From: Scott Morris (smorris@internetworkexpert.com)
Date: Tue Feb 10 2009 - 17:01:52 ARST
I've never really thought of myself as defeatist before, but ok...
The original question: "*Assume these are dedicated switch ports with only
one device on the other end. *"
was answered simply by pinging the broadcast address and looking at
associated ARP table and CAM table entries.
When it came to having another inteface like the port-channel with LOTS of
devices/IPs/MACs/whatever on the other end, then suddenly things changed. I
shouldn't need to point out that Cisco's not the only networking devices
that exist, right? Even "servers" such as the Dell blade servers, or HP
blade servers have little switches in them. So what's to say that's not the
directly connected device?
The concept isn't defeatist. It's more pragmatic to not make any
assumptions about a design we don't know anything about. I learned long ago
that evaluating others based on the way >I< may do something isn't
necessarily a good idea. :)
We can though, as you point out, make some educated guesses about how things
are done. Still, this begs the question about why you'd want or need to do
that if you weren't able to manage the devices anyway, where a simple phone
call could save lots of time. but that's just my thought...
Ever noticed that we call many things "common sense" but usually we have to
call it that because clearly it isn't? ;)
But yes, we'll see if it works (whatever the end result is supposed to be)!
Scott
-----Original Message-----
From: Jared Scrivener [mailto:jscrivener@ipexpert.com]
Sent: Tuesday, February 10, 2009 1:30 PM
To: smorris@internetworkexpert.com; 'Luan Nguyen'; 'Jersey Guy'
Cc: 'Cisco certification'
Subject: RE: what's on the other side of a GigE port?
Sure there are situations where it wouldn't work. As you explained there are
situations where the topology may be directly obscured from view or a
certain network design might make the process difficult.
However, given the described situation a common sense approach of
elimination down to the only MAC addresses that are issued from Cisco (by
contrast to what appear to be the MAC entries of a VMWare ESX server and the
underlying HP hardware) will probably elicit the desired result. Personally,
my general approach is to try the method, and if it works that's a solution.
If it isn't then an alternative approach (probably one involving phone
calls) could be entered into. However choosing not to try in a given
situation due to the chance that in a given topology the solution may not be
easy to achieve seems a bit defeatist, especially given that the effort
involved can be reduced to about 3-5 minutes of work.
Anyway, we can now all sit here with bated breath to see if it works. :)
Cheers,
Jared Scrivener CCIE3 #16983 (R&S, Security, SP), CISSP
Technical Instructor - IPexpert, Inc.
Telephone: +1.810.326.1444
Fax: +1.810.454.0130
Mailto: jscrivener@ipexpert.com
-----Original Message-----
From: Scott Morris [mailto:smorris@internetworkexpert.com]
Sent: Tuesday, 10 February 2009 1:17 PM
To: jscrivener@ipexpert.com; 'Luan Nguyen'; 'Jersey Guy'
Cc: 'Cisco certification'
Subject: RE: what's on the other side of a GigE port?
So let's assume that your edgerouter is connected to some SP's edge. IF
they are using something other than a /30 or /31, would it not be a
reasonable assumption that other devices you'd see on that edge would be
other edge routers? And along those lines of reasonable assumption, I'd
guess if we were picking out Cisco as an example there'd likely be more than
one of them!
Slice it and dice it any way you want to, but unless you know something
about the topology, or assume we will be lucky to find only one Cisco device
(which begs the question of "what the hell am I connecting to anyway?") then
getting a MAC and knowing the OUI code don't really help you narrow things
down. Too many what-ifs.
Likewise, you could bounce the port and see what MAC you actually learn
first. But even then can we 100% say that's the directly connected one?
Scott
-----Original Message-----
From: Jared Scrivener [mailto:jscrivener@ipexpert.com]
Sent: Tuesday, February 10, 2009 1:06 PM
To: 'Scott Morris'; 'Luan Nguyen'; 'Jersey Guy'
Cc: 'Cisco certification'
Subject: RE: what's on the other side of a GigE port?
