2009/3/12

Hot Standby Router Protocol (HSRP): Frequently Asked Questions

                                                                 
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 equals 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 3 and the holdtime timer defaults to 10.
 
 
 
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 router's own MAC address and it is filtered out if more than one router in a LAN becomes active. This behavior may change in future releases of Cisco IOSR.
 
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 a tracked interface's state 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 should decrement the priority. By default, it decrements its priority by 10.
 
 
 
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).
 
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. 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. From reading 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 track
 
 
 
Q. I am using HSRP and all hosts are using 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. You can change this by configuring a more desirable cost for the link you would like 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. For more information, refer to Using HSRP for Fault-Tolerant IP Routing.
 
 
 
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. To configure HSRP to use the interface's burnt-in address 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 host's RIF cache. 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 host's ARP table. 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. 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 of the failed router.
Q. Can I run NAT and HSRP together?
A. You can configure network address translation (NAT) and HSRP on the same router. However, a router that runs NAT holds state information for traffic that is translated through it. If this is the active HSRP router and the HSRP standby takes over, the state information is lost.
 
Note: Stateful NAT (SNAT) can make use of HSRP to fail over. For more information, refer to NAT Stateful Failover of Network Address Translation. Static NAT Mapping Support with HSRP for High Availability is another feature which makes NAT and HSRP interact. For more information refer to NATStatic Mapping Support with HSRP for High Availability.
 
 
 
Q. What are the IP source address and destination address of HSRP hello packets?
A. The destination address of HSRP hello packets is the all routers multicast address (224.0.0.2). The source address is the router's primary IP address assigned to the interface.
 
 
 
Q. Are HSRP messages TCP or UDP?
A. UDP, since HSRP runs on UDP port 1985.
 
 
 
Q. HSRP stops working when an Access Control List (ACL) is applied. How can I permit HSRP through an ACL?
A. HSRP hello packets are sent to multicast address 224.0.0.2 using UDP port 1985. Whenever an ACL is applied to an HSRP interface, ensure that packets destined to 224.0.0.2 on UDP port 1985 are permitted.
 
 

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