High availability and fault tolerance are two of the most critical technology areas in the networking world and they impact all the other technologies presented in a network.
High availability is often a factor taken into consideration when designing and implementing end-to-end solutions. This ensures redundancy for the network services and for the end-users and is accomplished by making sure that the network devices are reliable and fault tolerant.
You should design some level of high availability solutions into any network module that you want to propose. For example, the Access Layer can have multiple ways of connecting access devices, it can have multiple connections to multiple devices in the Distribution Layer, or the Distribution Layer can have some redundancy when connecting to the Core Layer.
Redundancy is a typical design element and high availability is a function of two parameters:
- The budget available
- How mission critical a particular service or application is to the business goals of the organization
There are many redundancy options that can be utilized in different modules of modern networks:
- Workstation-to-router redundancy at the Access Layer
- Server redundancy in the Server Farm module
- Route redundancy
- Media redundancy at the Access Layer
Each of these areas will be covered in detail in the following sections.
Workstation-to-Router Redundancy
The most important topic in the list above is workstation-to-router redundancy, because it is very important that the Access Layer devices maintain their default gateway information. Modern networks usually respect the 80/20 model, which states that 80% of the traffic will pass through the default gateway and 20% of the destinations will be local. This means that making the default gateway available is critical.
Workstation-to-router redundancy can be accomplished in multiple ways, including:
- Proxy ARP on routers
- Explicit configuration
- IRDP
- RIP
- HSRP
- VRRP
- GLBP
Proxy ARP involves a workstation that has no default gateway configured. To communicate with a remote host, the workstation sends ARP requests for the address of the host and the router that hears this request realizes that it can service that request and responds on behalf of the client using Proxy ARP. The router actually pretends to be the host so the workstation can send traffic destined to that specific client to the router.
Explicit configuration is the most common way of accomplishing workstation-to-router redundancy because some of the operating systems allow multiple default gateways to be configured. The problem with this is the latency that appears while the device figures out that one gateway is down and switches to another gateway. Another problem with the explicit configuration of multiple default gateways is that not all operating systems support this feature.
The optimal solution is one that can be used by every host in the infrastructure and, if possible, in a transparent way (i.e., configure network devices only). Some routers can run the ICMP Router Discovery Protocol (IRDP). If both routers and hosts can run IRDP, this is another option for hosts to dynamically discover an available default gateway. RIP is another protocol that can be used in this scenario, but this also means that all the hosts must know this technology and it implies some configuration on them.
The preferred solution is a technology that does not place any burden on the hosts and that is completely transparent to them, for example, the hosts just need to configure a single default gateway because the entire redundancy configuration is made on the routers. The protocols that can be used to accomplish this are generically called First Hop Redundancy Protocols (FHRPs) and they include the following:
- Hot Standby Router Protocol (HSRP)
- Virtual Router Redundancy Protocol (VRRP)
- Gateway Load Balancing Protocol (GLBP)
HSRP is a Cisco proprietary protocol that inspired IEEE to create the open standard protocol VRRP (their functionality is almost identical). GLBP is the most recent protocol, again a Cisco invention, and this is the most sophisticated FHRP.
HSRP
Analyzing Figure 26.7 below, the network has two gateway routers that connect to one Layer 2 switch that aggregates the network hosts:
Figure 26.7 – Hot Standby Router Protocol
Router 1 has one potential default gateway address (10.10.10.1) and Router 2 has another potential default gateway address (10.10.10.2). The two routers are configured in an HSRP group and they present to the clients a virtual default gateway address of 10.10.10.3. This host address is considered the default gateway address, although it is not assigned to any router interface because it is a virtual address.
One of the two routers is the active device (Router 1 in this example) and it is the one that is actually forwarding traffic for the 10.10.10.3 virtual address. Router 2 is the standby HSRP device. The two routers exchange HSRP messages to each other in order to check on each other’s health status. For instance, if Router 2 no longer hears from Router 1, it realizes that Router 1 is down and it will take over as the active HSRP device.
These devices transparently provide access for the clients, as they are transparently serving up the virtual default gateway address.
VRRP
VRRP works in a way that is similar to HSRP. The differences are that the two routers are configured in a VRRP group and one of them is the master device (instead of the active router) and it performs all the forwarding, while the other one is the slave device (instead of the standby router), as illustrated in Figure 26.8 below:
Figure 26.8 – Virtual Router Redundancy Protocol
As was the case for HSRP, the VRRP group presents a virtual IP address to the clients. An interesting aspect about VRRP is that you can utilize as the virtual IP address the same address that is on the master device (this is very useful if you are using public IP addresses). In this case, the virtual address is configured as 10.10.10.1, which is identical to the address on the Router 1 interface.
