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routing between secondary interfaces


I have just setup a secondary interface on my fa0/0 and am having trouble routing thru it.

I have a PC in one IP network, its default route is a 26xx (gateway to internal WAN), however I want it to go to a 16xx for its web connection. Problem is 16xx is on a different IP network.

My setup is as follows:

16XX (internet gateway)




------ Ethernet switch ----------

| |

| |

| |

26xx PC 1

gateway to WAN

16xx has fa0/0 ip address (gateway to internet)

PC1 has an address

Its default gateway is

26xx has only single Ethernet interface, fa0/0.

26xx setup:


interface FastEthernet0/0

ip address secondary

ip address secondary

ip address

duplex auto

!ip classless

How can I get PC1 to go to the 16xx for a internet connection?

instead of going to the default gateway?

I cannot ping the 16xx from the 26xx.....I thought once ip routing is enabled all connected routes are advertised.

Is there an issue with routing between a secondary address on the same int?

Any help I would greatly appreciate,




Re: routing between secondary interfaces

You need to run a routing protocol to advertise the routes between the two routers. right now the 1600 doesnt know how to reach 172 subnet.

New Member

Re: routing between secondary interfaces

Ah ok, currently I have just static routes on the 26xx, I dont manage the 16xx which explains the problem.

However your answer prompts me into another question. Maybe I mis-understand the fundamentals of how a router works here.

I cannot ping the 16xx from the 26xx fa0/0 interface. Even though the 26xx fa0/0 int has a secondary address in the same subnet as the 16xx eth int, 169.254.

My understanding:

I thought if ip routing (no routing protocol) is enabled, all directly connected networks on say RTR1 are reachable once a packet arrives at RTR1. So even though there is no dynamic routing once a router rcvs a packet destined for a network directly attached to it, it will route it correctly.

So, if I cant ping the 16xx eth int despite being in the same network, does this mean the 26xx always sptis out packets on its fa0/0 int with a src address of its PRIMARY address, not the secondary address..? This I think, is confirmed in a test I did.

Advanced trace from the 26xx fa0/0 to the 16xx eth int, I get a timeout. BUT if I do the exact same advanced trace but specify a src address in the 169.254 range the 16xx responds..! Does that prove my theory?

Re: routing between secondary interfaces

When you ping out an interface the ping packet always will use the primary ip address configured on the interface. Your primary address is and that is not in common subnet with the 1600.

Since the 1600 is not under your administration, it does not have any idea about network. Hence when you ping, it fails.

You can reconfigure the primary to be the 169.254 network (the same network the 1600 uses) and use the other two networks as secondary network.

The above mentioned reasons are the same, reason why you got a successful extended traceroute, with a source address in the 169.254 range specified.



Re: routing between secondary interfaces

Think about what you are trying to do in routing terms. All internet destinations are "unknown" networks unless your border router is one of the few the collects all routes from the internet. In the same sense, any local network that is not the same as the one the pc is in is also unknown to the pc. You are trying to distinguish between two types of "unkowns", local and remote. with a lot of work on the hosts you could define routes to the locals pointing to the 2600 and point the default to the internet, but you'd fail the test.

The design should make the routers make the routing decisions not the hosts. The host should have one default route pointing to the 2600. The 2600 should "know" all of the networks in its domain and should have a default route pointing toward the internet router. The internet router should be "above" the internal routers in the network hierarchy.


Re: routing between secondary interfaces

One thing to take note off is that your PC is using a private address. Hence, if it is the 16xx that is doing the NATting, it should be aware of the LAN address of the PC and the PC should be able to ping the 16xx. If it is the 2600, then consider what address it is NATting to. If you are using a proxy, where is it in your network

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