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Janis Veinbergs
Asked: 2011-01-20 03:24:33 +0800 CST

No answer on arp requests?

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I have a problem where my Layer 3 Dell PowerConnect 6248 Switch (main switch) will sometimes (that means sometimes it works, sometimes it won't) not get an arp response from a device behind Dell PowerConnect 2824 (edge switch).

The situation

- - - - -       ===============          ===============       - - - - - 
| client | ---- | Main Switch | -------- | Edge switch | ----- | Device |
- - - - -       ===============          ===============       - - - - -
  1. If the Client is on same vlan as Device, there is no problem and traffic always flows.
  2. If the Client is on different vlan, then only sometimes traffic will flow. This switch routes between vlans (inter-vlan routing).

  3. Main switch broadcasts (many) arp packets, but seems like no response?!

    990 719.323028  Dell_xx:xx:xx   Broadcast   ARP Who has 10.11.0.43?  Tell 10.11.0.1

    because when looking at main switch's arp table, it doesn't know coressponding mac address:

    #show arp
IP Address        MAC Address      Interface        Type        Age
---------------  -----------------  --------------  --------  -----------
10.11.0.43       0000.0000.0000     vlan 11         Dynamic    0h  0m  2s

  4. Main switch knows to which port traffic has to be sent to reach Device. There is an entry in address table that shows on which port the device is, the only problem is the switch doesn't know that particular IP corresponds to this MAC address :(

    Address table:

    VLAN ID     MAC Address           Interface  
VLAN 11     00:1c:c0:ef:e0:03       2/g5

  5. Edge switch is L2 switch and it appears that address tables are ok for this switch also.

  6. Some additional info about main switch:

    #show arp brief

Age Time (seconds)............................. 1200
Response Time (seconds)........................ 1
Retries........................................ 4
Cache Size..................................... 1024
Dynamic Renew Mode ............................ Disable
Total Entry Count Current / Peak .............. 68 / 99
Static Entry Count Configured / Active / Max .. 6 / 6 / 64

Where is the problem?

Sometimes the traffic will just start flowing. Does the problem lie within the Main Switch or Edge Switch? Is it device configuration maybe? Maybe something with network cables? Main switch and edge switch is connected with fibre optics if it helps. Where should i look into?

Workaround

Creating a static arp entry solves the problem... currently I need only 5 arp entries for my devices, but this is just a workaround not a solution.

routing networking switch arp

3 Answers

  1. You should check the expected source of the ARPs -- the device itself.

    Is the device configured with the IP address you listed? '10.11.0.43/16'? (is that subnet mask correct?)

    Is the device receiving the ARP requests from the router? (tcpdump)

    You mention changing VLANs. Are you changing them for the device or the client?

    What do your routing tables look like? Is the main switch the default route for both the 'client' and the 'device'?

    My initial guess is that your subnet masking on the device is /8 (255.0.0.0) when you need to set it to /16 (255.255.0.0) Either that or a misconfiguration of the IP might keep the device from responding to ARP requests.

    Best of luck.

    • 2
  2. Best Answer

    There is a known issue on the Dell firmware for the 6248. We experienced a similar issue that ARP entries were not being populated in the L3 ARP table. Sometimes they would work and other times they wouldn't.

    If we did static ARP entries (MAC/IP) traffic would flow just fine.

    Version 3.2.0.7 is what Dell indicated was the 'stable' version to back rev to. We are currently running 3.2.1.3 which has the symptoms mentioned above.

    Hope that helps someone who may come across this!

    • 2

  3. We had the same exact issue with a Cisco switch. It turns out that it was the Mellanox card on the EXI Box. When we replaced it with an Intel, ARP worked as it should.

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