Sure you'd see multiple ones, Scott. Fortunately every vendor has on OUI
code so unless there are a bunch of Cisco devices (which isn't too likely)
its generally pretty easy to filter down to the Cisco ones.
Cheers,
Jared Scrivener CCIE3 #16983 (R&S, Security, SP), CISSP
Technical Instructor - IPexpert, Inc.
Telephone: +1.810.326.1444
Fax: +1.810.454.0130
Mailto: jscrivener@ipexpert.com
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Scott Morris
Sent: Tuesday, 10 February 2009 12:50 PM
To: 'Luan Nguyen'; 'Scott Morris'; 'Jersey Guy'
Cc: 'Cisco certification'
Subject: RE: what's on the other side of a GigE port?
heheheeh.... If you can get someone to tell you what port number you are
plugged into on the other side, why can't they tell you the IP? :)
And MAC addresses are nice, but you'd see multiple MACs in a VLAN, wouldn't
you?!?
Scott
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Luan
Nguyen
Sent: Tuesday, February 10, 2009 11:38 AM
To: 'Scott Morris'; 'Jersey Guy'
Cc: 'Cisco certification'
Subject: RE: what's on the other side of a GigE port?
If you know what port that device connects to, do a show mac-address-table |
inc xxx to find out the mac address connect to that port, then do a show ip
arp *mac-address* that you found on the previous command to find out the IP
address.
Regards,
Luan Nguyen
Chesapeake NetCraftsmen, LLC.
[W] http://www.netcraftsmen.net
[M] luan@netcraftsmen.net
[Blog] http://cnc-networksecurity.blogspot.com/
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Scott Morris
Sent: Tuesday, February 10, 2009 11:25 AM
To: 'Jersey Guy'
Cc: 'Cisco certification'
Subject: RE: what's on the other side of a GigE port?
That's the point. You won't. As far as you can tell they are all equally
reachable.
That's part of why CDP is quite helpful.
What difference does it make? If that device isn't under your control
anyway?
Scott
From: Jersey Guy [mailto:guy.jersey@gmail.com]
Sent: Tuesday, February 10, 2009 10:55 AM
To: smorris@internetworkexpert.com
Cc: Cisco certification
Subject: Re: what's on the other side of a GigE port?
Scott,
It *is* an L3 device but how do you know *which* one it is? The LAN address
is a /23. When I do a broadcast ping, I get a 100 responses. Don't know what
response is coming from what port.
I want to see the IP address that's configured on the device at the other
end of po20.
A56M00S14#show run int po20
!
interface Port-channel20
switchport
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 326
switchport mode trunk
logging event trunk-status
load-interval 30
end
A56M00S14#show vlan brief
VLAN Name Status Ports
---- -------------------------------- ---------
-------------------------------
326 Access_VLAN326 active Gi1/1, Gi1/2, Gi1/3, Gi1/4
Gi1/5, Gi1/6, Gi1/7, Gi1/8
Gi1/9, Gi1/10, Gi1/11,
Gi1/12
Gi1/13, Gi1/14, Gi1/15,
Gi1/16
Gi1/17, Gi1/18, Gi1/19,
Gi1/20
Gi1/21, Gi1/22
thanks, GJ
On Tue, Feb 10, 2009 at 10:40 AM, Scott Morris
<smorris@internetworkexpert.com> wrote:
If an L2 device, you don't. If an L3 device, ping.
Otherwise, my vote is Gremlins.
Scott
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Jersey Guy
Sent: Tuesday, February 10, 2009 9:46 AM
To: Cisco certification
Subject: what's on the other side of a GigE port?
Ya I know, this is Networking-101, shame on me....but I can use some help
here....
Assuming there's no CDP running on the box, how do I find out the IP address
of the devices that are connected to GigE ports on my WS-C4948?
*GigabitEthernet1/23 unassigned YES unset up
up
GigabitEthernet1/24 unassigned YES unset up
up
*Assume these are dedicated switch ports with only one device on the other
end. *
*thanks, JG*
*
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