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3550 obtaining default route and dns addresses

Level 1
Level 1

ok, I may be missing something simple, but I have 2 3550's no ip routes, no default gateway set, no dns servers set, yet I can ping outside addresses, traceroute to them and resolve names miraculously, any ideas where these default routes might be coming from


interface Vlan30

ip address


interface Vlan39

ip address


ip classless

ip http server






line con 0

line vty 0 4

password 7 070A20581D0C1C09


line vty 5 15

password 7 02050D480809

no login



ussw01#sho ip route

Default gateway is not set

Host Gateway Last Use Total Uses Interface

ICMP redirect cache is empty


28 Replies 28


I just want to know about the traceroute output to outside addresses you mentioned.


Yes, lets see the PINGs and TRACES. Thanks.

And stop following me, Toshi!


Type escape sequence to abort.

Tracing the route to (

1 0 msec 0 msec 0 msec

2 33 msec 33 msec 17 msec

3 ( 25 msec 26 msec 50 msec

4 ( 17 msec 17 msec 50 mse


5 ( 143 msec 92 msec 51 msec

6 ( 58 msec 42 msec 34 msec


My guess is that this device is broadcasting an ARP request for a default gateway when you run the trace. Since it is in the same vlan as the next hop,, the next hop receives the request and responds, since it has proxy-arp enabled.

This device forwards to the next hop, the next hop routes the packet to its next hop - and so on, and then, on the return trip, the next hop does a L2 forwarding to this device, since they are on the same vlan.

This is my guess...


heres my problem with that, since is on a differnet subnet, he doesn't arp for, he should arp for, but how does he know that is his default gateway, proxy arp is usually when you have varied subnet mask and the host arps and the router forwards the arp request, in this case i don't think an arp request for would be answered by anyone even if it was proxied.

Then you have two choices:

1.) Turn on some debugging and see what the switch is doing when you execute a trace.

2.) Call a priest to perform an Exorcism on your switch because it may be possessed by a router demon. :-)




I'm a bit crazy right now.(grin) Is this lab environment? Would you please do "ping" and "debug ip packet detail"?

I just want to see which source ip address it is using. If it is something like 10.20.30.X. It should not do ARP for I would see "unroutable" in debug.


its not a lab environment, and you are correct it shouldn't arp for, it should arp for as that is the correct default gateway and it is working the problem being is how is it finding out what that default gateway is as it isn't configured, and how is it resolving names in the traceroute as there are no dns servers configured. My problem isnt that something isn't working and should be, it is that it is working and it shouldn't be.


"I'm a bit crazy right now."

Just "now"????

Or always?? :-)

actually i have 2 switches, and .22, correct default gateway is .1 using glbp, and it is sending to .2 and .3. they are both working in this manner, there are no default routes, no ip routes, no dns servers, yet I can ping remotely, reach them from remote subnets and ping by name. Go figure.


Translating ""...domain server ( [OK]

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:


Success rate is 0 percent (0/5)

no where in the config is a dns server configured, so how is it resolving?


I thought that would be an "ip domain lookup " command is on. It's using

However I'm waiting for "ping" and "debug ip packet detail".


cant do that in middle of day, but it should show me sending the packet to the virtual mac of my glbp routers, and if i do a sho ip arp that is what it has for all external addresses

Internet 54 0007.b400.0101 ARPA Vlan30

Internet 229 0007.b400.0101 ARPA Vlan30

Internet 84 0007.b400.0101 ARPA Vlan30

Internet 25 0007.b400.0101 ARPA Vlan30

Internet 8 0007.b400.0101 ARPA Vlan30

Internet 90 0007.b400.0101 ARPA Vlan30

Internet 172 0007.b400.0101 ARPA Vlan30

Internet 156 0007.b400.0101 ARPA Vlan30


Why it sent to one of gblp routers. How can it request that mac-address,0007.b400.0101 if it didn't configure the default-gateway. Properly be

I will open TAC case for Victor.

Victor, Are you there? (grin)


i just opened one.

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