sub interfaces.

Unanswered Question
Dec 22nd, 2007

Hi Folks,

(When we configure sub interface in router that will be a vlan number & that ip become as gateway am i right? but in my router one sub interface like f0/0.14 as been configured for vlan 14 and for that ip as 10.105.22.2 but now they are giving 10.105.22.1 as gateway for all ports which are in vlan 14 and they are working fine.

Here can any body explain why & how the sub interface ip & gateway of the vlan differently working.

Regards,

Naidu.

I have this problem too.
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glen.grant Sat, 12/22/2007 - 06:20

If thats the only ip configured and nothing like hsrp configured then probably proxy arp is on which will route packets without a gateway but is generally not recommended. I believe it is recommended that you turn off proxy arp if not absolutely needed as it is some type of security vulnerability left on . If you do a show ip interface command I think it will tell if proxy is turned on.

Richard Burts Sat, 12/22/2007 - 14:46

Naidu

Is it possible that 10.105.22.1 exists somewhere in the network and that the routing is being done by that device and not by your router?

If 10.105.22.1 does not exist, but that is the configured default gateway on the end stations in that VLAN then the question becomes what the end stations will do if they can not reach their configured default gateway. If they can not reach their configured default gateway and if they then ARP for the destination address, then Glen's explanation about proxy arp may be the correct answer.

Can you clarify whether 10.105.22.1 does exist? Also can you post the output from one of the end stations doing tracert (or traceroute depending on the OS of the end station) to some remote address? It would be very helpful to know what layer 3 device responded at the first hop.

It might also be helpful to post the arp table from the end station after it has successfully communicated with some remote device. (does the remote device show up in the ARP table, does the configured gateway show up in the arp table?)

HTH

Rick

ilnaiduccna Mon, 12/24/2007 - 08:16

Hi Glen,

Thanks for your reply,

Actually they have configured hsrp also and I checked with show ip int there is no proxy both proxy ARP, local proxy AP were disabled.

Now how they are working can you give idea.

Regards,

Naidu.

Richard Burts Mon, 12/24/2007 - 10:19

Naidu

If they have configured HSRP then there must be another router active on that VLAN. And that makes my suggestion that it is possible that 10.105.22.1 exists somewhere in the network even more likely.

Can you confirm the other router that is running HSRP and what IP address is it using for its physical interface and what address is the shared address?

HTH

Rick

ilnaiduccna Wed, 12/26/2007 - 08:46

Hi Rick,

Sorry for my delay.

As your description saying right there is another router and HSRP configured in both routers.

10.105.22.1 not somewhere in the network it is the IP of HSRP standby premt in both routers.

Rick can you clarify what is the "shared address" end of the sentence in your last post.

Regards,

Naidu.

Richard Burts Wed, 12/26/2007 - 13:22

Naidu

If 10.105.22.1 is the IP of HSRP standby then it does exist somewhere in the network. It is shared between the 2 physical router interfaces. And this is the shared address that I was referring to. One of the main attributes of a device that exists somewhere in the network is that if you ARP for its address you will get an ARP response giving the MAC address to use to access this device. This is certainly the case with the HSRP shandby address.

To review this thread: in your original post you state that you configured a router with 10.105.22.2 and wondered why end stations with configured default gateway of 10.105.22.1 were able to make connections since they were not using the address of your router. And the reason is that they are configured with the HSRP virtual address (shared address) which is shared by the routers and is used by the "active" router. so maybe we need to reformulate your original assumption:

When we configure sub interface in router that will be a vlan number & that ip become as gateway am i right?

and what we need to say is that the interface IP or the HSRP address on that interface will become the gateway.

HTH

Rick

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