HSRP
Short for Hot Standby Routing Protocol, a proprietary protocol from Cisco. HSRP is a routing protocol that provides backup to a router in the event of failure. Using HSRP, several routers are connected to the same segment of an Ethernet, FDDI or token-ring network and work together to present the appearance of a single virtual router on the LAN. The routers share the same IP and MAC addresses, therefore in the event of failure of one router, the hosts on the LAN are able to continue forwarding packets to a consistent IP and MAC address. The process of transferring the routing responsibilities from one device to another is transparent to the user.
In computer networking, the Hot Standby Router Protocol (HSRP) is a Cisco proprietary redundancy protocol for establishing a fault-tolerant default gateway. Version 1 of the protocol was described in RFC 2281 in 1998. There is no RFC for version 2 of the protocol.
The protocol establishes an association between gateways in order to achieve default gateway failover if the primary gateway becomes inaccessible. HSRP gateways send multicast hello messages to other gateways to notify them of their priorities (which gateway is preferred) and current status (active or standby).
HSRP Background and Operations
One way to achieve near-100 percent network uptime is to use HSRP, which provides network redundancy for IP networks, ensuring that user traffic immediately and transparently recovers from first hop failures in network edge devices or access circuits.
By sharing an IP address and a MAC (Layer 2) address, two or more routers can act as a single “virtual” router. The members of the virtual router group continually exchange status messages. This way, one router can assume the routing responsibility of another, should it go out of commission for either planned or unplanned reasons. Hosts continue to forward IP packets to a consistent IP and MAC address, and the changeover of devices doing the routing is transparent.
Dynamic Router Discovery Mechanisms
Below are descriptions of dynamic router discovery mechanisms that are available to hosts. Many of these mechanisms don’t provide the network resiliency required by network administrators. This may be because the protocol wasn’t initially intended to provide network resiliency or because it isn’t feasible for every host on a network to be running the protocol. In addition to what is listed below, it is important to note that many hosts only allow you to configure a default-gateway.
Proxy Address Resolution Protocol
Some IP hosts use proxy Address Resolution Protocol (ARP) to select a router. When a host runs proxy ARP, it sends an ARP request for the IP address of the remote host it wants to contact. A router, Router A, on the network replies on behalf of the remote host and provides its own MAC address. With proxy ARP, the host behaves as if the remote host were connected to the same segment of the network. If Router A fails, the host continues to send packets destined for the remote host to the MAC address of Router A even though those packets have nowhere to go and are lost. You can either wait for ARP to acquire the MAC address of another router, Router B, on the local segment by sending another ARP request, or reboot the host to force it to send an ARP request. In either case, for a significant period of time, the host can’t communicate with the remote host, even though the routing protocol has converged, and Router B is prepared to transfer packets that would otherwise go through Router A.
Dynamic Routing Protocol
Some IP hosts run (or snoop) a dynamic routing protocol such as the Routing Information Protocol (RIP) or Open Shortes Path First (OSPF) to discover routers. The drawback of using RIP is that it is slow to adapt to changes in the topology. Running a dynamic routing protocol on every host may not be feasible for a number of reasons, including administrative overhead, processing overhead, security issues, or lack of a protocol implementation for some platforms.
