The Internet Router Discovery Protocol (IRDP) is yet another way to handle dead-gateway detection. Routers that support IRDP use ICMP messages to advertise their presence. In Windows NT 4.0, Microsoft added IRDP support, which is disabled by default. You use registry modifications to enable IRDP individually for each NIC, as described in the Microsoft articles "Internet Router Discovery Protocol (IRDP) Client Support Added to Windows NT 4.0" (http://support.microsoft.com/?kbid=223756) and "Router Discovery Protocol Is Disabled by Default" (http://support.microsoft.com/?kbid=269734). After you enable IRDP, the protocol stack will listen for and request router advertisements and use them to set a default gateway.
Link aggregation. Several years ago, NIC vendors began to offer proprietary solutions to the single-NIC vulnerability. These solutions evolved into the IEEE 802.3ad Link Aggregation Control Protocol (LACP) standard. LACP supports multiple parallel switch-to-switch and server-to-switch connections. You can use this standard—variously called NIC teaming, port bonding, and link aggregation—to configure LACP-based products for fault tolerance, increased bandwidth, and load balancing across parallel links.
Figure 1 shows the concept of server-to-switch link aggregation. In this example, four NIC ports on the server connect to four ports on one switch. LACP static-mode support in the NIC driver and the switch combine the bandwidth of the four ports for a total effective bandwidth equal to the sum of the NIC speeds. Traffic across the four links is load-balanced, and when a link fails, the load-balancing algorithm quickly converges to balance the load across the remaining links. This configuration doesn't provide fault tolerance in the event of a switch failure.
Figure 2 shows a server-to-switch configuration that provides fault tolerance in the event of a switch failure but doesn't provide link aggregation or load balancing. This configuration requires that you enable the Spanning Tree Algorithm (STA) in both switches to ensure that only one link is active at a time, thereby preventing packets from circulating between the links.
Figure 3 shows a server-to-switch configuration that requires LACP Dynamic Mode support. Because the server has connections to two switches, the configuration provides switch fault tolerance. The server has multiple connections to each switch, and the connections to each switch are grouped together (i.e., teamed). In this configuration, the teamed connections to Switch A are active, whereas the teamed connections to Switch B remain in standby mode. LACP provides link aggregation, load balancing, and fault tolerance to link failures within the active team. In the event of a switch failure, LACP fails communications over to the standby team connected to Switch B.
Figure 4 shows a switch-to-switch configuration. This configuration supports additional switch-to-switch bandwidth, load balancing, and link-failure fault tolerance.
Redundant WAN Links
Whereas building redundancy into your LAN involves (typically Ethernet) server-to-switch and switch-to-switch connections, building redundancy into your WAN involves router-to-router connections. Let's look at the network architectures you can use to implement redundant paths to remote destinations and to implement fault tolerance in the event of WAN link or router failure.
Consider the simplest Internet-connection scenario, which Figure 5 shows. The local network connects to the ISP through a single link at Router A. If the local network is sufficiently simple, Router A can use static routing rather than run an interior gateway protocol such as the Routing Information Protocol (RIP) or the Open Shortest Path First (OSPF) protocol. However, this configuration offers no fault tolerance.
Figure 6 shows one level of fault tolerance: two independent connections from one site into the same autonomous system (AS) of one ISP. Routers A and B both run Border Gateway Protocol 4 (BGP-4). For information about the Interior Border Gateway Protocol (IBGP) and Exterior Border Gateway Protocol (EBGP), see the sidebar "A Glossary of Standards and Protocols Relevant to Redundant Networks." Although this configuration offers fault tolerance to the failure of either Router A or Router B or to either of their communications lines, the organization is still vulnerable to an outage. If both communication lines use the same "last-mile" communication path between the datacom provider (e.g., your local phone company) and the local network, anything—such as a backhoe—that inadvertently damages that path will take out both links. If both links terminate at the same ISP Point of Presence (POP), a problem at the ISP's location can also take down both links.
Figure 7 shows a more fault-tolerant configuration: a network connection to two ISPs. This connection might still be vulnerable to last-mile disruption but can survive an ISP outage. This configuration requires the local network to use a globally unique Autonomous System Number (ASN). For information about the ASN, see the sidebar "A Glossary of Standards and Protocols Relevant to Redundant Networks."
Organizations that have facilities in several locations around the country or around the world can take advantage of even more robust fault-tolerant configurations. Suppose a private network interconnects an organization's various locations. The private network connects each location to two or more other locations. At least two of these locations would have fault-tolerant ISP connections. Depending on the ISP facility's fault tolerance, the organization might be able to use the same ISP in several locations or might choose to contract with different ISPs. This network would be able to survive a regional outage (each organizational location would have at least two paths to reach other locations) and would be able to bypass regional Internet problems by connecting through an ISP outside the troubled region.
The Key To Success
A thorough analysis of your network communication paths is key to successfully implementing redundancy for fault tolerance. Do you know whether you can accomplish last-mile connections over more than one physical path? Does your ISP have fault-tolerant Internet connections?
Competent network administrators make the effort to correctly implement the components for which they are directly responsible. The best network administrators, however, look beyond that core responsibility by constantly searching for potential points of failure in the communication path.
End of Article
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