Dell S4048T-ON, Configuration Manual

Page: 1032 / 1146

• 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 the other router in a VLT domain. 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 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 displays in the
show
config command output.
An 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 allows 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 is supported on symmetric VLANs only. Proxy ARP is
enabled by default. To support proxy ARP, the routing table must be symmetrically configured. For example, consider a sample topology in
which you configure VLAN 100 on two VLT nodes, node 1 and node 2. You did not configure 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 different from the
received interface IP subnet. For example, if you configure VLAN 100 and 200 on the VLT peers, and if you configured the VLAN 100 IP
address as 10.1.1.0/24 and you configured 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 flushed. Peer down events cause the proxy
ARP to commence.
1032 Virtual Link Trunking (VLT)