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When designing a network topology for a customer who has critical systems, services, or network paths, you should determine the likelihood that these components will fail and design redundancy where necessary. Consider incorporating one of the following types of redundancy into your design:
- Workstation-to-router redundancy in the building-access layer
- Server redundancy in the server farm module
- Route redundancy within and between network components
- Media redundancy in the access layer
The following sections discuss each type of redundancy.
Workstation-to-Router Redundancy
When a workstation has traffic to send to a station that is not local, the workstation has many possible ways to discover the address of a router on its network segment, including the following:
- ARP
- Explicit configuration
- RIP
ARP
Some IP workstations send an ARP frame to find a remote station. A router running proxy ARP can respond with its data link layer address. Cisco routers run proxy ARP by default.
Explicit Configuration
Most IP workstations must be configured with the IP address of a default router, which is sometimes called the default gateway.
In an IP environment, the most common method for a workstation to find a server is via explicit configuration (a default router). If the workstation's default router becomes unavailable, you must reconfigure the workstation with the address of a different router. Some IP stacks enable you to configure multiple default routers, but many other IP implementations support only one default router.
RDP
RFC 1256 specifies an extension to Internet Control Message Protocol (ICMP) that allows an IP workstation and router to run RDP to let the workstation learn a router's address.
RIP
An IP workstation can run RIP to learn about routers. You should use RIP in passive mode rather than active mode. (Active mode means that the station sends RIP frames every 30 seconds.) Usually in these implementations, the workstation is a UNIX system running the routed or gated UNIX process.
HSRP
The Cisco HSRP provides a way for IP workstations that support only one default router to keep communicating on the internetwork even if their default router becomes unavailable. HSRP works by creating a phantom router that has its own IP and MAC addresses. The workstations use this phantom router as their default router.
HSRP routers on a LAN communicate among themselves to designate two routers as active and standby. The active router sends periodic hello messages. The other HSRP routers listen for the hello messages. If the active router fails and the other HSRP routers stop receiving hello messages, the standby router takes over and becomes the active router. Because the new active router assumes both the phantom's IP and MAC addresses, end nodes see no change. They continue to send packets to the phantom router's MAC address, and the new active router delivers those packets.
HSRP also works for proxy ARP. When an active HSRP router receives an ARP request for a node that is not on the local LAN, the router replies with the phantom router's MAC address instead of its own. If the router that originally sent the ARP reply later loses its connection, the new active router can still deliver the traffic.
Figure 2-14 shows a sample implementation of HSRP.
Figure 2-14. HSRP: The Phantom Router Represents the Real Routers
In Figure 2-14, the following sequence occurs:
GLBP
GLBP protects data traffic from a failed router or circuit, such as Hot Standby Router Protocol (HSRP), while allowing packet load sharing between a group of redundant routers. The difference in GLBP from HSRP is that it provides for load balancing between the redundant routers. It load balances by using a single virtual IP address and multiple virtual MAC addresses. Each host is configured with the same virtual IP address, and all routers in the virtual router group participate in forwarding packets. GLBP members communicate between each other through hello messages sent every three seconds to the multicast address 224.0.0.102, User Datagram Protocol (UDP) port 3222.
Server Redundancy
Some environments need fully redundant (mirrored) file and application servers. For example, in a brokerage firm where traders must access data to buy and sell stocks, two or more redundant servers can replicate the data. Also, you can deploy CallManager servers in clusters for redundancy. The servers should be on different networks and use redundant power supplies.
Route Redundancy
Designing redundant routes has two purposes: balancing loads and increasing availability.
Load Balancing
Most IP routing protocols can balance loads across parallel links that have equal cost. Use the maximum-paths command to change the number of links that the router will balance over for IP; the default is four, and the maximum is six. To support load balancing, keep the bandwidth consistent within a layer of the hierarchical model so that all paths have the same cost. (Cisco Interior Gateway Routing Protocol [IGRP] and Enhanced IGRP [EIGRP] are exceptions because they can load-balance traffic across multiple routes that have different metrics by using a feature called variance.)
