Cisco offers several architectural approaches to high availability, ranging from lower-layer concepts such as resilient packet ring and Multiprotocol Label Switching (MPLS) node protection up to protocol-intrinsic or application layer approaches.
The lower-layer concepts (Layers 1 through 3) are summarized under the Cisco Global Resilient IP Framework (GRIP). This framework consists of the following building blocks:
Stateful NAT (SNAT) for translation groups
IPSec stateful failover (VPN HA in combination with HSRP)
Multicast subsecond convergence
Nonstop forwarding with stateful switchover
MPLS fast reroute
Approaches that are relevant to the transport and application layers are discussed briefly in the following two subsections.
The Cisco IOS SLB feature is available for certain Cisco IOS routers and catalyst switches. It provides two load-balancing algorithms: weighted round-robin and weighted least connections. With SLB enabled, a virtual server (VIP) represents a cluster of real servers. Clients are configured to connect to the IP address of the virtual server (directed or dispatched redirection mode). DNS records usually point to the virtual IP address.
The Cisco IOS SLB intelligence picks a real server to satisfy the requesting client based on one of the load-balancing algorithms mentioned earlier. It can perform NAT, provide added security by hiding real servers, and provide rudimentary DoS protection such as maximum connection limits and SYNGuard (SYN flooding protection).
IOS SLB for Layer 3 switches works with HSRP to prevent single points of failure for virtual IP addresses. In contrast to crude round-robin approaches, the cluster constituents provide input into the IP load-balancing device by means of the Dynamic Feedback Protocol (DFP), indicating the level of CPU utilization, application, and user identity. DFP is implemented with workload agents (Windows, UNIX) that reside on IP server platforms. For further configuration information, consult the Cisco.com document "Configuring Server Load Balancing."
These devices, software and hardware, operate at Layers 4 through 7 and consist of the following products:
Local Director (local traffic distribution)
Distributed Director (geographically disperse traffic distribution)
Content routers (redirect the user to the most suitable site on a network based on a set of metrics such as delay topology, server load, and a set of policies such as location of content)
Content networking software (carries out the same duty without a dedicated appliance)
The main features of these approaches are caching, intelligent content delivery, traffic distribution, intelligent DNS services, and load balancing.