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Configuring IP Services
Configuring the Hot Standby Router Protocol
IPC-106
Cisco IOS IP Configuration Guide
Figure 18 Network Supporting the HSRP ICMP Redirection Filter
If the host wants to send a packet to another host on Net D, then it first sends it to its default gateway,
the virtual IP address of HSRP group 1.
The following is the packet received from the host:
dest MAC = HSRP group 1 virtual MAC
source MAC = Host MAC
dest IP = host-on-netD IP
source IP = Host IP
Router R1 receives this packet and determines that router R4 can provide a better path to Net D, so it
prepares to send a redirect message that will redirect the host to the real IP address of router R4 (because
only real IP addresses are in its routing table).
The following is the initial ICMP redirect message sent by router R1:
dest MAC = Host MAC
source MAC = router R1 MAC
dest IP = Host IP
source IP = router R1 IP
gateway to use = router R4 IP
Before this redirect occurs, the HSRP process of router R1 determines that router R4 is the active HSRP
router for group 3, so it changes the next hop in the redirect message from the real IP address of router
R4 to the virtual IP address of group 3. Furthermore, it determines from the destination MAC address of
the packet that triggered the redirect message that the host used the virtual IP address of group 1 as its
gateway, so it changes the source IP address of the redirect message to the virtual IP address of group 1.
The modified ICMP redirect message showing the two modified fields (*) is as follows:
dest MAC = Host MAC
source MAC = router R1 MAC
dest IP = Host IP
source IP* = HSRP group 1 virtual IP
gateway to use* = HSRP group 3 virtual IP
This second modification is necessary because hosts compare the source IP address of the ICMP redirect
message with their default gateway. If these addresses do not match, the ICMP Redirect message is
ignored. The routing table of the host now consists of the default gateway, virtual IP address of group 1,
and a route to Net D through the virtual IP address of group 3.
43140
R3
Net B
e1
e0
R1
Active 1
Standby 2
e1
e0
R2
Active 2
Standby 1
R7
e0
R8
Listen 1
Host
Net C
R6
Net D
R4
Active 3
Standby 4
R5
Active 4
Standby 3
Net E
Net A
Net F Net G
Default gateway:
virtual IP 1