A hop-based routing protocol does load balancing over unequal-bandwidth paths as long as the hop count is equal. After the slower link becomes saturated, packet loss at the saturated link prevents full utilization of the higher-capacity links; this scenario is called pinhole congestion. You can avoid pinhole congestion by designing and provisioning equal-bandwidth links within one layer of the hierarchy or by using a routing protocol that takes bandwidth into account.
IP load balancing in a Cisco router depends on which switching mode the router uses. Process switching load-balances on a packet-by-packet basis. Fast, autonomous, silicon, optimum, distributed, and NetFlow switching load-balance on a destination-by-destination basis because the processor caches information used to encapsulate the packets based on the destination for these types of switching modes.
Increasing Availability
You should keep bandwidth consistent within a given design component to facilitate load balancing. Another reason to keep bandwidth consistent within a layer of a hierarchy is that routing protocols converge much faster on multiple equal-cost paths to a destination network.
By using redundant, meshed network designs, you can minimize the effect of link failures. Depending on the convergence time of the routing protocols, a single link failure cannot have a catastrophic effect.
You can design redundant network links to provide a full mesh or a well-connected partial mesh. In a full-mesh network, every router has a link to every other router, as shown in Figure 2-15. A full-mesh network provides complete redundancy and also provides good performance because there is just a single-hop delay between any two sites. The number of links in a full mesh is n(n–1)/2, where n is the number of routers. Each router is connected to every other router. A well-connected partial-mesh network provides every router with links to at least two other routing devices in the network.
Figure 2-15. Full-Mesh Network: Every Router Has a Link to Every Other Router in the Network
A full-mesh network can be expensive to implement in WANs due to the required number of links. In addition, groups of routers that broadcast routing updates or service advertisements have practical limits to scaling. As the number of routing peers increases, the amount of bandwidth and CPU resources devoted to processing broadcasts increases.
A suggested guideline is to keep broadcast traffic at less than 20 percent of the bandwidth of each link; this amount limits the number of peer routers that can exchange routing tables or service advertisements. When planning redundancy, follow guidelines for simple, hierarchical design. Figure 2-16 illustrates a classic hierarchical and redundant enterprise design that uses a partial-mesh rather than a full-mesh topology. For LAN designs, links between the access and distribution layer can be Fast Ethernet, with links to the core at Gigabit Ethernet speeds.
Figure 2-16. Partial-Mesh Design with Redundancy
Media Redundancy
In switched networks, switches can have redundant links to each other. This redundancy is good because it minimizes downtime, but it can result in broadcasts continuously circling the network, which is called a broadcast storm. Because Cisco switches implement the IEEE 802.1d spanning-tree algorithm, you can avoid this looping in Spanning Tree Protocol (STP). The spanning-tree algorithm guarantees that only one path is active between two network stations. The algorithm permits redundant paths that are automatically activated when the active path experiences problems.
Because WAN links are often critical pieces of the internetwork, WAN environments often deploy redundant media. As shown in Figure 2-17, you can provision backup links so that they become active when a primary link goes down or becomes congested.
Figure 2-17. Backup Links Can Provide Redundancy
Often, backup links use a different technology. For example, a leased line can be in parallel with a backup dialup line or ISDN circuit. By using floating static routes, you can specify that the backup route have a higher administrative distance (used by Cisco routers to select routing information) so that it is not normally used unless the primary route goes down. This design is less available than the partial mesh presented previously. Typically, on-demand backup links reduce WAN charges.
Note
When provisioning backup links, learn as much as possible about the physical circuit routing. Different carriers sometimes use the same facilities, meaning that your backup path might be susceptible to the same failures as your primary path. You should do some investigative work to ensure that your backup really is acting as a backup.
You can combine backup links with load balancing and channel aggregation. Channel aggregation means that a router can bring up multiple channels (for example, ISDN B channels) as bandwidth requirements increase.
Cisco supports Multilink Point-to-Point Protocol (MPPP), which is an Internet Engineering Task Force (IETF) standard for ISDN B channel (or asynchronous serial interface) aggregation. MPPP does not specify how a router should accomplish the decision-making process to bring up extra channels. Instead, it seeks to ensure that packets arrive in sequence at the receiving router. Then, the data is encapsulated within PPP and the datagram is given a sequence number. At the receiving router, PPP uses this sequence number to re-create the original data stream. Multiple channels appear as one logical link to upper-layer protocols.
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