brouter - bridging non routable (layer 3?!) addresses - terminology question

brouter - bridging non routable (layer 3?!) addresses - terminology question
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Subject Author Date
brouter - bridging non routable (layer 3?!) addresses - terminology question q_q_anonymous 02-28-2006
Posted by snertking on March 1, 2006, 1:22 pm
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Rich Seifert wrote:
>
>
>>q_q_anonymous@yahoo.co.uk wrote:
>>
>>>A brouter is said to bridge non routable prtocols. But aren't non
>>>routable protocols still Layer 3?
>>
>>Some are, some are not. I would not call Spanning Tree a layer 3
>>protocol. It is a protocol, and it is not routable (nor would one WANT
>>to route it!) and it carries ethenet specific information of interest
>>only to connected switches.
>>A counter expample is Netbios/Netbui over erthernet: not routable
>>either, but it IS a layer 3 protocol.
>
>
> That's arguable. NetBIOS/NetBEUI is more of an application-layer
> interface to the Data Link than an internetworking (Layer 3) protocol.
>
>
>>DEC LAT is another good example of
>>a non-routable layer 3 protocol.
>>
>
>
> DEC LAT is a good example of an Application layer protocol (or arguably,
> a Transport (Layer 4) protocol). It provides terminal services analogous
> to Telnet, although it is restricted to a single Data Link due to the
> fact that it interfaces directly to the Ethernet and incorporates no
> internetworking (Layer 3) capability. It is most definitely not a Layer
> 3 protocol.


Thanks. Problem was with my understanding of OSI layers apparrently.

Got it now, I think.

Posted by glen herrmannsfeldt on March 1, 2006, 2:45 pm
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> Thanks. Problem was with my understanding of OSI layers apparrently.

It helps if you think of the layers as approximations.
(though 1-3 aren't so bad.)

There is a story about when the OSI layer model was being
developed, the Europeans wanted six and US wanted 8 layers
(or the other way around) and compromised on seven.

Another story is that the ATM 48 byte payload is a compromise
between Europe's 32 and US 64 (or the other way around).

-- glen


Posted by on February 28, 2006, 10:07 pm
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snertking wrote:
> q_q_anonymous@yahoo.co.uk wrote:
> > A brouter is said to bridge non routable prtocols. But aren't non
> > routable protocols still Layer 3?
>
> Some are, some are not. I would not call Spanning Tree a layer 3
> protocol. It is a protocol, and it is not routable (nor would one WANT
> to route it!) and it carries ethenet specific information of interest
> only to connected switches.
> A counter expample is Netbios/Netbui over erthernet: not routable
> either, but it IS a layer 3 protocol.

Layer 3 because it's a logical address?

so if a device 'relays/forwards' using NETBIOS/netbeui over eth, is it
bridging or routing?

A Brouter is said to bridge nonroutable protocols. That indicates that
the word bridging is being used at Layer 3
Yet later, you say bridging is layer 2 only.

<snip>
Generally what BROUTER means is that it routes the
> protocols it knows how to handle at layer 3, and bridges everything else
> at layer 2.

but surely it's not bridging everything else at layer 2. NETBIOS you
said is a layer 3 so if it deals with that one, it's bridging at layer
3.
I wonder if the term non routable isn't OSI. I guess it's only called
non routable because it's flat. But maybe OSI considers it routable
'cos it's layer 3. (assuming layer 3 means logical)

> >
> > so a brouter does either bridging at layer 3 or routing at layer 3 -
> > depending on the protocol.
>
> bridging is layer 2 by definition.

in which case, saying 'bridging netbios' would mean ignoring the layer
3 netbios part, and bridging the - say - MAC addresses.

<snip>

thanks


Posted by Albert Manfredi on March 1, 2006, 10:28 am
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q_q_anonymous@yahoo.co.uk wrote:

> A Brouter is said to bridge nonroutable protocols. That indicates that
> the word bridging is being used at Layer 3
> Yet later, you say bridging is layer 2 only.

When one says "bridging if it can't route," what that means is that
whatever frames arrive with an unknown Layer 3 header, or no Layer 3
header at all, remain within the domain of the local Link Layer. In an
IP example, that would mean they stay in the same IP subnet as the
brouter port on which they arrived. They are switched through the
brouter as if this were a bridge, using only the MAC address
information, and not forwarded to the port(s) that lead beyond the IP
subnet of the port on which these frames arrived.

> I wonder if the term non routable isn't OSI. I guess it's only called
> non routable because it's flat. But maybe OSI considers it routable
> 'cos it's layer 3. (assuming layer 3 means logical)

Layer 3 means beyond the scope of the local Link Layer protocol. In an
IP over Ethernet example, beyond the boundaries of that local Ethernet
catenet, for example. Layer 3 addresses tend to be hierarchical, but
then again, some addresses that are used as Layer 2 can also be (e.g.
NSAPAs used with ATM). And I suppose one could have non-hierarchical
Layer 3 addresses too, in principle. It would require complete routing
tables at each router.

Bert


Posted by on March 2, 2006, 8:47 am
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Albert Manfredi wrote
> When one says "bridging if it can't route," what that means is that
> whatever frames arrive with an unknown Layer 3 header, or no Layer 3
> header at all, remain within the domain of the local Link Layer. In an
> IP example, that would mean they stay in the same IP subnet as the
> brouter port on which they arrived. They are switched through the
> brouter as if this were a bridge, using only the MAC address
> information, and not forwarded to the port(s) that lead beyond the IP
> subnet of the port on which these frames arrived.
>

that sounds very odd.

If the brouter sends the packet back out the interface where it came,
then preumably, it means it was a mistake for the comp to send it there
in the first place with that interface's Dest MAC.

If the brouter knows the correct MAC then it must have MACs for that
funny layer 3 protocol, at every interface, for each comp on that
interface.

If the brouter has the correct MAC, it means the host is there on that
subnetwork, and the packet was sent with the wrong MAC.

If it doesn't have the correct MAC, i guess it'd drop it.

this is rather inefficient. If what i've said is correct, the brouter
has all MACs for all hosts on all interfaces at each interface
(otherwise who is it going to send the packet back to? itself? the
original dest mac on the packet was itself)
So, if it has them all, and the packet is for a subnet local to the
brouter (though not local to the host) then our brouter would be able
to send it out the correct interface with comp's MAC. Yet you are
saying it wouldn't do that, since I suppose that'd be considered
routing. (though surely it shouldn't have gone to the brouter in the
first place, the sending comp should've had the MAC)

minor additional point - if by the brouter bridging you mean sending
back out the same interface , that is very unbridge like. Bridges send
out a diff interface. They choose the correct interface.

<snip>

thanks


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