router interfaces on the same network

[reggae3227]

I have been trying to found this documented somewhere. I am not sure why anyone would want to

config a router this way? I get an error when I attempted to add same network on 2 router interfaces. "ipaddr" OVERLAPS WTIH FAST ETHERNET 0/0. I would like to see documentation that router interfaces cannot be on the same network. Wouldn't this be considered bridging or switching.

[dbellaze]

Yes this would be considering bridging/switching.

I think I do recall a document some where that does state that you cannot have two interfaces on a router in the same network.

But given the function of a "router" that already gives you the answer. Routers route between layer 3 domains and each interface is considered to be a part of a unique domain.

We have come a long way with routers and we can now use them in ways that go beyond the base router function.

Daniel

[reggae3227]

When is a router not a router? when it's a bridge or switch! Can you bridge different media say one interface token-ring and other ethernet in the same router. Wouldn't this functionality be better served by another device, switch or bridge. If you can point me to documentation that would be great.

[Richard Burts]

The easiest way to make a router into not a router is to enter the command no ip routing. With that you have created a pretty expensive bridge. You can then enable bridging on the interfaces and if you want to put two interfaces into the same subnet then you can do so.

A more complex way would be to configure Integrated Routing and Bridging. This allows you to route on some interfaces and bridge on some interfaces. If you want to put two interfaces into the same subnet, you could not put IP addresses on those interfaces, configure bridging on the interfaces, configure a BVI interface which will process the bridging of the interfaces.

But why would you want to do this in the first place?

HTH

Rick

[reggae3227]

I am trying to justify NOT doing it and using the router for what it is intended. Any more bullets for my gun would be appreciated

[dbellaze]

A router can bridge between ethernet and token ring media, but it requires translational bridging because the frame's are not read the same for the two media's.

Another use for bridging is the ability to bridge on WAN interfaces, a lot of the time you can't buy a layer 2 switch with WAN interfaces like T1's. In situations like this you would need to use the router's bridging capabilities.

Most bridging scenarios I see (other than IRB) are when administrators do not want to re-number or have a routed IP network internally. They want to use 10.0.0.0/8 for there entire network including all remote sites. This isn't good practice because a lot of bandwidth is wasted on broadcast traffic.

Daniel

[reggae3227]

I found the following document. It explains translational bridging between ethernet and tokenring

is only pssble for non routable protocols. It is not

possible to translational bridge IP with ether/tokenring media. Can someone give me a second opinion?

http://www.cisco.com/en/US/customer/tech/tk331/tk660/technologies_tech_note09186a0080093d4d.shtml

Translational Bridging

Translational bridging allows you to bridge between dissimilar LANs, commonly Ethernet and Token Ring, or Ethernet and FDDI. In the case of Ethernet and Token Ring bridging, translational bridging only allows connectivity for non-routable protocols such as Local-Area Transport (LAT), Maintenance Operation Protocol (MOP), and Network Basic Input/Output System (NetBIOS).

The translation for bridging between Ethernet/Token Ring and Ethernet/ FDDI requires bit order reversal because the internal representation of MAC addresses is different on Ethernet, Token Ring, and FDDI. Ethernet is little endian (it transmits least order bit first), and Token Ring and FDDI are big endian (transmit high order bit first). For example, address 0000.0cxx.xxxx on Ethernet would appear as 0000.30yy.yyyy on Token Ring since every byte needs to be bit-swapped. Both Ethernet and Token Ring use the first transmitted bit of a frame's destination address to determine whether the frame is unicast or multicast. With no address conversion, a unicast frame (a frame that has only one destination) on one network may appear as multicast address (an address for more than one station) on another network.

Remember that Ethernet and Token Ring bridging is only possible with non-routable protocols. At times, MAC addresses are carried in the data portion of a frame. For instance, Address Resolution Protocol (ARP) places the hardware address in the data portion of the the link-layer frame. It is simple to convert source and destination addresses in the header, but conversion of hardware addresses that may appear in the data portion is more difficult. When performing source route transparent or source route translational bridging between Ethernet and Token Ring, Cisco does not search for instances of hardware addresses in the data portion. Only non-routable protocols work with Ethernet and Token Ring bridging.

Translational bridging between Ethernet and FDDI carries the issue of bit reversal a little further since few protocols work across the FDDI and Ethernet barrier. One reason for this is the concept of a canonical address above the MAC layer - any address that is above the MAC layer on FDDI should be ordered canonically according to Ethernet order. This is how IP is done on FDDI, and it is why Cisco can bridge it when going from Ethernet to FDDI. Unfortunately, other protocols do not do this.

The protocols below can be translationally bridged between Ethernet and FDDI.

IP

OSI

DECnet

Non-routable protocols (NetBIOS, MOP, and LAT)

John F.

[Richard Burts]

John

In my experience the document that you reference is pretty much correct. While in the mechanical sense you could bridge between Token Ring and Ethernet (take frames from one media, adjust the bit order, rewrite the header, and deliver to the other media, the reality is that IP will not work. There are a number of issues that impact this and the document highlights one of the biggest problems which is that ARP would not work if bridged between Token Ring and Ethernet.

HTH

Rick

[reggae3227]

Ok so I have found another document stating that SR/TLB works with ethernet to tokenring. It doesnot

state that it doesnot work specifically with IP.

http://www.cisco.com/en/US/customer/tech/tk331/tk660/technologies_tech_note09186a00800947b2.shtml

can someone verify this config for IP actually works.