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This is seen as the MAC address of the router, and it is filtered out if more than one router in a LAN becomes active. This behavior can change in future releases of Cisco IOS ®. Note: 4x00 series and earlier do not have the hardware required to support more than one MAC address at a time on Ethernet interfaces. However, the Cisco 2600. If your Apple Developer Program membership expires, your apps will no longer be available for download and you will not be able to submit new apps or updates. You will lose access to pre-release software, Certificates, Identifiers & Profiles, and Technical Support Incidents. This site contains user submitted content, comments and opinions and is for informational purposes only. Apple may provide or recommend responses as a possible solution based on the information provided; every potential issue may involve several factors not detailed in the conversations captured in an electronic forum and Apple can therefore provide no guarantee as to the.

Introduction

This document addresses the most frequently asked questions related to Hot Standby Router Protocol (HSRP).

Q. Will the standby router take over if the active router LAN interface state is 'interface up line protocol down'?

A. Yes, the standby router takes over once the holdtime expires. By default, this is equivalent to three hello packets from the active router having been missed. The actual convergence time depends on the HSRP timers configured for the group and possibly on routing protocol convergence. The HSRP hellotime timer defaults to three and the holdtime timer defaults to ten.

Q. Can I configure more than one standby group with the same group number?

A. Yes. However, Cisco does not recommend it on lower-end platforms such as the 4x00 series and earlier. If the same group number is assigned to multiple standby groups, it creates a non-unique MAC address. This is seen as the MAC address of the router, and it is filtered out if more than one router in a LAN becomes active. This behavior can change in future releases of Cisco IOS^®.

Note: 4x00 series and earlier do not have the hardware required to support more than one MAC address at a time on Ethernet interfaces. However, the Cisco 2600 and Cisco 3600 do support multiple MAC addresses on all Ethernet and Fast Ethernet interfaces.

Q. When an active router tracks serial 0 and the serial line goes down, how does the standby router know to become active?

A. When the state of a tracked interface changes to down, the active router decrements its priority. The standby router reads this value from the hello packet priority field, and becomes active if this value is lower than its own priority and the standby preempt is configured. You can configure by how much the router must decrement the priority. By default, it decrements its priority by ten.

Q. If there is no priority configured for a standby group, what determines which router is active?

A. The priority field is used to elect the active router and the standby router for the specific group. In the case of an equal priority, the router with the highest IP address for the respective group is elected as active. Furthermore, if there are more than two routers in the group, the second highest IP address determines the standby router and the other router/routers are in the listen state.

Note: If no priority is configured, it uses the default of 100.

Q. What are the limiting factors that determine how many standby groups can be assigned to a router?

A. Ethernet: 256 per router. FDDI: 256 per router. Token Ring: 3 per router (uses reserved functional address).

Q. Which HSRP router requires that I configure preempt?

A. An HSRP-enabled router with preempt configured attempts to assume control as the active router when its Hot Standby priority is higher than the current active router. The standby preempt command is needed in situations when you want an occurring state change of a tracked interface to cause a standby router to take over from the active router. For example, an active router tracks another interface and decrements its priority when that interface goes down. The standby router priority is now higher and it sees the state change in the hello packet priority field. If preempt is not configured, it cannot take over and failover does not occur.

Q. Based on the documentation, it looks like I can use HSRP to achieve load-balancing across two serial links. Is this true?

A. Yes, refer to Load Sharing with HSRP for more information.

Q. Does HSRP support DDR, and if so, how will it know to dial?

A. No, HSRP does not support Dial-on-Demand Routing (DDR) directly. However, you can configure it to track a serial interface and swap from the active to the standby router in case of a WAN link failure. The command used to track the state of an interface is standby <group#> track <interface>.

Q. I use HSRP and all hosts use the active router to forward traffic to the rest of my network. I have noticed that the return traffic comes back through the standby router. Will this cause problems with HSRP or my applications?

A. No, normally this is transparent to all hosts and/or servers on the LAN and can be desirable if a router experiences high traffic. In order to change this, configure a more desirable cost for the link you want the distant router/routers to use.

Q. How does DECnet traffic fit into the HSRP scenario?

A. DECnet and XNS are compatible with HSRP and multiple HSRP (MHSRP) over Ethernet, FDDI, and Token Ring on the Cisco 7000 and Cisco 7500 routers only. Refer to Using HSRP for Fault-Tolerant IP Routing for more information.

Q. Can a Cisco 2500 and Cisco 7500 router on the same LAN segment use HSRP, or do I have to replace one of the routers so the platforms are identical?

A. You can mix the platforms with HSRP, but you are not able to support multiple HSRP (MHSRP) due to the hardware limitations of the lower-end platform.

Q. If I use a switch, what do I see on the CAM tables for the HSRP?

A. The content-addressable memory (CAM) tables provide a map for the HSRP MAC address to the port on which the active router is located. In this way, you can determine what the switch perceives the HSRP status to be.

Q. What is the standby use-bia command and how does it work?

A. By default, HSRP uses the preassigned HSRP virtual MAC address on Ethernet and FDDI, or the functional address on Token Ring. In order to configure HSRP to use the burnt-in address of the interface as its virtual MAC address, instead of the default, use the standby use-bia command.

For example, on Token Ring, if Source Route Bridging is in use, a Routing Information Field (RIF) is stored with the virtual MAC address in the RIF cache of the host. The RIF indicates the path and final ring used to reach the MAC address. As routers transition to the active state, they send gratuitous Address Resolution Protocols (ARPs) in order to update the ARP table of the host. However, this does not affect the RIF cache of the hosts that are on the bridged ring. This situation can lead to packets being bridged to the ring for the previous active router. In order to avoid this situation, use the standby use-bia command. The router now uses its burnt-in MAC address as the virtual MAC address.

Note: Using the standby use-bia command has these disadvantages:

When a router becomes active the virtual IP address is moved to a different MAC address. The newly active router sends a gratuitous ARP response, but not all host implementations handle the gratuitous ARP correctly.
Proxy ARP breaks when use-bia is configured. A standby router cannot cover for the lost proxy ARP database o

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