Notes: Route Selection in Cisco Routers

This post will examine how a router chooses which route is the best among those presented by routing protocols, manual configuration, and various other means.

Processes Involved

Three components involved in building and maintaining the routing table:

• processes that run a routing protocol (such as EIGRP, OSPF, BGP, and IS-IS)
• the routing table itself (accepts information from the routing protocols and replies to requests from forwarding processes)
• the forwarding process, which requests information from the routing table to make a packet forwarding decision

Building the Routing Table

Three main considerations when building the routing table:

• Prefix length - Each unique prefix is considered as a separate destination. 
• Administrative distance - The measure of trustworthiness of the source of the route. If the route is learned from multiple sources, the lowest administrative distance (the believability of the source) is preferred. 
• Metric - The measure used by the routing protocol to calculate the path cost to a given destination. Each routing protocol uses a different metric.

Backup Routes

• What happens to routes that are not installed in the routing table?
• What if the most preferred route fails?
• Two approaches to solve this problem:
  1. Have the routing process attempt to install its best route periodically.
  2. Have the routing protocol keep the route in its own database, and instruct the routing process to report if the best path fails. 
• If the routing protocol does not have its own database, it attempts to install the updated information in the routing table.
• If there's already a route to the same destination, the installation attempt fails.
• For protocols that have their own database (EIGRP, OSPF, BGP, IS-IS, and RIP), a backup route is registered when the initial attempt to install the route fails.
• If the best path fails, the routing table process calls each routing protocol process that has registered a backup route, and asks them to reinstall the route (the lowest administrative distance wins).

Adjusting the Administrative Distance

• Changing the administrative distance can be very dangerous!
• For an entire routing protocol, the change is quite easy; simply specify the distance under the routing process.
• For static routes, the distance can be configured for each route individually.

How Metrics Determine the Route Selection Process

• Routes are chosen in the routing table based on the lowest administrative distance.
• If there are multiple paths to the same destination from a single routing protocol, the best path is selected based on the lowest metric.
• The parameters used to determine the metrics differ for each routing protocol.
• If there are multiple paths to the same destination with equal metrics, all routes are installed in the routing table and load balancing is done between them.

Prefix lengths

• Let's assume that a router has learned the following routes:

1. EIGRP (internal): 192.168.32.0/26
2. RIP: 192.168.32.0/24
3. OSPF: 192.168.32.0/19

• What will be installed in the routing table?
• All of them, because they have different prefix lengths, so they are considered different destinations.

Making Forwarding Decisions

• Longer prefixes are always preferred!
• Let's look at the three routes previously installed in the routing table.

router# show ip route
     ....
     D   192.168.32.0/26 [90/25789217] via 10.1.1.1
     R   192.168.32.0/24 [120/4] via 10.1.1.2
     O   192.168.32.0/19 [110/229840] via 10.1.1.3
     ....

• A packet destined to 192.168.32.1 matches all routes but it is forwarded according to the EIGRP route because of the longest prefix length.
• Similarly, a packet to 192.168.32.254 would be forwarded 10.1.1.2 (RIP route) because of the most specific match.

IP Classless

• Only affects the forwarding process, not the building of the routing table.
• If IP classless isn't configured (no ip classless), the router won't forward packets to supernets.
• If one part of a major network is known, but the subnet toward which the packet is destined within that major network is unknown, the packet is dropped.
• The router only uses the default route if the destination major network doesn't exist in the routing table at all.
• Configuring ip classless allows the router to ignore the classful boundaries of the networks in its routing table and simply route to the longest prefix match it can find.

Summary

• Making a forwarding decision actually consists of three processes: the routing protocols, the routing table, and the forwarding process.
• Installing a route into the routing table: the lowest administrative distance wins; if there are multiple paths to the same destination, compare the metric (install all equal-cost routes).
• Making a forwarding decision: the longest prefix wins.
• The process is illustrated in the diagram below.

References

Route Selection in Cisco Routers
What is Administrative Distance?