Path Selection

the logic a router uses to identify the best route and install it in the routing table.

• A router identifies the path a packet should take by evaluating the prefix length that is programmed in the FIB.
• The FIB is programmed through the routing table, which is also known as the RIB.
• This section will cover prefix length, administrative distance, metrics, equal cost multipathing, and unequal-cost load balancing.

Path Selection

Path selection has three main components:

• Prefix length – represents the number of leading binary bits in the subnet mask that are in the on position.
• Administrative distance – is a rating of the trustworthiness of a routing information source. If a router learns about a route to a destination from more than one routing protocol, and all the routes have the same prefix length, then the AD is compared.
• Metrics – A metric is a unit of measure used by a routing protocol in the best-path calculation. The metrics vary from one routing protocol to another.

Prefix Length

Assume that a router has the following routes with various prefix lengths in the routing table: 10.0.3.0/28, 10.0.3.0/26, and 10.0.3.0/24. Each of these routes, also known as prefix routes or simply prefixes, has a different prefix length (subnet mask). The routes are considered to be different destinations, and they will all be installed into the RIB, also known as the routing table. The routing table also includes the outgoing interface and the next-hop IP address (unless the prefix is a connected network). Table 6-2 shows this routing table.

If a packet needs to be forwarded, the route chosen depends on the prefix length, where the longest prefix length is always preferred. The forwarding decision is a function of the FIB and results from the calculations performed in the RIB. The RIB is calculated through the combination of routing protocol metrics and administrative distance.

Administrative Distance

The RIB is programmed from the various routing protocol processes. Every routing protocol presents the same information to the RIB for insertion: the destination network, the next-hop IP address, the AD, metric values. The RIB accepts or rejects a route based on the following logic:

• If the route does not exist in the RIB, the route is accepted.
• If the route exists in the RIB, the AD must be compared. Routes with the lower AD values are installed in the routing table, routes with higher AD values are rejected and the submitting routing process is notified.

  Each of these three protocols attempts to install the route to 10.3.3.0/24 into the routing table. Because the prefix length is the same, the next decision point is the AD, where the routing protocol with the lowest AD installs the route into the routing table. Because the EIGRP internal route has the best AD, it is the one installed into the routing table.

The routing protocol that failed to install their route into the table (in this example, OSPF and IS-IS) hang on to the route and tell the routing table process to report to them if the best path fails so that they can try to reinstall this route.

Metrics – Equal-Cost Multipathing

The logic for selecting the best path for a routing protocol can vary. Most IGPs prefer internally learned routes over external routes and further prioritize the path with the lowest metric. Equal-Cost Multipathing If a routing protocol identifies multiple paths as a best path and supports multiple path entries, the router installs the maximum number of paths allowed per destination. This is known as equal-cost multipathing (ECMP) and provides load sharing across all links. RIP, EIGRP, OSPF, and IS-IS all support ECMP. ECMP provides a mechanism to increase bandwidth across multiple paths by splitting traffic equally across the links. This figure shows an example and the routing table.

Metrics- Unequal-Cost Load Balancing

By default, routing protocols install only routes with the lowest path metric. However, EIGRP can be configured to install multiple routes with different path metrics. This allows for unequal- cost load balancing across multiple paths. Traffic is transmitted out the router’s interfaces based on that path’s metrics in ratio to other the interface’s metrics. The delay has been incremented on R1’s Gi0/2 interface from 1 μ to 10 μ. R1 sees the two paths with different metrics. The path from R1 to R3 via R1–R2–R3 has been assigned a path metric of 3328, and the path via R1–R4–R3 has been assigned a path metric of 5632.   Note: The explicit path must be viewed to see the traffic ratios with unequal-cost load balancing.       Other useful information:

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