Use of secondary ip address on Cisco router interface.

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Nov 15th, 2002
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I have two Cisco routers (A & B) that reside on the same ethernet hub. Cisco router A is configured with a primary and secondary address. The secondary address is in the same IP range as the IP address for Router B's ethernet interface. Router B's ethernet is configured with only a primary address.

From Router B, I can ping the secondary address of Router A's ethernet and it's serial interface's IP address and beyond.

From Router A, I am not able to ping Router B's ethernet.

Any ideas?

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vcjones Fri, 11/15/2002 - 15:57
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This is an illegal configuration. Cisco routers require all routers on a LAN to have identical views of addressing on the LAN (you can make exceptions, but you had better know what you're doing or you'll get strange results like you describe).

In your case, the pings from Router A are being sent with a return address of the the primary address of the Ethernet interface and Router B has no route back to that IP address. The correct way to fix the problem is to correctly configure Router B so its primary address is on the same subnet as the primary address of Router A, and both have secondary addresses on the second subnet. Or you could patch it by installing a static route on Router B so it knows how to get to Router A's primary address. Or you could use extended ping to set the source address to Router A's secondary IP rather than the primary which is unknown to Router B.

Be aware that ping is not the only thing broken in your configuration. Routing protocols like OSPF will refuse to establish neighbor relations if the primary IPs are not on the same subnetwork.

Good luck and have fun!

VIncent C Jones

prafuljaded Sun, 11/17/2002 - 00:23
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That's right, according to last update from Vincent C Jones, without a return path for router B, it simply drops the packet.


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