question about IP unnumbered
ideal.lee
puzzled by:
"
IP Routing for an Interface
IP unnumbered (serial only)
If you plan to use IP routing on a serial interface, you must
determine whether it will be unnumbered.
Default: No.
Action: If you plan to use IP unnumbered routing on an interface,
write Yes on the worksheet. If not, write No. Repeat this for each
installed serial interface you plan to configure.
"
Who can explain the meaning of IP unnumbered in detail.
Thanks a lot.
Barry Margolin
>Who can explain the meaning of IP unnumbered in detail.
IP unnumbered is an option for point-to-point interfaces. Anything sent
out a PtP link will automatically be received by the device at the other
end; no addressing is necessary (compare this to an Ethernet, where you
have to look up the IP address of the destination in the ARP cache). You
can use it on Serial interfaces, frame relay subinterfaces, tunnel
interfaces, etc.
When you configure an interface to be unnumbered, you associate one of the
other (numbered) interfaces with it, e.g.
interface Serial0
ip unnumbered Ethernet0
The address of the associated interface is used as the source address of
any packets originated by the router out that interface (e.g. if you
traceroute through the router, the Ethernet0 address will show up, since
there's no Serial0 address).
If you need to configure static routes pointing to the router on that
Serial line, they should use the interface name as the next hop, e.g.
ip route 10.1.2.0 255.255.255.0 Serial0
The benefit of IP unnumbered is that you don't waste a subnet address block
for a link that doesn't require addressing. The problem is that you can no
longer distinguish different interfaces, which is mostly a concern in
troubleshooting; for instance, if a router has multiple serial interfaces
that are all "ip unnumbered Ethernet0", you can't tell which one was used
in a traceroute, and you can't ping specific interface addresses to see
whether they're up.
--
Barry Margolin, bar...@bbnplanet.com
GTE Internetworking, Powered by BBN, Burlington, MA
*** DON'T SEND TECHNICAL QUESTIONS DIRECTLY TO ME, post them to newsgroups.
Please DON'T copy followups to me -- I'll assume it wasn't posted to the group.
Jason Bomar
you wanted to use your registered IP address internally and it was a class
C, you might need to conserve address space so you would use Unnumbered on
the serial interfaces. The way it works, simply put, is that the serial
interface "borrows" an IP address from an existing interface, usually a LAN
interface. So you would type:
IP Unnumbered Ethernet 0
Most people reccomend NOT using unnumbered unless you have to for some
reason.
Hope his helps.
Regards,
Jason
ideal.lee <lix...@990.net> wrote in message
news:7o6ug3$gfa$2...@news.cz.js.cn...
> Hello,everyone,I am reading the configuration for cisco router.I was
> puzzled by:
> "
> IP Routing for an Interface
>
> IP unnumbered (serial only)
>
> If you plan to use IP routing on a serial interface, you must
> determine whether it will be unnumbered.
> Default: No.
> Action: If you plan to use IP unnumbered routing on an interface,
> write Yes on the worksheet. If not, write No. Repeat this for each
> installed serial interface you plan to configure.
> "
> Who can explain the meaning of IP unnumbered in detail.
> Thanks a lot.
Paul Woodford
Jason Bomar wrote:
I would not use eth 0 for IP unnumbered but set up a loop back interface with a
reserved IP address. This is in case your Ethernet interface goes down you will
also bring down the Serial connection.
Paul
Dan Swartzendruber
>
>
> The benefit of IP unnumbered is that you don't waste a subnet address block
> for a link that doesn't require addressing. The problem is that you can no
> longer distinguish different interfaces, which is mostly a concern in
> troubleshooting; for instance, if a router has multiple serial interfaces
> that are all "ip unnumbered Ethernet0", you can't tell which one was used
> in a traceroute, and you can't ping specific interface addresses to see
> whether they're up.
Not to mention that if the ethernet is down for some reason, you can't
gain access to the router, even over a serial link that is up...