1. Home
  2. Cisco Systems
  3. One way arp?

One way arp?

I've a network that involves multiple VLANs, so there is a lot of VLAN trunking going on. Up until now, this hasn't been a problem.

But I'm now in a situation where two sets of systems are having a very asymmetric behavior. One set cannot get the MAC addresses for IPs in the other set. The other set can get the MAC addresses for IPs in the first set.

If a machine in the other set proactively connects to a machine in the first set, then that machine now has the MAC address and can connect back to the machine in the other set.

So ARP requests appear to work only in one direction.

These machines are connected by a chain of switches speaking over 802.1q trunk ports. What could I possibly have done to cause this behavior? I wouldn't even know how to recreate this if I wanted to!

Thanks...

Andrew

Reply to
Andrew Gideon
Loading thread data ...

Do you have a subnet mask issue? If the first set only had say a /25 as their mask, then they would arp for their gateway for say a .130 address. If however they were arped FOR, they should respond with their mac. I presume that all of these machines are in the same vlan/ subnet? Are the 'groups' all on two different or specific switches, or its a logical group off of multiple devices w/ no rhyme or reason?

Reply to
Trendkill

No.

There is actually a chain of switches. The first switch into which some were connected. The second switch which sits between the first and third. And the third switch, into which some others were connected.

The problem was always between systems connected between switches one and three.

However, I found that someone had blocked ethernet broadcast packets on a piece of equipment, typically invisible to me, between switches one and two. Broadcasts one way were permitted; the other way were blocked.

I had them remove the block, and all is working perfectly.

Thanks...

Andrew

Reply to
Andrew Gideon

Cabling-Design.com Forums website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.