Addendum

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 to 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. 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. If
VLT nodes are configured with private VLANs, and the ARP request for private VLAN IP address reaches
the wrong peer, when the ICL link or peer is down, then the wrong peer responds to the ARP request
with the peer MAC address.
IP address of the VLT node VLAN interfaces is synchronized with the VLT peer over ICL when VLT peers
are up. Whenever an IP address is added or deleted, 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 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 becomes
operationally up, proxy ARP will be stopped 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 proxy ARP to
be commenced.
When a VLT node detects peer up, it will not perform proxy ARP for the peer IP addresses. IP address
synchronization occurs again between the VLT peers.
Proxy ARP is enabled only if peer routing is enabled on both the VLT peers. If you disable peer routing by
using the no peer-routing command in VLT DOMAIN node, a notification is sent to the VLT peer to
disable the proxy ARP. If peer routing is disabled 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 VLT domain is removed on one of the VLT nodes, the peer routing configuration removal will be
notified to the peer. In this case VLT peer node will disable the proxy ARP. When the ICL link is removed
on one of the VLT nodes by 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 further LAG hashed ARP
requests.
VLT Nodes as Rendezvous Points for Multicast Resiliency
You can configure virtual link trunking (VLT) peer nodes as rendezvous points (RPs) in a Protocol
Independent Multicast (PIM) domain on the S4810, S4820T, and Z9000 platforms. This capability enables
VLT resiliency and robustness for multicast routing operations.
PIM uses a VLT node as the root or a Rendezvous Point (RP) of the share tree distribution tree to
distribute multicast traffic to a multicast group. Messages to join the multicast group (Join messages) are
sent towards the RP and data is sent from senders to the RP so receivers can discover who are the
senders and begin receiving traffic destined to the multicast group.
To enable an explicit multicast routing table synchronization method for VLT nodes, you can configure
VLT nodes as RPs. Because multicast routing requires the incoming interface for each route to be
identified, 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 by entering 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 to the device. For the (S, G) routes that are
synchronized from the VLT peer, after the RP starts receiving multicast traffic via the (S,G), these (S, G)
routes are considered valid and are downloaded to the device. Only (S,G) routes are used to forward the
multicast traffic from the source to the receiver.
Virtual Link Trunking (VLT)
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