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New Member

Routing with secondary ip address

I have a setup with the following:

s0 <- 1720 -> fa0 -> 5 port basic switch <- e0 <- 1720 -> fa0 -> LAN

| | |

VoIP phone adapters (the adapters should acatually be hanging off the switch, but my format is off in the diagram)

1. The s0 interface is connected to a T1

2. The 1720 fa0 is connected to a 5 port switch.

3. The 1721 e0 is connected to the same switch

4. There are 3 VoIP phone adapters connected to the same switch

5. The 1721 fao is the default gateway of the LAN.

The config of the 1720 is:

interface s0

Interface fa0 (subnet for VOip phone adapters) secondary

ip route

The config of the 1721 is:

Interface e0

Interface fa0


With this setup, I cannot ping, telnet or have any communication between the 1720 fa0 and the 1721 e0. I believe part of the problem is that the secondary address is not seen as routable so the gateway on the 1721 should actually be (the primary of the interface). Can anyone help me with this config? Do I need a switch that does VLANs?




Re: Routing with secondary ip address


I believe your problem is related to your default route on the 1720 is sending all traffic there is no route for out the s0 interface. try adding a static route on the 1720

ip route

New Member

Re: Routing with secondary ip address

There are a couple of things to look at...

Does your routing protocol support VLSM? is the network number, not a valid address(unless you use IP SUBNET ZERO). Use 10 and 11 as the host addresses, 9 is the network number and 12 is the broadcast address.

There is a technical issue when you configure one router with a primary and secondary address and another router on the same segment with only the secondary. I cannot remember the TAC document, but it can cause some routing issues and in general is probably not a good practice.

If you had to keep the config, make the network as your primary and make the network as the secondary on the WAN router only. That way, your primary(WAN) is talking to the only address on the LAN router.



Re: Routing with secondary ip address

the correct subnet is .9 & .10 are hosts .11 is the broadcast would be a new network


New Member

Re: Routing with secondary ip address

Thanks for correcting me.... sorry for the confusion.

I need to wear those glasses and not store them in my desk drawer.

New Member

Re: Routing with secondary ip address

I just checked the config in the lab and you need to have the interface with two ip addresses setup such that the primary address matches the other end routers only ip address.

The error is "not on common subnet" for your current configuration.

I felt complelled to check it after goofing up the network address/mask answer.

New Member

Re: Routing with secondary ip address

Did you try changing primary and secondary address definitions of fast 0 interface? This would help you to access both addresses. Generally, using highest address as secondary from the similar subnets would cause some network routing problems. Probably adding a static route for inaccessible network to the router which has two interface address, will solve the problem but I'm not sure and I am unable to try on my lab now.

Hope this helps.

New Member

Re: Routing with secondary ip address

Thanks for all the posts. I found out that I have to have a secondary ip address on both routers in the same subnet and a primary ip address on both routers in the same subnet.


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