[c-nsp] trunks, vlans and a metroLAN

Peter Rathlev peter at rathlev.dk
Thu May 1 17:17:17 EDT 2008


On Thu, 2008-05-01 at 15:36 -0400, Sridhar Ayengar wrote:
> Peter Rathlev wrote:
> > On Thu, 2008-05-01 at 17:06 +0200, Benny Amorsen wrote:
> >> Eric Van Tol <eric at atlantech.net> writes:
> >>> Are /31 subnets valid for an ethernet network nowadays?
> >> See RFC 3021.
> > 
> > So the answer is: No, not unless Ethernet is "point-to-point", which it
> > isn't.
> 
> It can be, can't it?  How would you describe an ethernet with two nodes 
> on it, using an RFC 3021 addressing scheme?

It still technically wouldn't be a "point-to-point" link. It would be a
"broadcast, multiple access" segment that would happen to have only two
participating nodes. If you configure a /31 netmask on an
Ethernet-interface on e.g. a C6k SXF you get:

PE2(config)#interface GigabitEthernet4/9
PE2(config-if)#ip address 10.0.0.0 255.255.255.254
% Warning: use /31 mask on non point-to-point interface cautiously
PE2(config-if)#

You can configure it, but the message says that the GE interface (this
one is on a WS-X6516-GBIC LAN card) is not "point-to-point".

So to sum it up: You can use /31 netmasks also on Ethernet links, but
RFC3021 treats only "point-to-point" links, and thus technically not
Ethernet. (Section 2.2.)

Using an Ethernet line as "point-to-point" is definately possible. And
e.g. ISIS has the "isis network point-to-point" interface command to
force the IGP to assume that a link is "point-to-point". But you can't
hide behind RFC3021 if you encounter problems...

Regards,
Peter




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