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Beginner

ARP resolution with port channels

Hello folks-

I'm looking to configure port channels between leaf and spine in a L3 underlay. In general, how is ARP resolved over a port channel? That is, for a packet routed out on a member link, what will be the destination MAC? Is the ARP resolved per member interface of the port channel?

Thanks for your time!

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Hall of Fame Cisco Employee

Re: ARP resolution with port channels

Hi Satish,

I am assuming that we are still talking about Layer3 routed port-channels where you have the no switchport and an IP address configured right on the Port-channel interface. Is this correct?

If so then the answer to your question is: When a Port-channel interface is created, it is assigned its own MAC address. It can be a unique MAC address from the set of MAC addresses allocated for the whole switch, or it can be a MAC address that is shared with some of the existing physical interfaces - that does not really matter. You can see the MAC address when you pull out the show interface port-channel ... command. What matters is that the Port-channel as a whole has one MAC address on its own, independently of the MAC addresses of the physical members, and the DMAC of the frame would be the address of the Port-channel. It would not matter what physical member port receives this frame because under a Port-channel, all physical members are required to listen to the MAC address owned by the Port-channel interface itself.

Please feel welcome to ask further!

Best regards,
Peter

5 REPLIES 5
Hall of Fame Cisco Employee

Re: ARP resolution with port channels

Hello Satish,

It is best to think of port-channels simply as ports like any other. To most of the higher protocols - ARP, IP, IPv6, even STP, a port-channel is a single port, no matter how many physical members it has.

Regarding ARP: We need to clarify - ARP is mapping IPs to MACs. ARP is not concerned with particular interfaces, and is not mapping IPs or MACs to interfaces. However, on NX-OS, if you execute the show ip arp detail command, you will see the routed interface where the IP/MAC binding was learned, and possibly a switched interface toward it if such interface exists. On routed port-channels, the show ip arp detail will show you the name of the port-channel interface, never the physical members. The particular member link for the IP traffic routed out that port will be determined on a per-packet basis, based on source and destination IP addresses of the packets and the particular load balancing method that is currently active.

Please feel welcome to ask further!

Best regards,
Peter

Beginner

Re: ARP resolution with port channels

Hi Peter-

thanks for getting back. Let me give an example. 

Lets assume we have a port channel (10) with member interfaces eth1, eth2 and eth3. Eth1 is connected to eth1 on the peer switch, eth2 to eth2 and eth3 to eth3. 

Now, we have a route over this port channel and based on the hash, eth2 is picked. What will be the DMAC on this packet going out? Will it be the dmac of the peer eth2 interface?

thanks. 

Hall of Fame Cisco Employee

Re: ARP resolution with port channels

Hi Satish,

I am assuming that we are still talking about Layer3 routed port-channels where you have the no switchport and an IP address configured right on the Port-channel interface. Is this correct?

If so then the answer to your question is: When a Port-channel interface is created, it is assigned its own MAC address. It can be a unique MAC address from the set of MAC addresses allocated for the whole switch, or it can be a MAC address that is shared with some of the existing physical interfaces - that does not really matter. You can see the MAC address when you pull out the show interface port-channel ... command. What matters is that the Port-channel as a whole has one MAC address on its own, independently of the MAC addresses of the physical members, and the DMAC of the frame would be the address of the Port-channel. It would not matter what physical member port receives this frame because under a Port-channel, all physical members are required to listen to the MAC address owned by the Port-channel interface itself.

Please feel welcome to ask further!

Best regards,
Peter

Beginner

Re: ARP resolution with port channels

Thanks Peter. That was exactly what I was looking for.
Highlighted
Beginner

Re: ARP resolution with port channels

Peter, Great explanation as always. 

 

Hope you are doing good. Its been a long time since we spoke.

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