From: Joseph Brunner (joe@affirmedsystems.com)
Date: Tue Oct 09 2007 - 16:50:11 ART
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 <http://30.30.30.2/>
sw2-vlan50: 30.30.30.1 <http://30.30.30.1/>
sw2-vlan50: 50.50.50.1 <http://50.50.50.1/>
sw3-vlan50: 50.50.50.2 <http://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.
This archive was generated by hypermail 2.1.4 : Fri Nov 16 2007 - 13:11:13 ART