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New Member

Regarding HSRP issues

Hi,

1) In HSRP, we will have one Active router and one standby. is it possible to have multiple standby in a single group. If so what is the advantage.

2) When Active router fails and preempt enabled here, and when it regains its state, it will take over the active state because of preempt enabled.

If preempt is not enabled what happens and how active gains the role without preempt enabled.

3) When we make a gateway for client as HSRP Virtual IP. It prefers to send to Active router in a LAN network. If the standby is made as Root bridge, all the switches will reach standby. so how it takes care of switching in this scenario.

4) Wether Active only forwards the frames or even standby participates in forwarding. please clarify my confusion in HSRP concepts.

5) If we track an interface in HSRP in Active router we can make it to decrement the priority then it switch over to standby. Again how Active device will take over once it regains its state.

Thanks in advance

Naveen

3 REPLIES
Hall of Fame Super Silver

Regarding HSRP issues

Naveen

Here are my answers to your questions:

1)  It is possible to have more than 2 routers in a HSRP group. One router will be active, one router will be standby, and the other routers will be listeners. If the active router fails then the standby router becomes active and one of the listeners will become standby. The advantage of this is to provider greater failover capability and to have an alternative that will provide service if both the original active and original standby router should both fail then there is still a router that will provide the functionality of failover.

2) If preempt is not enabled when the original router comes back into service after a failure it will become the standby router (or a listener router if there are at least two other routers in the HSRP group). The original active router will not gain the active role unless the current active router (former standby router) has its own failure.

3) Spanning tree provides a loop free path to each of the devices within the spanning tree. So if the root of the spanning tree is the standby router then there is still a viable path from all switches to the active router. It may not be the most optimum path, but it is a viable path and data will flow correctly to the active router.

4) The active router has the virtual IP and the MAC address associated with the virtual IP. So only the active router will forward traffic sent to the virtual IP. The standby router does not forward any of that traffic.

5) When we track an interface in HSRP we can have the router decrement its priority if the tracked interface goes down. When the priority goes down the standby can take over only if it is configured with preempt. If the standby is not configured with preempt then it will not be able to take over in an interface track situation. When the tracked interface on the original active router comes back up then the router resumes its original priority. If that priority is higher and if the router is configured with preempt then it will resume the role as active router. If the former active router is not configured with preempt then it will not resume as active router when the down interface becomes up again.

HTH

Rick

New Member

Regarding HSRP issues

Hi Richard,

Thanks for your wonderful explaination.

5) With respect to preempt, if preepmt is not configured on both active and standby, when active fails it will fail over to standby right ? if not stanby has to be rebooted the only active will take over its role, please correct me if my understanding is wrong.

4) in case of Standby router, even it maintains ARP entries, so why it will not forward even it has a better hop.

3) If RB is stanby router, then forward and reverse path for the client will be same right?

Hall of Fame Super Silver

Regarding HSRP issues

Naveen

I am not sure that I understand your further question about 5). But let me offer some additional explanation and if it does not clear your issue then you can provide some clarification of the question. If the active router fails then the standby router will take over and there is no issue of preempt or not preempt. When the original active router comes back after the failure, and assuming that it has higher priority than the other router (previously standby and currently active) if the router is configured with preempt then it will resume as active and if it is not configured with preempt then it will take the role of standby.

Since the original 5) was about tracking interfaces let me give a little more explanation about that. Assuming that router A is active and is tracking an interface and that router B is standby. When the tracked interface on router A goes down then router A will lower its HSRP priority. If router B now has priority higher than router A and if router B is configured with preempt then it will take over as active and router A will become standby. If router B now has priority higher than router A but router B is not configured with preempt then router B will not take over as active and router A will remain active.

I do not understand your comment about 4). The standby router maintains its own ARP table and will know the MAC address of other devices in the subnet. But the standby router does not have the MAC address that corresponds to the virtual IP for HSRP. Only the active router has that MAC address. So when a PC sends a packet to its gateway address (which is the virtual IP) then the destination MAC address is the virtual MAC and only the active router will receive that packet. Since the standby does not own that MAC address it can not forward the packet.

3) Usually the forward and reverse paths would be the same. But there are scenarios in which the forward and reverse paths could be different.

HTH

Rick

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