Reference Guide

private-vlan mapping secondary-vlan vlan-list
The list of secondary VLANs can be:
● Specified in comma-delimited (VLAN-ID,VLAN-ID) or hyphenated-range format (VLAN-ID-VLAN-ID).
● Specified with this command even before they have been created.
● Amended by specifying the new secondary VLAN to be added to the list.
Proxy ARP Capability on VLT Peer Nodes
The proxy ARP functionality is supported on VLT peer nodes.
A proxy ARP-enabled device answers the ARP requests that are destined for another host or router. The local host forwards the
traffic to the proxy ARP-enabled device, which in turn transmits the packets to the destination.
By default, proxy ARP is enabled. To disable proxy ARP, use the no proxy-arp command in the interface mode. To re-enable
proxy ARP, use the ip proxy-arp command in INTERFACE mode. To view if proxy ARP is enabled on the interface, use the
show config command in INTERFACE mode. If it is not listed in the show config command output, it is enabled. Only
nondefault information is displayed in the show config command output.
ARP proxy operation is performed on the VLT peer node IP address when the peer VLT node is down. The ARP proxy stops
working either when the peer routing timer expires or when the peer VLT node goes up. Layer 3 VLT provides a higher resiliency
at the Layer 3 forwarding level. VLT peer routing enables you to replace VRRP with routed VLT to route the traffic 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 supported on symmetric VLANs only. Proxy
ARP is enabled by default. Routing table must be symmetrically configured to support proxy ARP. For example, consider a
sample topology in which VLAN 100 is configured on two VLT nodes, node 1 and node 2. ICL link is not configured 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 will not perform the ARP request for 20.1.1.2. Proxy ARP is
supported only for the IP address 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 VLAN 100 and 200 are configured on the
VLT peers, and if the VLAN 100 IP address is configured as 10.1.1.0/24 and the VLAN 200 IP address is configured 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 peer routing is enabled on both the VLT peers. If peer routing is disabled 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 the 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, then 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 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 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 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 the proxy ARP to commence.
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.
826
Virtual Link Trunking (VLT)