Dynamic NAT Mapping with HSRP feature supports stateless redundancy using HSRP
with dynamic Network Address Translation (NAT), Port Address Translation (PAT),
and interface overload configuration. Dynamic NAT, PAT and interface overload
support HSRP with and without virtual routing and forwarding (VRF) instances.
All these configurations are supported in the Carrier Grade NAT (CGN) mode.
Hot Standby Router
Protocol (HSRP) provides high network availability by providing first-hop
routing redundancy for IP hosts on networks configured with a default gateway
IP address. HSRP is used in a group of routers for selecting an active device
and a standby device. HSRP provides redundancy for routing IP traffic without
being dependent on the availability of a single router. In a group of device
interfaces, the active device is the device of choice for routing packets; the
standby device is the device that takes over when the active device fails or
when preset conditions are met.
HSRP send and receive multicast UDP-based hello packets to detect router
failure and to designate active and standby devices. Selection of active and
standby devices is based on the assigned priority. The device with the highest
priority is selected as the active device. After failover, a new active device
sends a gratuitous Address Resolution Protocol (ARP) request to LAN users to
notify about the change in MAC address for the virtual IP address (VIP).
To enable this
feature, both the active and standby devices must be configured with the same
NAT rules and HSRP must be configured on both the devices. Based on the
configured priority one of the devices will be active and the other standby.
This feature supports VRF-aware NAT translation and Carrier Grade NAT (CGN)
supports the LAN-LAN topology as well as the LAN-WAN topology. In the LAN-WAN
topology, only symmetric routing is supported.
When an Address
Resolution Protocol (ARP) query is triggered for an address that is configured
with dynamic NAT mapping and owned by the device, NAT responds with the
burned-in MAC (BIA MAC) address on the interface to which the ARP is pointing.
You must enable and configure the NAT inside interfaces of the active and
standby devices to belong to a group.
In Cisco IOS XE
Denali 16.3 release, the Allow same ACL/router-map on multiple NAT statements
feature was introduced to support usage of same ACL for configuring both
dynamic mapping and static mapping in NAT. Dynamic mapping is given the
precedence over static mapping regardless of the configuration order. The
precedence of dynamic mapping over static mapping using the sequence number of
the class ensures class order consistency in NAT.