No default-gateway but I can still ping outside of subnet?!?

Answered Question
Nov 28th, 2007
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I have two 3750 switches in a stack that does not have a default-gateway statement yet when I telnet to that stack, I can ping a device that is on a different subnet. The switches are L2 only and are uplinked to two distribution layer 6509 routers that are running GLBP between them.


Isn't the default-gateway command necessary for communications to subnets other than what the stack has locally? I've only had one cup of coffee this morning and maybe that's where the problem is.


CSB-SW73-15-216#sh run int vlan 73

Building configuration...


Current configuration : 65 bytes

!

interface Vlan73

ip address 168.77.73.15 255.255.255.128

end


CSB-SW73-15-216#sh ip route

Default gateway is not set


Host Gateway Last Use Total Uses Interface

ICMP redirect cache is empty


CSB-SW73-15-216#ping 168.77.44.25


Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 168.77.44.25, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/4/9 ms

CSB-SW73-15-216#

Correct Answer by Edison Ortiz about 9 years 6 months ago

I'm sure the 6509 has proxy-arp enabled hence forwarding those packets in behalf of those switches.


Disable proxy-arp and see the differences.

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Correct Answer
Edison Ortiz Wed, 11/28/2007 - 08:45
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I'm sure the 6509 has proxy-arp enabled hence forwarding those packets in behalf of those switches.


Disable proxy-arp and see the differences.

Richard Burts Wed, 11/28/2007 - 09:36
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Jim


I believe that it is true that for RELIABLE access to outside subnets there should be a default gateway. As you have discovered switches such as the 3750 will ARP for remote destinations if they do not have a default gateway. And as Edison has pointed out for the connection to work the next hop router must have proxy arp enabled. And if proxy arp is disabled then you will not have remote connectivity without a default gateway configured.


HTH


Rick

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