). The hosts on
the HSRP VLAN use this MAC address as the default gateway.
■ Although HSRP is enabled on an interface, each route processor still maintains its
own unique IP and MAC addresses on the VLAN interface. These addresses are
used by other routers for routing protocol traffic.
■ When an HSRP group is enabled, the highest-priority HSRP device at that time
becomes the active router, whereas the second-highest-priority stays in the standby
state. All other HSRP devices in the group maintain a “listening” state, waiting for the
active device to fail. A new active router election occurs only when the active device
fails. The previous active router (having the highest priority) might reclaim its active
role by preempting the other HSRP routers in the group.
■ HSRP devices communicate by sending a hello message over UDP at multicast
address 224.0.0.2. These messages are sent every three seconds by default.
■ Devices on a VLAN use the HSRP address as their default gateway. If one of the
HSRP devices fails, there will always be another one to take its place as the default
gateway address.
Section 8-3
Displaying Information About SSO and NSF
You can use the switch commands in Table 8-1 to display helpful information about SSO
and NSF.
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