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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.
Virtual Link Trunking (VLT)
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Troubleshooting
