From: John Moor (johmoor@gmail.com)
Date: Tue Oct 09 2007 - 17:36:23 ART
So you accept this without understanding why this happenes? Could anybody
just in 2 sentences give me an idea how sw3 behaves when it tries to send a
ping to sw1?
Like: sw1 have to send a packet, it know the destination ip, doesn't know
the mac address, doesn't know the default gateway. It sends the arp request
and by proxy arp reply makes address resolution. But it doesn't have still
default gateway configured. Why does it know where to forward packet??
Something like this. How the box behave to forward a packet?
Thanks.
On 10/9/07, Joseph Brunner <joe@affirmedsystems.com> wrote:
>
> I had this happen for years in a real network it was one of those
> "mysteries" ccna's just accept I never wondered why with
>
> "no default gateway configured" my 3550's grabbed ntp from
> tick.usno.navy.mil in never figured out my msfc was proxy-arp'ing
>
>
>
> "probably yes it is enabled by default."
>
>
>
> Probably? Probably gets very expensive $1,400 a shot
>
>
>
> Default behavior(3560)
>
>
>
> Vlan11 is up, line protocol is up
>
> Internet address is 1.1.11.11/24
>
> Broadcast address is 255.255.255.255
>
> Address determined by non-volatile memory
>
> MTU is 1500 bytes
>
> Helper address is not set
>
> Directed broadcast forwarding is disabled
>
> Multicast reserved groups joined: 224.0.0.9
>
> Outgoing access list is not set
>
> Inbound access list is not set
>
> Proxy ARP is enabled <-------------------------- MIRA ESTO!
>
> Local Proxy ARP is disabled
>
> Security level is default
>
> Split horizon is enabled
>
> ICMP redirects are always sent
>
> ICMP unreachables are never sent
>
> ICMP mask replies are never sent
>
> IP fast switching is enabled
>
> IP CEF switching is enabled
>
> IP CEF switching turbo vector
>
> IP Null turbo vector
>
> IP multicast fast switching is enabled
>
> IP multicast distributed fast switching is disabled
>
> IP route-cache flags are Fast, CEF
>
> Router Discovery is disabled
>
> IP output packet accounting is disabled
>
> IP access violation accounting is disabled
>
> TCP/IP header compression is disabled
>
> RTP/IP header compression is disabled
>
> Probe proxy name replies are disabled
>
> Policy routing is disabled
>
> Network address translation is disabled
>
> BGP Policy Mapping is disabled
>
> WCCP Redirect outbound is disabled
>
> WCCP Redirect inbound is disabled
>
> WCCP Redirect exclude is disabled
>
>
>
>
>
>
> ------------------------------
>
> *From:* John Moor [mailto:johmoor@gmail.com]
> *Sent:* Tuesday, October 09, 2007 3:39 PM
> *To:* Joseph Brunner
> *Cc:* Cisco certification
> *Subject:* Re: interesting behaviour of the switches
>
>
>
> to guys: No everything is configured just as simple as I described. No
> errors in the configuration. IP proxy arp is not explicitly configured but
> probably yes it is enabled by default.
> Let's look again into the topology
>
>
>
> sw1-vlan30-------vlan30-sw2-vlan50------------------vlan50--sw3
>
>
>
> Between sw1,sw2 and sw3 is just a simple dot1q trunk.
>
> Configuration:
>
> sw1-vlan30: 30.30.30.2
>
> sw2-vlan50: 30.30.30.1
>
> sw2-vlan50: 50.50.50.1
>
> sw3-vlan50: 50.50.50.2
>
> On all trunks configuration is really very simple:
>
> switchport
>
> switchport trunk enc dot1q
>
> sw mode trunk
>
>
>
> on vlans like:
>
> vlan 30
>
> ip address 30.30.30.2
>
> switch 2.
>
> vlan 30
>
> ip address 30.30.30.1
>
> vlan 50
>
> ip address 50.50.50.1
>
> ...
>
>
>
> sw1 vlan 30 int can bing vlan 3 vlan 50 int withou any default gateway or
> routing configured. Proxy arp.. yes maybe.
>
> But question is still the same:
>
> sw3 can receive proxy arp reply from sw2 but how can it forward packet
> without having any routing entry in its table? The PC can actually send a
> packet out it's interface without default gateway configured if it receives
> proxy arp reply. Maybe it is the same with the switch?
>
>
>
> Any thoughts?
>
>
>
> P.S http://www.tildefrugal.net/tech/arp.php look here what PC do if it
> don't have default-gateway configured...
>
>
>
> On 10/8/07, *Joseph Brunner* <joe@affirmedsystems.com> wrote:
>
> Proxy arp is known for this "interesting behavior"
>
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> John
> Moor
> Sent: Monday, October 08, 2007 12:57 PM
> To: Cisco certification
> Subject: interesting behaviour of the switches
>
> Hello, guys!
> I have mentioned interesting behaviour. Could you pleas confirm this or
> tell
> that this is incorrect?
> sw1-vlan30-------vlan30-sw2-vlan50------------------vlan50--sw3
>
> Between sw1,sw2 and sw3 is just a simple dot1q trunk.
> Configuration:
> sw1-vlan30: 30.30.30.2
> sw2-vlan50: 30.30.30.1
> sw2-vlan50: 50.50.50.1
> sw3-vlan50: 50.50.50.2
>
> I can ping from 50.50.50.2 to 30.30.30.2 even without configuring
> default-gateway on sw 3. I assume that it happenes because when sw3
> doesn't
> know the route it just broadcasts the ip packet and the same does sw1 in
> opposite to the router.
>
> Is it correct? Or there is some other explanation of this. Thank you.
>
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