L3 links to dual core switches , failover scenarios

[kamal kumar]

Hello ,

we have two 4948 cisco switches acting as L2/L3 switch . HSRP is defined for all VLAN. These switches connects to a pair of cisco 3750 stackable switches via layer 3 routed links .

Each 4948 is having default routes pointing to 3750 switches . 3750 switches are pointing LAN side subnets to each L3 routed link . Under normal conditions things works perfectly . But when one links goes down , even though 3750 is having LAN side subnets in routing table pointing to another 4948 , but we are not getting response from the HSRP Virtual IP address .

How best the HA and failover can be configured , so that once link goes down , we can use the alternate path automatically . Is some routing protocol required or IP SLA , Pls suggest .

Thanks .

[Jon Marshall]

Kamal

But when one links goes down , even though 3750 is having LAN side subnets in routing table pointing to another 4948 , but we are not getting response from the HSRP Virtual IP address .

Can you clarify exactly what is isn't working ie. if you are not getting a response from HSRP virtual IP is that from the clients or from the 3750 switches ?

Jon

[kamal kumar]

Hi Jon,

Thanks for response .

There is no response form the HSRP virtual IP when pinging from 3750 . From server side its fine , they can reach default gateway in case of failure also . I can even reach the vlan interface IP , but the VIP is not reach able .

Is there any alernate way to make it work , achieving HA and automatic failover to alternate link .

[Jon Marshall]

Kamal

So when HSRP fails overs do you lose network connectivity for clients/servers or is it just you can't ping the virtual IP from the 3750s.

Jon

[kamal kumar]

Jon,

Just can't ping Virtual IP from 3750 . Once the primary switch (4948) goes down , floating routes comes into picture in 3750 pointing Server subnets to secondary link and things start working . Its just a initial deployment so I was testing for ping during failure cases . 

If we can make it in some better way , then I can change design right away . what you suggest ?  is it OK to fine tune same design , or shall I modify and put ospf between 3750 and 4948 .

Thanks .

[Jon Marshall]

Is it the switch going down or the link between the 4948 and the 3750s ? 

If the switch goes down then HSRP will swap, but if the L3 link goes down then it will stay on the active 4948.

If it is the link then can you ping the virtual IP from the standby 4948 switch ?

I'm not entirely sure why you can't ping the VIP if the L3 link goes down, but it depends how much you want to troubleshoot this as it may or may not be an issue for you.

One thing to notice is that at the moment because each switch only has one default route then traffic from that switch will go direct to the 3750s. So looking at your diagram if the 4948 on the left hand side is HSRP active for vlan 8 then all traffic from clients/servers in that vlan goes to the left hand 4948. This switch only has a default route direct to the 3750s so you are only using that link.

Is this what you want ?

As for statics vs routing protocol, if this is the entire setup it's probably not going to make much difference. I generally use routing protocols (EIGRP/OSPF) rather than statics and if you have other devices that will need to know the routes to these subnets a routing protocol would be the way to go.

How many vlans/IP subnets are there going to be on the 4948 ?

You do need to decide how you want traffic to flow though from the 4948s to the 3750s. Do you want to utilse both uplinks for each vlan or only use one per vlan.  This can make a big difference in how you configure the setup.

Also, what does your floating static look like on the active 4948 ie. what is the next hop IP used  ?

Jon

[Joseph W. Doherty]

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Posting

Normally, when routing between devices, neither L3 device should be using virtual IPs for next hops.  Are you doing this?  (It's a bit unclear from your description.)

From what you've posted, it appears each 4948 only has a single default to its p2p interface.  If true, if that link goes down, that 4948 won't have a default route.  If true, you should have a secondary (could be floating) default route on both 4948s to use the other 4948 for a path to the 3750 stack.

[kamal kumar]

Hi Joseph,

The default route is pointing to next hop IP of the routed interface . I have added a floating default route in each 4948 , pointing to alternate p2p link next hop IP address . During failure , the 4948 inserts the floating route to routing table . But during ping test , the rechablity is only for vlan physical IP , not for the VIP .

[Joseph W. Doherty]

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

I'm confused how you're using your virtual IP with routing.  Normally, you would not (when you don't you might need to use mHSRP).  Could you post all the relevant parts of the configs, i.e. all interfaces and route statements?