Secondary IP Addresses

[network_dude]

Hi,

Quick question regarding secondary IP Addresses...

I have a customer who has multiple gateways per vlan across a couple switches (don't ask). In order to help migrate them with a more seamless transition to the new network I'm installing for them, I was wondering about secondary addresses. If I create an SVI for say, vlan 100. Could I put secondary addresses on it which can also be used as gateway for that network?

ex

Interface vlan100

ip address 1.1.1.1 255.255.255.0

ip address 1.1.1.2 255.255.255.0 secondary

Could both of those be used as gatways if I have 10 machines set as 1.1.1.1 as the gateway and 10 others set to 1.1.1.2?

Thanks,

Scott

[glen.grant]

Yes you can do that...

[network_dude]

Thank you for your reponse, something else to add...can I use HSRP with this set up?

interface vlan 100

ip address 1.1.1.2 255.255.255.0 secondary

ip address 1.1.1.3 255.255.255.0 secondary

ip address 1.1.1.4 255.255.255.0

no ip redirects

standby 100 ip 1.1.1.1

standby 100 priority 110

standby 100 preempt

or

interface vlan 100

ip address 1.1.1.4 255.255.255.0

no ip redirects

standby 100 ip 1.1.1.2 secondary

standby 100 ip 1.1.1.3 secondary

standby 100 ip 1.1.1.1

standby 100 priority 110

standby 100 preempt

Thanks again,

Scott

[Richard Burts]

Scott

I certainly agree with Glen that your first question would work. In this post your option 1 with a primary, several secondaries, and an HSRP address would certainly work. And it would give you some flexibility to migrate end stations which are configured with .2 or .3 or .4 as their gateway to be configured with .1 as the gateway.

(as a side note I am not sure why you are setting priority 110. perhaps if we had more context of what you are configuring it would make better sense. but my immediate reaction to priority 110 is that if you are doing track interface with default values it will decrement to priority 100 and create a tie with the standby interface which will not allow the standby interface to take over. I much prefer priority 105.)

I am not sure that your option 2 with a single configured address and with multiple secondary addresses in HSRP would work. I do not know that it would not. But I suspect that there are problems with it.

HTH

Rick

[network_dude]

Thanks Rick...this is how I would have both routers set up.

Option 1

interface vlan 100

ip address 1.1.1.2 255.255.255.0 secondary

ip address 1.1.1.3 255.255.255.0 secondary

ip address 1.1.1.4 255.255.255.0

no ip redirects

standby 100 ip 1.1.1.1

standby 100 priority 110

standby 100 preempt

interface vlan 100

ip address 1.1.1.5 255.255.255.0 secondary

ip address 1.1.1.6 255.255.255.0 secondary

ip address 1.1.1.7 255.255.255.0

no ip redirects

standby 100 ip 1.1.1.1

standby 100 priority 100

standby 100 preempt

Option 2

interface vlan 100

ip address 1.1.1.3 255.255.255.0

no ip redirects

standby 100 ip 1.1.1.2 secondary

standby 100 ip 1.1.1.5 secondary

standby 100 ip 1.1.1.6 secondary

standby 100 ip 1.1.1.7 secondary

standby 100 ip 1.1.1.8 secondary

standby 100 ip 1.1.1.1

standby 100 priority 110

standby 100 preempt

interface vlan 100

ip address 1.1.1.4 255.255.255.0

no ip redirects

standby 100 ip 1.1.1.2 secondary

standby 100 ip 1.1.1.5 secondary

standby 100 ip 1.1.1.6 secondary

standby 100 ip 1.1.1.7 secondary

standby 100 ip 1.1.1.8 secondary

standby 100 ip 1.1.1.1

standby 100 priority 100

standby 100 preempt

I'm wondering which would be the best way to migrate the users...and seamlessly begin to remove the secondary IPs once everyone is migrated. It seems to me that option 2 (If it's possible) would be the best for true redundancy.

Thanks again,

Scott

[Richard Burts]

Scott

I have no doubt that option 1 would work. I have not seen option 2 where you define a secondary HSRP group but no secondary address. I have seen configs with a secondary address and a secondary HSRP group but not the secondary group without a secondary address configured. It might work - I just am not sure. It would be a good thing to create a test environment for and check it out.

I can see that it would provide redundancy. But I am not clear what IOS does when it has a secondary HSRP group but no secondary address. I wonder if you would need something like this:

interface vlan 100

ip address 1.1.1.9 255.255.255.0 secondary

ip address 1.1.1.10 255.255.255.0 secondary

ip address 1.1.1.11 255.255.255.0 secondary

ip address 1.1.1.12 255.255.255.0 secondary

ip address 1.1.1.13 255.255.255.0 secondary

ip address 1.1.1.3 255.255.255.0

no ip redirects

standby 100 ip 1.1.1.2 secondary

standby 100 ip 1.1.1.5 secondary

standby 100 ip 1.1.1.6 secondary

standby 100 ip 1.1.1.7 secondary

standby 100 ip 1.1.1.8 secondary

standby 100 ip 1.1.1.1

standby 100 priority 110

standby 100 preempt

HTH

Rick