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New Member

Cat6K MAC Table

In the 6k isn't there only 1 MAC address table? I had someone tell me that there was 1 for each VLAN? Also, how does the 6k handle dup MAC address appearing on different VLANs? My understanding is that when the 6k receives a packet and the MAC table shows the same MAC address in 2 different locations, that the 6K will send the packet to both interfaces, i.e. FastEthernet 8/10, and FastEthernet 8/25 in my case? Is this correct?

13 REPLIES
Bronze

Re: Cat6K MAC Table

Hmmm, I can't tell you if they are physically different tables but entries are identified by vlan. The switch will only forward packets inside the vlan they are received on. The only exception I know of is the multicast vlan and I dont know if that exists in the 6k yet. Mine are on older sw images.

New Member

Re: Cat6K MAC Table

'The switch will only forward packets inside the vlan they are received on'

What about the 6K receiving a packet on the MSFC and needs to forward to a host on the switch? It will do an ARP lookup to resolved the MAC address then a MAC table lookup to find the port it needs to forward on, now it finds the same MAC address on 2 different ports and 2 different VLANS. What does the switch do then? My understanding is it will forward on both ports?

Re: Cat6K MAC Table

The MSFC is a daughter card on the SUP. It doesnt have any physical ports to receive packets. All the packets come in only through the switch ports. The Sup forwards the packets that need to be routed to the MSFC.

Re: Cat6K MAC Table

The MSFC is a daughter card on the SUP. It doesnt have any physical ports to receive packets. All the packets come in only through the switch ports. The Sup forwards the packets that need to be routed to the MSFC.

New Member

Re: Cat6K MAC Table

I understand that. My question was a layer 3 question, not a port question regarding the MSFC. When the MSFC get a packet in from subnet A (doesn't matter what the physical port is), and needs to send it to a host on on subnet B which is one of the switches FE ports. The MSFC/PFC will go through th whole ARP table lookup and then the MAC table lookup and determine which port this host resides on. What happens when this MAC address is on 2 differnt ports?

Bronze

Re: Cat6K MAC Table

I believe the only MAC address entries considered are ones that map to the destination VLAN (which incidentally is the only VLAN that should see the ARP request to begin with). The MAC table may be segmented on a per-VLAN basis, or the switch may just 1) search for all entries matching a given MAC address, and then 2) check the corresponding VLAN for each entry before making a forwarding decision.

So if the same MAC address appears on separate VLANs, I wouldn't expect that to cause a problem -- presumably, the switch sends a given packet to the appropriate host (i.e., not both hosts) depending on what the destination VLAN is. There would be serious security concerns if a switch were to forward such packets to multiple hosts across multiple VLANs, and the same MAC address appearing on multiple ports within a single VLAN is considered an error condition on every Cisco switch that I've worked with.

New Member

Re: Cat6K MAC Table

MAC address table cannot contain entries for different ports with same MAC address within single VLAN. At layer 3 the process ends at ARP lookup (looking in ARP cache or sending ARP request). Forwarding is made at layer 2 and this is a supervisor's job.

Look at the process of MAC address learning closely. The switch puts the MAC addess in it's table when it sees traffic coming from host to any of switch's port. Example:

Ethernet frame received on port 3/12 belonging to vlan 1 of the switch looks like this:

source mac dest. mac data

0011.2233.4455 0054.5432.3210 dddddddddd

and the switch puts entry in the table like this:

mac address port vlan

0011.2233.4455 3/12 1

If the frame with the same src.mac address (0011.2233.4455) comes to a different port (say 3/4) on the same VLAN previous entry is rewriten to a new port like this:

mac address port vlan

0011.2233.4455 3/4 1

This could be indication of layer 2 loop or NIC misconfiguration and is considered an error condition.

Forwarding dicission is made by looking in MAC address table. If the switch finds matching entry forwards the frame to that port only. If there is no matching entry the frame is flooded to all ports within vlan.

Well, maybe I wrote too much but I understand it that way.

New Member

Re: Cat6K MAC Table

I may be stupid, but how do you have the same mac address out 2 different ports, ethernet macs are unique and token ring macs should be unique unless you make a mistake.

New Member

Re: Cat6K MAC Table

By localy administrating the address and yes the adminstator made a mistake

New Member

Re: Cat6K MAC Table

A MAC address may be unique to a *device* - this

is the default for Sun workstations, for instance.

Thus a Sun with 2 Ethernet ports will have the same

MAC address on both ports by default. This is

explicitly allowed by the Ethernet standard (ask

Rich Seifert in comp.dcom.lans.ethernet).

New Member

Re: Cat6K MAC Table

If the same MAC address is seen to be swapping between ports on a rapid basis 1 or both of the ports will be disabled (because the implication is that there is a loop). If the MAC address swaps ports relatively infrequently, then a frame for that 'moving' MAC address will be sent to the port the MAC address was last seen on.

Have a nice day.

New Member

Re: Cat6K MAC Table

i have two 2950-48-EI with giga uplink on 6509 hybrid

with pfc2 MSFC2 . i have 2 host same ip, same mac(LAA), but separate vlan . first vlan hsrp routed with some others, second vlan totally isolated for test of disaster-recovery.I got problem with this and i need to change one mac-address then the two host are ok.

New Member

Re: Cat6K MAC Table

Hi,

Having the same MAC address exist in 2 separate vlans is completely normal. In fact, on a Sup2 system, the same MAC address is used on all MSFC vlan interfaces.

-Robert

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