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

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.

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.

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.

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.

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.

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.

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> .

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.

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.

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.

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 of the failed router.

A. The destination address of HSRP hello packets is the all routers multicast address (224.0.0.2). The source address is the primary IP address of the router assigned to the interface.

A. UDP, since HSRP runs on UDP port 1985.

A. HSRP hello packets are sent to multicast address 224.0.0.2 with 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.

A. If routers are configured in HA mode (that run HSRP in-between them), then the active and standby routers act as one logical unit and share the same IP and MAC address. Only the active router generates the accounting record with a particular virtual IP address and updates the TACACS/RADIUS server. If the standby generates the accounting record with the same address, there is duplicate data in the backend RADIUS/TACACS server. Therefore, in order to avoid duplication of data, the standby router does not generate accounting records.

A. It is not possible to use the HSRP configuration to track the GRE tunnel interface. However, the tunnel interface never goes down and the track never triggers failover.

A. The only way to make a failover without an interface shut down is to manually change the priority in the HSRP configuration.

A. Yes it is possible to run HSRP on the interfaces configured for 802.1q. Make sure to verify that both sides of the trunk are configured to use the same native VLAN and verify that VLANs are not pruned and in the STP state for router-connected ports.

A. Yes, it is possible to run HSRP on two interfaces on two different routers. In order to have HSRP on two interfaces on two different routers, two HSRP groups are needed .

A. HSRP and OSPF are two different protocols. The OSPF that runs on the router advertises the two physical interfaces and not the virtual IP address. When this router becomes active, it broadcasts a gratuitous ARP packet with the HSRP virtual MAC address to the affected LAN segment. If the segment uses an Ethernet switch, this allows the switch to change the location of the virtual MAC address so that packets go to the new router instead of the one that is no longer active. End devices do not actually need this gratuitous ARP if the routers use the default HSRP MAC address.

A. When a reply for traceroute is received from a hop that runs HSRP, the reply must contain the active physical IP adddress and not the virtual ip address. If there is an asymmetric routing in the network due to which standby router IP address is seen in the reply for the traceroute.

A. GLBP provides load balancing over multiple routers (gateways) using a single virtual IP address and multiple virtual MAC addresses. Members of the GLBP group select one of them to become the active virtual gateway for the group.

With HSRP in a single router (gateway), one interface is used as the active interface and the other interface is in standby. The active interface is used for all the traffic and the standby interface just waits for the active interface to fail without any traffic.

A. If router A is the HSRP active router and then loses a link, which causes it to become standby router, and then the link comes back, the delay command causes router A to wait before it becomes active again. In this case, it waits for 60 seconds for the router to become active.

A. Yes. You can run HSRP on Sub-Interfaces.

A. Use this configuration:

track 10 ip route 0.0.0.0 0.0.0.0 reachability
!
interface fa0/1
glbp 50 ip [ip address]
glbp 50 priority 210
glbp 50 preempt
glbp 50 weighting track 10