From: Carlos G Mendioroz (tron@huapi.ba.ar)
Date: Mon Jul 07 2008 - 07:16:04 ART
The BUG seems to be in the design.
That the router lets you put some configuration that in this case
leads to a non operative implementation is a consequence of flexibility.
Having the same IP in more than one interface is useful on some
situations, some have been mentioned already (bundling), there are
others (e.g. unidirectional links).
So answering your question (two different semantics):
Q1: Why does it let you put the same address ?
A1: Beacuse sometimes it's needed
Q2: Why not on ethernets ?
A2: Because someone at cisco does not think A1 holds here...
-Carlos
Fahad Khan @ 07/07/2008 05:25 -0300 dixit:
> here is the scenario
> R1(s1/0 - 10.0.0.1/8) ---------------(s1/0-10.0.0.2/8) R2
> R1(s1/1 - 10.0.0.1/8) ---------------(s1/0-10.0.0.3/8) R3
>
> R1# sh ip route
>
> C 10.0.0.0/8 is directly connected, Serial1/0
> is directly connected, Serial1/1
>
>
> -when R2 pings to R1(10.0.0.1), its fine, but
>
> -when R3 pings to R1(10.0.0.1), it wont get reply since the "debug ip
> packet" on R1 shows that R1 goes to reply R3 from its serial1/0 interface
> where R2 is connected, "debug ip packet" on R2 shows that R2 is getting
> reply .
>
>
> is this a bug? now router is confused to send traffic....
>
> Dear Roman,
>
> Kindly tell me the logic in case of Frame-relay subinterfaces..?
>
>
> On Sun, Jul 6, 2008 at 5:40 PM, Jason Madsen <madsen.jason@gmail.com> wrote:
>
>> BVIs seem to work just fine too.
>>
>> Jason
>>
>>
>> On Sun, Jul 6, 2008 at 2:00 PM, Edison Ortiz <edisonmortiz@gmail.com>
>> wrote:
>>
>>> Additionally, 'ip unnumbered'?
>>>
>>>
>>>
>>> Let's consider assigning IP addresses to the interfaces of a router using
>>> a
>>> class B network that has been subnetted using eight bits of subnetting.
>>> Every interface requires a unique subnet. Although each point-to-point
>>> serial connection has only two end points to address, if we assign an
>>> entire
>>> subnet to each serial interface, we use 254 available addresses for each
>>> interface where only two addresses are needed. If we use IP unnumbered on
>>> each serial interface, we save address space; the address of a LAN
>>> interface
>>> is "borrowed" and used as the source address for routing updates and
>>> packets
>>> sourced from the serial interface. In this way, address space is
>>> conserved.
>>> IP unnumbered only makes sense for point-to-point links.
>>>
>>> A router receiving a routing update installs the source address of the
>>> update as the next hop in its routing table. Normally, the next hop is a
>>> directly-connected network node. This is no longer the case if we use IP
>>> unnumbered because each serial interface "borrows" their IP address from a
>>> different LAN interface, each in a different subnet and possibly in a
>>> different major network. When IP unnumbered is configured, routes learned
>>> through the IP unnumbered interface have the interface as the next hop
>>> instead of the source address of the routing update. Thus we avoid an
>>> invalid next hop address problem due to the source of the routing update
>>> coming from a next hop that is not directly connected.
>>>
>>>
>>> http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080
>>> 094e8d.shtml#ip_ip_un<http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080094e8d.shtml#ip_ip_un>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> Edison Ortiz
>>>
>>> Routing and Switching, CCIE # 17943
>>>
>>>
>>>
>>> -----Original Message-----
>>> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
>>> Fahad Khan
>>> Sent: Sunday, July 06, 2008 3:45 PM
>>> To: Sadiq Yakasai
>>> Cc: Joseph Saad; Cisco certification
>>> Subject: Re: IP address on serial interfaces of a router
>>>
>>>
>>>
>>> What is the logic behined that I can assign 10.0.0.1/24 (exactly the same
>>> ip
>>>
>>> address) on two serial interfaces of a single router???
>>>
>>>
>>>
>>>
>>>
>>> R1#sh run
>>>
>>>
>>>
>>> ---output omitted---
>>>
>>>
>>>
>>> interface Serial1/0
>>>
>>> ip address 10.0.0.1 255.0.0.0
>>>
>>> serial restart-delay 0
>>>
>>> !
>>>
>>> interface Serial1/1
>>>
>>> ip address 10.0.0.1 255.0.0.0
>>>
>>> serial restart-delay 0
>>>
>>>
>>>
>>>
>>>
>>> Wont the router be confused when it will forward traffic for
>>>
>>> 10.0.0.0/24network??
>>>
>>>
>>>
>>> Thanks and regards,
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> On 7/6/08, Sadiq Yakasai <sadiqtanko@gmail.com> wrote:
>>>
>>>> Huh????
>>>> And how is this exactly related to ethernet again? :-)
>>>> We digress again!
>>>> _______________________________________________________________________
>>>> Subscription information may be found at:
>>>> http://www.groupstudy.com/list/CCIELab.html
>>>
>>>
>>>
>>>
>>> --
>>>
>>> Fahad Khan
>>>
>>>
>>>
>>>
>>>
>>> _______________________________________________________________________
>>>
>>> Subscription information may be found at:
>>>
>>> http://www.groupstudy.com/list/CCIELab.html
>>>
>>>
>>> _______________________________________________________________________
>>> Subscription information may be found at:
>>> http://www.groupstudy.com/list/CCIELab.html
>>>
>>>
>>>
>>>
>>>
>
>
-- Carlos G Mendioroz <tron@huapi.ba.ar> LW7 EQI Argentina
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