topic Re: Determine the IP address of a connected switch in General Topics

Determine the IP address of a connected switch

Niall_Lynch — Mon, 24 Jan 2022 18:41:33 GMT

Hello,

I am hoping someone may be able to help me out here. Thanks in advance.

I have a cluster of 6200 devices. I have a netgear switch which is connected to one of the interfaces but I do not know the IP address of the switch. Is it possible to determine the IP Address of the switch if I have the MAC address of the switch please? I have performed an arp -a but I could not see the MAC address of the device.

Many thanks.

Re: Determine the IP address of a connected switch

_Val_ — Tue, 25 Jan 2022 07:51:17 GMT

Are you sure that Switch even have an IP? It might be that it does not. Why not to look on the switch itself?

Re: Determine the IP address of a connected switch

Guy_Grundman — Tue, 25 Jan 2022 08:21:39 GMT

if you are referring to the switch management IP, running lldpneighbors in expert mode might help.

Re: Determine the IP address of a connected switch

Niall_Lynch — Tue, 25 Jan 2022 09:22:07 GMT

Thanks Val, I tried the default IP address of the switch with no luck.

Re: Determine the IP address of a connected switch

Niall_Lynch — Tue, 25 Jan 2022 09:26:49 GMT

Thanks Guy, I tried lldpneighbors but it did not work. I got lldpneighbors couldn't connect to the OpenLDP transport socket. Is lldpd running?

My cluster is running on R80.40

Re: Determine the IP address of a connected switch

_Val_ — Tue, 25 Jan 2022 09:55:04 GMT

Connect console, look locally on the switch. It may not have ANY IP address to begin with.

Re: Determine the IP address of a connected switch

Guy_Grundman — Tue, 25 Jan 2022 12:06:11 GMT

Unfortunately LLDP is support from R81.

Re: Determine the IP address of a connected switch

Chris_Atkinson — Tue, 25 Jan 2022 13:17:27 GMT

Do you see different results if you ping the broadcast address of the subnet first?

Re: Determine the IP address of a connected switch

Ruan_Kotze — Tue, 25 Jan 2022 13:18:01 GMT

This is not something I tried myself, but what about using tcpdump to see if you can capture lldp packets on the wire?

This certainly looks promising - https://medium.com/@getGroovy/lldp-packet-sniffing-38f999e733fa

Re: Determine the IP address of a connected switch

Timothy_Hall — Tue, 25 Jan 2022 14:17:21 GMT

There will be two ways to determine the management IP address of the switch: active and passive. Passive depends on the switch emanating some identifying information such as CDP or LLDP. My Max Power 2020 book covered how to capture CDP and/or LLDP traffic if it is there; the techniques are listed below. The switch should always emanate BPDUs for Spanning Tree purposes and you will be able to see these frames in a capture, but they do not contain Layer 3 information such as an IP address to my knowledge. Ensure that any capture you run is allowed to execute for at least 60 seconds.

As far as active techniques, as Chris said you can try pinging the network broadcast address like this: ping -b 192.168.1.255; then check your ARP cache with arp -an to see if the switch IP has showed up. You can also try ping -b -I (interfacename) 255.255.255.255. But some devices won't answer a broadcast ping for whatever reason, so you could run something like this then check the ARP cache afterward:

#!/bin/sh

CNTR=1

while [ $CNTR -lt 254 ]
do
ping 192.168.1.$CNTR -c 1 -t 1 -n
CNTR=$(( $CNTR + 1 ))
done