Users Guide

forwarding level. VLT peer routing allows you to replace VRRP with routed VLT to route the trac from Layer 2 access nodes. With proxy
ARP, hosts can resolve the MAC address of the VLT node even when VLT node is down.
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 congured. For example, consider a sample topology in
which you congure VLAN 100 on two VLT nodes, node 1 and node 2. You did not congure 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 dierent from the
received interface IP subnet. For example, if you congure VLAN 100 and 200 on the VLT peers, and if you congured the VLAN 100 IP
address as 10.1.1.0/24 and you congured 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 ushed. 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-
routing
command in VLT DOMAIN node, a notication is sent to the VLT peer to disable the proxy ARP. If you disable peer routing when
ICL link is down, a notication 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 conguration removal is notied 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, ushes the peer IP addresses and does not perform proxy ARP for the additional LAG hashed ARP requests.
Virtual Link Trunking (VLT)
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