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New Member

no ARP entries for VLAN X

I have a question about ARP. I have a number of VLANs configured on a 6500 switch most of the vlans routing interfaces are also on the 6500. One of the VLANS use a static route pointing to a remote router for the routing interface. My question is: APP works fine for all vlans that are reouted localy by the 6500, but there are no arp entries for VLAN X that is routed remotely. I thought ARP was a L2 not L3. If someone could clear this up for me it would be great. Thanks

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Accepted Solutions

Re: no ARP entries for VLAN X

If you have a route to another router on the same VLAN, then the 6500 will ignore any incoming ARP requests for IP addresses on the VLAN except its own address.

I presume that the hosts on the VLAN have been configured with the other router as default gateway. In that case, the traffic from that VLAN would never go near the 6500.

However, if a host did send a packet to the 6500 destined for an address that is off the VLAN, then the 6500 would forward it in the normal way. It would then depend whether you have ICMP re-directs enabled on that VLAN interface. If you do not, then the 6500 would have no reason to put the host in its ARP cache. But if you have ICMP re-directs enabled, then the 6500 would have to ARP to find the MAC address of the host in order to send its ICMP re-direct.

In fact, the 6500 will only make an ARP table entry if it has a packet to send to the host, either because it has to forward a packet that came from outside VLAN, or because it needs to send an ICMP re-direct to the host to tell it to use the other router.

Does that make sense?

Kevin Dorrell

Luxembourg

6 REPLIES

Re: no ARP entries for VLAN X

If you have a route to another router on the same VLAN, then the 6500 will ignore any incoming ARP requests for IP addresses on the VLAN except its own address.

I presume that the hosts on the VLAN have been configured with the other router as default gateway. In that case, the traffic from that VLAN would never go near the 6500.

However, if a host did send a packet to the 6500 destined for an address that is off the VLAN, then the 6500 would forward it in the normal way. It would then depend whether you have ICMP re-directs enabled on that VLAN interface. If you do not, then the 6500 would have no reason to put the host in its ARP cache. But if you have ICMP re-directs enabled, then the 6500 would have to ARP to find the MAC address of the host in order to send its ICMP re-direct.

In fact, the 6500 will only make an ARP table entry if it has a packet to send to the host, either because it has to forward a packet that came from outside VLAN, or because it needs to send an ICMP re-direct to the host to tell it to use the other router.

Does that make sense?

Kevin Dorrell

Luxembourg

New Member

Re: no ARP entries for VLAN X

Hi Kevin, thanks very much for your responce,

All my servers are on the 6500 in VLAN X. Should the 6500 not hold ARP entries for the machines connected locally even if they are pointing to the other router as the DFGW.

Im still confused

Re: no ARP entries for VLAN X

Not necessarily. The 6500 will only create an ARP entry for a host if it has a packet to send to that host. If it doesn't get any packets from the outside to be routed to the host, then it does not need to find out the MAC address. And if all the hosts are using the other router as DFGW, then the 6500 does not even need to issue any re-directs. So it just ignores the hosts. It will not create ARP entries just by observing the exchange between the host and the other router.

Kevin Dorrell

Luxembourg

New Member

Re: no ARP entries for VLAN X

Ok I think I am getting it now, what about if the servers talk to each other? What would I have to do to get the 6500 to record the MAC?

Thanks again

Re: no ARP entries for VLAN X

The 6500 will not record the MAC in the (layer-3) ARP table unless it is talking directly to the host. However, you will find the host MACs in the layer-2 forwarding table. show mac-address-table

Kevin Dorrell

Luxembourg

New Member

Re: no ARP entries for VLAN X

Thanks very much Kevin you have been a great help

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