Connectivity Guide

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.
When you remove the VLT domain on one of the VLT nodes, the peer routing configuration removal is notified to the peer. In
this case, the VLT peer node disables the proxy ARP. When you remove the ICL link on one of the VLT nodes using the no
peer-link command, the ICL down event is triggered on the other VLT node, which in turn starts the proxy ARP application.
The VLT node, where the ICL link is deleted, flushes the peer IP addresses and does not perform proxy ARP for the additional
LAG hashed ARP requests.
VLT Nodes as Rendezvous Points for Multicast
Resiliency
You can configure VLT peer nodes as rendezvous points (RPs) in a Protocol Independent Multicast (PIM) domain.
PIM uses a VLT node as the RP to distribute multicast traffic to a multicast group. Messages to join the multicast group (Join
messages) and data are sent towards the RP, so that receivers can discover who the senders are and begin receiving traffic
destined for the multicast group.
To enable an explicit multicast routing table synchronization method for VLT nodes, you can configure VLT nodes as RPs.
Multicast routing needs to identify the incoming interface for each route. The PIM running on both VLT peers enables both the
peers to obtain traffic from the same incoming interface.
You can configure a VLT node to be an RP using the ip pim rp-address command in Global Configuration mode. When you
configure a VLT node as an RP, the (*, G) routes that are synchronized from the VLT peers are ignored and not downloaded
Virtual Link Trunking (VLT)
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