nx.os.and.cisco.nexus.switching.2nd.edition.1587143046

with the same site-id. The site-id configured is advertised in the IS-IS hello packets sent;
this advertisement is over both the overlay and on the site VLAN.
Note
If you run the OTV with versions of NX-OS prior to 5.2.1, In-Service Software
Upgrade (ISSU) cannot be performed. The OTV Edge Device is forced into a down
state until the site-id is configured the same on both AEDs in the same data center.
OTV and ARP
OTV introduces several enhancements to reduce the overall amount of traffic that traverses
the inter-DC links. One of these enhancements is ARP optimization. ARP optimization
enables each AED to snoop and cache initial ARP replies from hosts located at a remote
site. Because the reply information is cached, a local AED can then reply in proxy to ARP
requests for remote hosts, thus reducing ARP traffic across the inter-DC links. The caching
process functions as follows:
1. A device in Datacenter 2 site sources an ARP request to determine the MAC of the
host with address IP A.
2. The ARP request is a Layer 2 broadcast frame and is sent across the OTV overlay to
all remote sites, eventually reaching the machine with address IP A in Datacenter 1,
which creates an ARP reply message. The ARP reply is sent back to the originating
host in the Datacenter 2.
3. The OTV Edge Device in Datacenter 2 snoops the ARP reply and caches the
contained mapping information (MAC 1, IP A) in a local data structure named ARP
Neighbor-Discovery (ND) Cache.
4. A subsequent ARP request is originated from Datacenter 2 for the same IP A address.
5. The request is not forwarded to the remote sites but is locally answered via proxy by
the local OTV Edge Device on behalf of the remote device IP A.
This caching process does, however, introduce some potential caveats. In a situation in
which the ARP aging timer is set longer than the CAM aging timer, traffic might be
blackholed because of OTV dropping unknown unicast frames. Recommended practice
dictates that the ARP aging-timer is set lower than the CAM aging-timer to eliminate this
caveat. The default aging timers on the Nexus 7000 follow:
• OTV ARP aging-timer: 480 seconds / 8 minutes
• MAC aging-timer: 1800 seconds / 30 minutes
These timers differ on other Cisco platforms, such as the Nexus 5000 or an IOS-based
device such as a Catalyst 6500. In deployments in which the host’s default gateway is
placed on a device different than the Nexus 7000, you must set the ARP aging-timer of the
device to a value lower than its MAC aging-timer. The OTV ARP caching behavior is
enabled by default. It can be disabled via the otv suppress-arp-nd command inside of the
OTV overlay interface subconfiguration.