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New Member

ARP Learning Problem

Hi Friends,

Will appreciate if u can help me out in this.

I have a main switch where have multiple vlans defined in it. I have port 11 under access mode vlan 107 serving downstream switch, port 13 under vlan 103 serving a downstream switch. One of the ports say f0/9 ( access vlan 101) is connected to a downstream switch which is having the actual hosts in vlan 101.

Now on this main switch am learning a mac address of a host in vlan 101 from all the 3 interfaces i.e 0/9,0/11,0/13...correct me if iam wrong but it shud learn only from f0/9 which is under vlan 101 rite?. This suggests its a loop sumwhere dwn in the network where both the vlan domains are getting mised up. My scope of work doesnt allow me to dig in that aprt of network so as of now can any1 suggest a workaround of forceful mac learning thru one interface

Can u please suggest as to whether we can force a mac learning thru sum particular interface...in this f0/9

Thanks in advance

Vicky

6 REPLIES

Re: ARP Learning Problem

Hello Vicky,

just adding my 2 cents.

If you do have a "bridge" problem the best would be to find it and stop this physically.

This is never good, and could lead into many problems, I dont think filtering MAC will be the best option on this.

vlad

Silver

Re: ARP Learning Problem

It is not common practice to connect a switch to an access port. I would use a trunk to comply to the requirement of chaining switches.

If you issue show mac-address-table interface Fa0/9, can you confirm that there are hosts belonging to another VLAN.

Do you have spanning-tree setup properly? Issue show spanning-tree vlan 101 detail.

Can you provide a simple diagram?

--Leon

Re: ARP Learning Problem

Hi Vicky,

Can you please confirm if the downstream switch is same for all the interface which you have mentioned and if yes than the ports in downstream switch are in which vlan?

As you mentioned on main switch port 11 is in vlan 107, port 13 is vlan 103 and they both connectes to downstream switch. What I want to know is that if the downstream switch for both the ports are same and if yes what vlan the downstream ports are configured for which are connected for upstream main switch? If the downstream switch ports conneted to port 11 and port 13 on main switch are in vlan 101 then it is the expected behavior.

Regards,

Ankur

New Member

Re: ARP Learning Problem

Hi Guys,

Thanks for ur suggestions. Problem what i was facing was some hosts in vlan 101 were losing connectivity to the network. This has been resolved after the span sessions revelaed lot of broadcast/arp requests coming from a couple of hosts ...which now have been taken out from the network. Probably all this happened coz of broadcast storm which again points out to some loop down in the network. As i said b4, not everything is in my scope of work :), so have informed the concern to revamp & reconsider the physcial design of their network.

Regarding the forceful mac learning, if i use the below command will it serve my purpose?. Will try it in non-production hours but wanted to check if anyone had used this before.

arp 10.10.10.10 a11d.vdef.be08 arpa int Fast Ethernet 0/9 ( will this static arp entry make the switch to fwd the request out on f0/9 only?)

Have rated all the posts

Please find the info required by all of u:

VLAD: Seems to be a bridge problem, but as i said not much i can do since not all is in my administrative rights.

Leon: I was getting mac addresses of hosts in other vlan on the downstream switch & hence wanted to restrict mac learning on the main switch itself thru only one interface

Ankur: All the main switch ports are connected to 3 different downstream switches & i agree if they were in the same switch with the same vlan memeberships it wud hv been normal behaviour. But this is exactly whts happening sumwher down in the network. Seems these switches are definelty interconnected which is why am learning mac from 3 diff interfaces on the main switch.

Thanks & Regards,

Vicky

Re: ARP Learning Problem

Hello Vicky,

If you are willing to add static ARP entried for each server/host (lots of administrative job), then that's ok.

But, if you have control on the MAC level (if you know the macs from the hosts you need to control) then you could use a layer 2 filter to block specific MACs.

Check the link:

http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12225see/scg/swacl.htm#wp1177176

Vlad

Silver

Re: ARP Learning Problem

As this is a layer 2 switch, the arp entry will not prevent the swtich from forwarding the pakets in or out other ports. Note that the switch will use the arp entry when the switch tries to make a connection to the device. For forwarding of packets, the mac address table is used.

To achieve your objective, try configuring port security. Check the following link

http://www.cisco.com/en/US/products/hw/switches/ps628/products_configuration_guide_chapter09186a00800d6a38.html#xtocid7

However, remember that as mentioned previously, dealing with the physical problem is best.

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