Addendum

Proxy ARP Capability on VLT Peer Nodes
The proxy ARP functionality on VLT peer nodes is supported on the S4810, S4820T, Z9000, I/O
Aggregator, and MXL platforms.
Proxy ARP enables hosts with knowledge of the network to accept and forward packets from hosts that
contain no knowledge of the network. Proxy ARP makes it possible for hosts to be ignorant of the
network, including subnetting.
Virtual Link Trunking (VLT) is a mechanism that enables the physical links between two devices that are
called VLT nodes or peers, and within a VLT domain, to be considered as a single logical link to external
devices that are connected using LAG bundles to both the VLT peers. This capability enables redundancy
without the implementation of Spanning Tree Protocol (STP), thereby providing a loop-free network with
optimal bandwidth utilization.
A Proxy ARP-enabled device answers the ARP requests that are destined for another host or router. This
phenomenon operates by causing the local host to consider that the Proxy ARP-enabled device is the
originator or the owner of the IP address, and the local host forwards the traffic to the proxy ARP-
enabled device, which in turn transmits the packets to the real 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 non-default information is displayed in the show
config command output.
ARP proxy operation is performed for the IP address of the peer VLT node when the peer VLT node is
down. The working of ARP proxy is stopped 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.
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 if the ICL link is down. 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, connsider 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 IP
address of VLAN 100 in node 1 is 10.1.1.1/24 and IP address of VLAN 100 in node 2 is 20.1.1.2/24. In this
case i,f the ARP request for 20.1.1.1 reaches node 1, Node 1 will not perform ARP 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 is configured on the VLT peers, and if IP address of VLAN 100 is configured as
10.1.1.0/24 and IP address of VLAN 200 is configured as 20.1.1.0/24, Proxy ARP is not performed if the
VLT node receives 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 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 owing to peer routing timer expiry. The source
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Virtual Link Trunking (VLT)