Users Guide

If the ICL link is down when a VLT node receives an ARP request for the IP address of the VLT peer, owing to
LAG-level hashing algorithm in the top-of-rack (ToR) switch, the incorrect VLT node responds to the ARP
request with the peer MAC address. Proxy ARP is not performed when the ICL link is up and the ARP request
the wrong VLT peer. In this case, ARP requests are tunneled to the VLT peer.
Proxy ARP supported on both VLT interfaces and non-VLT interfaces. Proxy ARP is supported on symmetric
VLANs only. Proxy ARP is enabled by default. To support proxy ARP, the routing table must be symmetrically
configured. For example, consider a sample topology in which you configure VLAN 100 on two VLT nodes,
node 1 and node 2. You did not configure the ICL link between the two VLT nodes. Assume that the VLAN
100 IP address in node 1 is 10.1.1.1/24 and VLAN 100 IP address in node 2 is 20.1.1.2/24. In this case, if the ARP
request for 20.1.1.1 reaches node 1, node 1 does not perform the ARP request for 20.1.1.2. Proxy ARP is
supported only for the IP address that belongs to the received interface IP network. Proxy ARP is not
supported if the ARP-requested IP address is different from the received interface IP subnet. For example, if
you configure VLAN 100 and 200 on the VLT peers, and if you configured the VLAN 100 IP address as
10.1.1.0/24 and you configured the VLAN 200 IP address as 20.1.1.0/24, the proxy ARP is not performed if the
VLT node receives an ARP request for 20.1.1.0/24 on VLAN 100.
Working of Proxy ARP for VLT Peer Nodes
Proxy ARP is enabled only when you enable peer routing on both the VLT peers. If you disable peer routing on
one of the VLT peers, proxy ARP is not performed when the ICL link goes down. Proxy ARP is performed only
when the VLT peer's MAC address is installed in the database. Proxy ARP is stopped when the VLT peer's MAC
address is removed from the ARP database because of peer routing timer expiry. The source hardware
address in the ARP response contains the VLT peer MAC address. Proxy ARP is supported for both unicast and
broadcast ARP requests. Control packets, other than ARP requests destined for the VLT peers that reach the
undesired and incorrect VLT node, are dropped if the ICL link is down. Further processing is not done on
these control packets. The VLT node does not perform any action if it receives gratuitous ARP requests for the
VLT peer IP address. Proxy ARP is also supported on secondary VLANs. When the ICL link or peer is down, and
the ARP request for a private VLAN IP address reaches the wrong peer, the wrong peer responds to the ARP
request with the peer MAC address.
The IP address of the VLT node VLAN interface is synchronized with the VLT peer over ICL when the VLT
peers are up. Whenever you add or delete an IP address, this updated information is synchronized with the
VLT peer. IP address synchronization occurs regardless of the VLAN administrative state. IP address addition
and deletion serve as the trigger events for synchronization. When a VLAN state is down, the VLT peer might
perform a proxy ARP operation for the IP addresses of that VLAN interface.
VLT nodes start performing Proxy ARP when the ICL link goes down. When the VLT peer comes up, proxy
ARP stops for the peer VLT IP addresses. When the peer node is rebooted, the IP address synchronized with
the peer is not flushed. Peer down events cause the proxy ARP to commence.
When a VLT node detects peer up, it does not perform proxy ARP for the peer IP addresses. IP address
synchronization occurs again between the VLT peers.
Proxy ARP is enabled only if you enable peer routing on both the VLT peers. If you disable peer routing by
using the no peer-routingcommand in VLT DOMAIN node, a notification is sent to the VLT peer to disable
the proxy ARP. If you disable peer routing when ICL link is down, a notification is not sent to the VLT peer and
in such a case, the VLT peer does not disable the proxy ARP operation.
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