GLBP
GLBP is the most unique of the FHRPs. With GLBP, not only do you have the ability to achieve redundancy but also you have the ability to perform load balancing and it is much easier to use more than two devices. This scenario is depicted in Figure 26.9 below:
Figure 26.9 – Gateway Load Balancing Protocol
In Figure 26.9 above, there are three routers configured in a GLBP group, which is assigned a virtual default gateway address (10.10.10.4) that is also configured on the clients. One of the devices (Router 1 in this example) is elected as the AVG (Active Virtual Gateway) and the other devices are AVFs (Active Virtual Forwarders).
When the hosts send ARP requests for the 10.10.10.4 MAC address, the AVG responds to the ARP requests and round robins with the virtual MAC addresses of the AVF machines. Router 1 responds to the first ARP request it receives with its own virtual MAC address, then it responds to the second ARP request it receives with Router 2’s virtual MAC address, and then with Router 3’s virtual MAC address. In other words, the AVG round robins the traffic over the available AVF devices. The simplistic round robin balancing approach can be changed within the configuration to other load balancing techniques for GLBP.
| Note: The AVG can also function as an AVF and it usually does so.
|
Server Redundancy
Server-based redundancy technologies can be implemented in server farms or data centers. This is often needed to ensure high availability for key server functions, like file or application sharing. One way to achieve this is to mirror multiple servers so in case of a failure of one server you can dynamically fail over to another server.
Another example of redundancy technology is VoIP, which has a critical component called the PBX (or Call Manager in Cisco networks). Because PBXs are so critical for routing call traffic, you will typically configure them in a cluster with the same idea as in server redundancy: if one device fails, the other device starts servicing the VoIP clients transparently.
Route Redundancy
If you wanted to configure redundancy between the campus infrastructures in a WAN, you can achieve this by implementing load balancing at the routing protocol level.
Figure 26.10 – WAN Route Redundancy
This increases availability because in the case of a direct path failure between Site 1 and Site 2 in Figure 26.10 above, the two sites can still reach each other by going through another location (Site 1 > Site 3 > Site 2).
Although it features high availability functionality, the scenario presented above has a downside regarding its full-mesh connectivity. This implies configuration overhead and a high cost because of the number of circuits that need to be created. This is one of the reasons full-mesh topologies are not implemented very often.
To calculate the necessary number of circuits for a full-mesh architecture, you can use the n*(n-1)/2 formula, where “n” equals the number of sites (nodes). In Figure 26.10 above, there are four sites, so the number of connections equals 4*3/2 for a total of six circuits.
Details concerning the specific capabilities of high availability and load balancing for each particular protocol will not be covered in this chapter or in the remainder of the manual.
Media Redundancy
Another type of redundancy is media redundancy, which occurs in places where devices with multiple paths are connected. This is always useful in case one of the links fails. Media redundancy demands the configuration of Spanning Tree Protocol at Layer 2 to avoid loops that can bring the network down.
In a WAN environment, there might be floating static routes, meaning static routes that point to a backup path, as illustrated in Figure 26.11 below:
Figure 26.11 – Floating Static Routes
Analyzing Figure 26.11 above, there are two sites (Site 1 and Site 2) connected via a direct WAN circuit and via a slower connection that involves more hops as a backup path (via Site 3). The traffic from Site 1 to Site 2 should be direct (the primary path), which in this example the route is learned via EIGRP and it has an AD (administrative distance) of 90.
To achieve redundancy over the backup path, you can create a static route on Site 1 pointing to Site 3. To make this a floating static route, you need to intentionally set its administrative distance higher than the AD of the main path. In this example, the floating static route is AD 91 so it will be less preferred than the EIGRP route. This means that the static route will not be used unless the EIGRP route goes away. In other words, the floating route will not be used unless there is a loss of the primary route.
The floating static route is a technique often used in WAN scenarios to provide circuit redundancy. Another technology used to achieve media redundancy is EtherChannel. This is a Layer 2 or Layer 3 logical bundling or channel aggregation technique that can be used between switches. The bundled links can appear as one single link on the specific devices, as shown in Figure 26.12 below:
Figure 26.12 – EtherChannel Example
EtherChannels are important when using the Spanning Tree Protocol because when all of the links look like one link, STP will not consider them a possible loop threat and will not shut down any link from that specific bundle. Therefore, the links in an EtherChannel can be dynamically load balanced without STP interfering in this situation.
As explained previously in Chapter 19, EtherChannels come in two forms:
- PAgP (Port Aggregation Protocol): a Cisco proprietary protocol
- LACP (Link Aggregation Control Protocol): an open standard protocol
Channel aggregation techniques can also be used in WANs. An example in this regard is the Multilink PPP (MPPP) technology.