Cisco Systems 78-11741-02 manual Redirects to Passive Hsrp Routers, Redirects to Non-HSRP Routers

Configuring IP Services

Configuring the Hot Standby Router Protocol

Redirects to Passive HSRP Routers

Redirects to passive HSRP routers are not permitted. Redundancy may be lost if hosts learn the real IP addresses of HSRP routers.

In the previous example, redirects to router R8 are not allowed because R8 is a passive HSRP router. In this case, packets from the host to Net D will first go to router R1 and then be forwarded to router R4, that is, they will traverse the network twice.

A network configuration with passive HSRP routers is considered a misconfiguration. For HSRP ICMP redirection to operate optimally, every router on the network that is running HSRP should contain at least one active HSRP group.

Redirects to Non-HSRP Routers

Redirects to routers not running HSRP on their local interface are permitted. No redundancy is lost if hosts learn the real IP address of non-HSRP routers.

In the example, redirection to router R7 is allowed because R7 is not running HSRP. In this case, the next hop IP address is unchanged. The source IP address is changed dependent upon the destination MAC address of the original packet. You can specify the no standby redirects unknown command to stop these redirects from being sent.

Passive HSRP Router Advertisements

Passive HSRP routers send out HSRP advertisement messages both periodically and when entering or leaving the passive state. Thus, all HSRP routers can determine the HSRP group state of any HSRP router on the network. These advertisements inform other HSRP routers on the network of the HSRP interface state, as follows:

•Dormant—Interface has no HSRP groups, single advertisements sent once when last group is removed

•Passive—Interface has at least one non-active group and no active groups, advertisements sent out periodically

•Active—Interface has at least one active group, single advertisement sent out when first group becomes active

You can adjust the advertisement interval and holddown time using the standby redirects timers command.

Redirects Not Sent

If the HSRP router cannot uniquely determine the IP address used by the host when it sends the packet that caused the redirect, the redirect message will not be sent. The router uses the destination MAC address in the original packet to make this determination. In certain configurations, such as the use of the standby use-biainterface configuration command specified on an interface, redirects cannot be sent. In this case, the HSRP groups use the interface MAC address as their virtual MAC address. The router now cannot determine if the default gateway of the host is the real IP address or one of the HSRP virtual IP addresses that are active on the interface.

Using HSRP with ICMP redirects is not possible in the Cisco 800 series, Cisco 1000 series, Cisco 1600 series, Cisco 2500 series, Cisco 3000 series, and Cisco 4500 series routers because the Ethernet controller can only support one MAC address.

Cisco IOS IP Configuration Guide

IPC-107