Can CDP route if a switch has no default gateway,

Answered Question
Feb 26th, 2009

I have a Cisco 3750 switch and there is no default gateway set on it.

SW01#show ip route

Default gateway is not set

Host Gateway Last Use Total Uses Interface

ICMP redirect cache is empty

SW01#show cdp ne

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone

Device ID Local Intrfce Holdtme Capability Platform Port ID

CORE01 Gig 1/0/1 144 R S I WS-C3750G Gig 1/0/6

FASW01.local.com

Gig 1/0/3 155 S I WS-C3750- Gig 1/0/3

SW01#

SW01#traceroute 172.24.223.113

Type escape sequence to abort.

Tracing the route to 172.24.223.113

1 172.24.175.18 1007 msec 0 msec 0 msec

2 172.24.175.35 0 msec 0 msec 0 msec

3 192.168.107.117 9 msec 16 msec 17 msec

4 192.168.247.141 17 msec 8 msec 25 msec

5 192.168.247.142 17 msec 17 msec 33 msec

6 172.24.223.113 34 msec 17 msec *

SW01#

How is this possible for me to traceroute from a switch that isn't acting as a layer-3 device?

SW01#show ip int br

Interface IP-Address OK? Method Status Protocol

Vlan1 172.24.175.26 YES NVRAM up up

FastEthernet1/0/1 unassigned YES unset down down

FastEthernet1/0/2 unassigned YES unset down down

FastEthernet1/0/3 unassigned YES unset down down

<snip>

GigabitEthernet1/0/1 unassigned YES unset up up

GigabitEthernet1/0/2 unassigned YES unset down down

GigabitEthernet1/0/3 unassigned YES unset up up

GigabitEthernet1/0/4 unassigned YES unset down down

SW01#

Thanks,

John

I have this problem too.
0 votes
Correct Answer by Edison Ortiz about 7 years 10 months ago

172.24.175.18 has proxy-arp enabled (default setting) and it's handling any request from devices w/o gateway on that segment.

If you want to prove this concept, go to this device and disable proxy-arp then try communicating from the switch to other segments.

HTH,

__

Edison.

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royalblues Thu, 02/26/2009 - 01:09

Do a "sh run | i ip default-gateway" and see whether you have anything setup

Narayan

jholding09 Thu, 02/26/2009 - 01:40

Hello Narayan,

Here is the output:

SW01#sh run | i ip default-gateway

SW01#

I have never in my life seen a device do this without a default gateway.

Thanks,

John

Correct Answer
Edison Ortiz Thu, 02/26/2009 - 06:23

172.24.175.18 has proxy-arp enabled (default setting) and it's handling any request from devices w/o gateway on that segment.

If you want to prove this concept, go to this device and disable proxy-arp then try communicating from the switch to other segments.

HTH,

__

Edison.

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