Cisco Systems 78-11741-02 manual IPC-106

Configuring IP Services

Configuring the Hot Standby Router Protocol

Figure 18 Network Supporting the HSRP ICMP Redirection Filter

Net B		R3	Net C	Net D		R6	Net E
e1	R1	e1	R2		R4		R5
e0	Active 1	e0	Active 2		Active 3		Active 4
	Standby 2		Standby 1		Standby 4		Standby 3
							Net A

	e0	Listen 1
	R7	R8
Net F		Net G

Host

Default gateway: virtual IP 1

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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.

Cisco IOS IP Configuration Guide

IPC-106