Information Technology Reference
In-Depth Information
Usually in these implementations, the workstation is a UNIX system running the routed or
gated UNIX process.
HSRP
The Cisco HSRP provides a way for IP workstations that support only one default router
to keep communicating on the internetwork even if their default router becomes unavail-
able. HSRP works by creating a virtual router that has its own IP and MAC addresses. The
workstations use this virtual IP address as their default router.
HSRP routers on a LAN communicate among themselves to designate two routers as ac-
tive and standby. The active router sends periodic hello messages. The other HSRP routers
listen for the hello messages. If the active router fails and the other HSRP routers stop re-
ceiving hello messages, the standby router takes over and becomes the active router. Be-
cause the new active router assumes both the phantom's IP and MAC addresses, end
nodes see no change. They continue to send packets to the phantom router's MAC ad-
dress, and the new active router delivers those packets.
HSRP also works for proxy ARP. When an active HSRP router receives an ARP request for
a node that is not on the local LAN, the router replies with the phantom router's MAC ad-
dress instead of its own. If the router that originally sent the ARP reply later loses its con-
nection, the new active router can still deliver the traffic.
Figure 2-15 shows a sample implementation of HSRP.
Workstation
E0
E0
Phantom Router
Virtual IP:
192.168.1.1/24
Router A Ethernet 0
IP: 192.168.1.2/24
Router B Ethernet 0
IP: 192.168.1.3/24
S0
S0
Figure 2-15
HSRP: The Phantom Router Represents the Real Routers
In Figure 2-15, the following sequence occurs:
1. The workstation is configured to use the phantom router (192.168.1.1) as its default
router.
2. Upon booting, the routers elect Router A as the HSRP active router. The active router
does the work for the HSRP phantom. Router B is the HSRP standby router.
3.
sponds with the phantom router's MAC address.
 
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