HSRP question

[4everlearning]

Hi Guys,

Just a quick question, let's say i have connectivity where only one physical link is provided (physical redundancy is not there) can i still configure HSRP on my both routers to avoid doing any configuration if the physical link that is connected to router 1 goes down. and in case router one goes down, someone will go to site physically and plug the link to router 2. where it is already configured with the virtual IP and HSRP configuration. Would that work?

Would bringing the physical link up to router 2 make it work right away?

Thanks,

[Aaron Harrison · Accepted solution]

Hi

You wouldn't need HSRP.

If you have the same IP address (e.g. your default GW) configured on both, then if R2 has it's interface disconnected (down) it will be inactive. When you disconnect R1, it goes down/inactive, and then when you connect R2, R2 would come up and start servicing.

You would need to ensure that R2 advertises out routes to the subnet, i.e. you don't have a static route pointing to R1 on the 'other side' or 'outside' of these routers.

That said, you could still configure HSRP to avoid problems if both routers were connected at once.

Also, this would be quite unusual - if you have two routers on a site, why would you not connect them both to the LAN?

Aaron

[4everlearning]

Thanks Aaron,

they both in the same site, but due to some limitation in the network we are connected to, they can only provide 1 physical link for now. 

[Jon Marshall]

Not sure this is going to work as you want it.

If you configure the same IP on both routers and router 1 goes down  all the internal devices that forward to that router will have the wrong mac address for that IP ie. they will still be using the old mac not router 2's mac address.

Are you saying that both routers have LAN connections but only one has a WAN connection ?

Or are you saying that you only have one LAN connection to router 1 ?

I assume there is no L3 switch at the site ?

Jon

[4everlearning]

Yes, both routers have LAN connection configured with HSRP for Up link traffic. 

However, for Down link traffic, only one physical link is available. Hence i was thinking to configure HSRP to avoid doing any configuration if router 1 goes down.

[Jon Marshall]

Okay now I am more confused :-)

Both routers have a LAN connection with HSRP configured.

When you say down link do you mean WAN connection ie. only router 1 has a WAN connection ?

So are you talking about configuring HSRP on the WAN side ?

Apologies but the terminology you are using is a bit confusing.

Jon

[4everlearning]

Yes,I am thinking to configure an HSRP for the WAN side, sot hat if R1 goes down, only changing the physical link and plug it to R2 would shift the traffic.

[Jon Marshall]

Okay  it really depends on the WAN connection and how it is setup. If it is in effect a point to point link you may well not face the same issue with the mac addresses that I mentioned.

Difficult to say without knowing more about the WAN connection, would probably need some testing.

If there were issues with the mac address to IP mapping using HSRP may be the best solution but again it would need testing.

The other issue you need to be aware of is the WAN interface on router 1 fails but the LAN interface doesn't. If you just plug the WAN connection to router 2 the clients will still be sending the traffic to router 1.

What happens then is down to whether router 1 and router 2 are exchanging routes with each other.

Not trying to confuse the issue but it's difficult to say what will and won't work properly without knowing more about the setup.

Jon

[ghostinthenet · Accepted solution]

As long as both units are on the same LAN, you can configure HSRP with pre-emotion between the two and track the physical WAN interface, reducing the priority of the router if it goes down. With this approach, pulling the physical WAN interface and plugging it into the other router will automatically cause the other unit to take over the gateway. Is that what you're looking for?

If so, you could just do something like this:

Primary Router

interface GigabitEthernet0/0 ! LAN
standby ip 192.168.0.1
standby version 2
standby preempt
standby track GigabitEthernet0/1 ! WAN

Backup Router

interface GigabitEthernet0/0 ! LAN
standby ip 192.168.0.1
standby version 2
standby preempt
standby priority 95

This will cause the primary router to drop its priority to 90 if the WAN is disconnected, causing the backup router to take the default gateway if the WAN cable is physically unplugged. Should the physical cable be moved back to the primary router, it will once again take over the default gateway. Obviously, you'll need to adjust your addressing accordingly.

[Ketanbelekar@tsys.com]

I have used the same setup in my case, but the question i have is in our testing scenario we kept 2 WAN links down and now when source (server) sends the traffic to routers its getting time out after 20 seconds, so how would i reduce this time?