Core Issue
Hot Standby Router Protocol (HSRP) provides redundancy for IP networks, ensuring that user traffic immediately and transparently recovers from first hop router failures. HSRP allows multiple routers on a single LAN to share a virtual IP and MAC address configured as the default gateway on the hosts. From the group of routers configured in an HSRP group, there is one router elected as the active router and another as a standby router. The active router assumes the role of forwarding packets sent to the virtual IP address. If the active router fails, the standby router takes over as the new active router. HSRP routers send HSRP packets to the multicast group address 224.0.0.2, to which all routers listen and elect the active and standby routers.
LAN Emulation (LANE) is a standard used to connect two different LAN segments across an ATM cloud. There are different components in LANE that emulate the nature of a broadcast capable LAN on an ATM network, which by itself does not support broadcasting. Since HSRP was designed for use over multicast or broadcast capable LANs, there are certain issues to be handled when running HSRP over LANE. These issues are automatically addressed by the Cisco IOS Software running on the devices.
Resolution
In LANE, the devices acting as LANE Clients (LEC) establish ATM virtual circuits between them to exchange data traffic. To establish the virtual circuits, the destination MAC address of a host in the LAN frame must be mapped to an ATM Network Service Access Point (NSAP) address used by the LEC to which the destination host is attached. This is done through the LAN Emulation ARP (LE-ARP) process by using the services of a LAN Emulation Server (LES), which maintains this mapping when the LEC joins a particular Emulated LAN (ELAN). The MAC-to-NSAP address mapping obtained through the LE-ARP process is maintained in the LE-ARP table on the LECs.
When HSRP operates over LANE, the same virtual MAC address must be mapped to a new ATM NSAP address when a new router becomes the active router. The LE-ARP table must be updated on the LECs with the new information without waiting for the default timeout values which would otherwise defeat the purpose of redundancy provided by HSRP. These are different ways in which routers acting as HSRP devices are connected over an ATM network implementing LANE:
- All HSRP routers directly connected to the ATM cloud functioning as an LEC
- All HSRP routers connected to a LAN switch attached to the ATM cloud and functioning as a LANE client
- A combination of both the above
To address the different HSRP over LANE scenarios, certain mechanisms have been introduced into the LANE specification to send additional messages and refresh the LE-ARP table. For more information, refer to Implementing HSRP Over LANE.
