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Router subinterfaces w/o 802.1Q

I'm routing between subnets using a 2620 router with one fast ethernet port. I assigned the port two IP addresses. One address is in subnet A range. One is in subnet B range and is secondary. I'm nost using trunking. All hosts on both subnets are attached to one switch. Subnet A uses a modem for a gateway address to get to the internet. The modem interface address is in subnet A. Subnet B uses the router as a gateway, with a default route set to the IP addr of the subnet A modem. I was unable to get hosts from subnet B to the internet until I configured a static route in subnet A modem that routes all traffic for subnet B to the router's subnet A address. Why is this? I thought the router would replace the source address of a web request from subnet B to it's subnet A interface address?

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: Router subinterfaces w/o 802.1Q

Patrick

The feature that you are describing is proxy arp. With proxy arp, if the router receives an arp request from a device in subnet A (such as the modem) looking for an address in subnet B, then if proxy arp is enabled the router will respond to the arp request with its own MAC address and can then forward the packet to the destination in B.

An important question is whether the modem will ARP for a destination that is not in its local subnet. By default a device (such as the modem) will ARP for the MAC address of devices within its own subnet. Some devices (and perhaps some configurations) can ARP for devices in remote subnets. But the behavior you describe implies that the modem is not arping for any remote addresses. That is why you need a route in the modem for the subnet of B.

HTH

Rick

5 REPLIES
Hall of Fame Super Silver

Re: Router subinterfaces w/o 802.1Q

Patrick

Based on what you have described so far I would say that this is pretty normal behavior. I am puzzled why you would think that the router would replace the source address as it forwards a web request.

A router will normally rewrite the layer 2 MAC addresses as it forwards at layer 3. But normal behavior in forwarding layer 3 is to keep the addresses the same. The exception would be if Network Address Translation were configuered. But you have not described NAT as part of this environment.

HTH

Rick

New Member

Re: Router subinterfaces w/o 802.1Q

You're right my bad! A router replaces MAC addresses. So a web request from subnet B would enter subnet A with a source IP address of subnet B. The modem would try replying to the subnet B address. But wouldn't the router answer on behalf of the subnet B host using its own router MAC address? Then you wouldn'nt need to config a static route on the modem.

Hall of Fame Super Silver

Re: Router subinterfaces w/o 802.1Q

Patrick

The feature that you are describing is proxy arp. With proxy arp, if the router receives an arp request from a device in subnet A (such as the modem) looking for an address in subnet B, then if proxy arp is enabled the router will respond to the arp request with its own MAC address and can then forward the packet to the destination in B.

An important question is whether the modem will ARP for a destination that is not in its local subnet. By default a device (such as the modem) will ARP for the MAC address of devices within its own subnet. Some devices (and perhaps some configurations) can ARP for devices in remote subnets. But the behavior you describe implies that the modem is not arping for any remote addresses. That is why you need a route in the modem for the subnet of B.

HTH

Rick

New Member

Re: Router subinterfaces w/o 802.1Q

Genius. Thanks so much! That explains it well.

Hall of Fame Super Silver

Re: Router subinterfaces w/o 802.1Q

Patrick

I am glad that my explanations helped you to resolve your question. Thank you for using the rating system to indicate that your question was resolved (and thanks for the rating). It makes the forum more useful when people can read a question and can know that there was a response which did resolve the question.

The forum is an excellent place to learn about Cisco networking. I encourage you to continue your participation in the forum.

HTH

Rick

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