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Primary and Secondary ip address on a single interface

Unanswered Question


I need advice urgently.

I have 3 routers.When I configure primary and secondary ip addresses on a single physical interface,

I do not want to use sub-interfaces because I will have to buy an additional switch.

Q1) Is it true that at any point in time, only one of the ip address is in use?

In other words, if primary address is in used, then the secondary ip address cannot be used?

Scenario: I have 3 routers, 1 router is configured with primary and secondary ip address on a single FE interface. The other 2 routers are configured normally with 1 ip address per FE interface.

Router2 is connected to router1 via router1's primary ip address.

Router3 is connected to route1 via router1's secondary ip address.

Problem1: I discover that I can ping from router1 to router2 and vice versa.

But I can only ping from router3 to router1's ip address, but not from router1 to router3.

Q2)Is it because it is secondary ip address that router1 cannot ping to router3?

Q3) I also discover that my ospf routes are distributed only through the primary address of router1 but not the secondary address of router1.

Pls advice how I can resolve all my above problems

Thank you.


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amit-singh Wed, 11/09/2005 - 23:06
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1. Nope, you can use both the ip's simultanouesly.

2. Yes, itcould be reason is in point 3.

3. OSPF makes adjacency only on primary IP addresses. So you router 3 will not be in router 1's neighbor table. Do " show ip ospf nei ".Now you R3 can ping R1 interface, but will not able to ping R2 and all.

Try setting up static routes on your routers and then advertise that in OSPF, It should take care of it.


-amit singh


Although I run ospf on all 3 routers, but router1 is directly connected to router2 and router3.

So when I do "show ip route" on router1, I can see a "C" beside the route between router1 to router3, likewise I see the same thing when I do "show ip route" on router3.

Although OSPF does not make adjacency with secondary ip addresses, but router1 and router3 both run ospf and are directly connected, so when router3 can ping router1, shouldn't router1 be able to ping router3?

Pls advice.

Do I need to create any policy routing for it to work?


amit-singh Thu, 11/10/2005 - 00:11
User Badges:
  • Blue, 1500 points or more


You should be able to ping the routers.

Do an extended ping using the source as the secondry Ip of the R1 interface. It might be that it is using the primary IP to ping the R3 and that's y its not able to ping it.

Let me know how it goes.


-amit singh


You are right!

I used extended ping from the secondary ip address of router1 and I can ping to router3.

But I do not understand.

Why can't I ping from primary address of router1 to router3?

Router1's all interfaces are already directly connected, under normal circumstances all interfaces can ping to all routers, isn't it?

I do not have any ACL implemented.

Another question, since I can only ping by using extended ping, does it affect my data transfer from router1's secondary ip address to router3?

Thank you!


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