administrative distance and routing protocols

What is a Routing Table -Explained

What is a Routing Table -Explained

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A routing table has information usually viewed in table format to decide where to send data packets. All IP-enabled devices, including routers, direct a packet to the destination using routing tables.

The router gets route information from the routing table and selects the best path for the destination. Each packet has information about its source and destination. The router examines the packet and matches it to the routing table entry, providing the best match for its destination. Then, the router sends the packet to the next hop on its route across the network.

We can configure routes manually or dynamically. The static routes do not change unless a network administrator manually changes them, but the dynamic routes automatically update and change according to routing protocols. The routing protocols exchange information about the network topology and network changes and update the routing table.

Dynamic routing protocols also allow devices to listen to the network and react to occurrences like device failures and network congestion. The routing table is a data file storing route information about directly connected and remote networks.

  • Directly connected routes—When configuring and activating the interface, the router adds adirectly connected route against the interface.
  • Remote routes—These are the routes from remote networks to other routers. We can configure these routes statically or dynamically.

Routing Table Sources

We can check the routing information on a Cisco router using the show ip route command. The router also provides additional route information, including the source of the route, with this command. The following are the different sources of the routing entries.

  • Local Route interfaces—The router adds the route when we configure and activate the router interface. This entry is available in all IOS for IPv6, but for IPv4, the option is available only in IOS 15 or newer versions.
  • Directly connected interfaces—The routes that were directly connected were added to the routing table when we activated and configured the interface.
  • Static routes– The static route is added to the routing table when a route is manually configured, and the exit interface is active.
  • Dynamic routing protocol—The routing protocols that dynamically learn the network’s information and add the information to the routing table, such as RIP, EIGRP, and OSPF.

We can find the routing entry sources with a code. The code tells us the source of the route information. The figure below illustrates the codes of the route sources, including the entries in a single route:

Routing Table

Some common codes are:

  • C–This code is for the directly connected network.
  • L– This code is for the Local Router/Switch Interface route.
  • S– We can find a static route with this code.
  • D– This is the identification code for a dynamically learned network from another EIGRP router.
  • O– This code Identifies a dynamically learned network from another router using the OSPF.
  • R– This code Identifies a dynamically learned network from another router using the RIP.
  • S*– This is the default route.

Remote Network Routing Entries

Understanding the content of an IPv4 and IPv6 routing table is most important. We have marked the route to destination network 172.16.17.0 in the above figure. The marked entry for 172.16.17.0 identifies the following information:

  • Route source—This entry identifies how the router adds this route. In this example, the entry is “D”, meaning the router learns this route from the dynamic routing protocol EIGRP.
  • Destination network– This is the entry for remote network Identification. In this example, the remote network is 172.16.17.0.
  • Administrative distance—This is the trustworthiness of the route source. Lower values indicate amore trustworthy route to the destination network.
  • Metric—The metric is the cost of each available route, so the router selects the most cost-effective path. The Lower values indicate preferred routes to the destination.
  • Next-hop– This is the IPv4 address of the next connected router to send the packet.
  • Route timestamp– This entry shows the timing since the route was added.
  • Outgoing interface–This entry identifies the exit interface of the router that sends a packet toward the destination.

Directly Connected Interfaces

A newly Installed router, without any configured and active interface, has an empty routing table, as shown in the figure below.

Before the interface state is up/up and added to the routing table, the interface must be assigned a valid IPv4 or IPv6 address and must not be shut down. It should also be able to receive the carrier signals from another device, e.g., router, switch, host, etc.

When the interface is up, the network of that interface is added automatically to the routing table as a directly connected network. For example, when we configure the interfaces of Router5 with IPv4 addresses and issue the no shutdown command, it receives the carrier signals from the router and hosts. It updates the routing table from an empty routing table, as shown in the figure below.

Directly Connected Routes (C) and Local Routes (L) Entries

The properly configured connected interface creates two routing table entries. The figure below displays the IPv4 routing table entries on Router5 for the directly connected network 172.16.19.0. The directly connected router interfaces’ routing entries contain the following information:

  • Route source– This entry identifies the route source. Directly connected interfaces have two route source codes. “C” and “L”. The “C” is for the directly connected network, and the “L” is for the IPv4 address assigned to the router interface.
  • Destination network– The address of the remote network.
  • Outgoing interface–This is the router’s outgoing interface for the destination network.

Another route it is showing is the Local (L) route. The difference between Local and Directly connected routes is that a directly connected route is a route to a network that is directly attached to the interface and Local is the route that belongs to the router/switch itself in the above example you can see that in the directly connected route the destination is 172.16.19.0/24 address but the destination in Local route is 172.16.19.1 which is configured on the same Router (Router5)

What is Administrative Distance – Explain

What is Administrative Distance – Explain

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In network environments, routers often receive multiple routes to the same destination from different sources, such as routing protocols (e.g., OSPF, EIGRP, RIP), static routes, or directly connected networks. To select the most trustworthy route, Cisco routers use Administrative Distance (AD), a critical concept in the Cisco Certified Network Associate (CCNA) curriculum. This article explains AD’s purpose, default values, and practical applications.

Note:- At the end of this article, the assessment test is waiting for you about the Administrative Distance

What is Administrative Distance (AD)?

Administrative Distance (AD) is a numeric value between 0 and 255 that routers use to evaluate the reliability of a routing information source. The lower the AD value, the more trustworthy the source. When multiple routes to the same network exist, the router compares their ADs and selects the route with the lowest AD. Only if ADs are equal does the router use metrics (e.g., hop count, cost) to choose the best path.

For example, if the routing table has more than one route source for the same destination network, a router configured for that network will use both Enhance Interior Routing Gateway Protocol (EIGRP) and Routing Information Protocol (RIP).

So, both routing protocols may decide a different path to the destination based on that routing protocol’s metrics because RIP selects a path based on hop count, and EIGRP selects a path based on its composite metric. So, the administrator distance is the value that tells the router which path is best and which path to use first.

As I initially asked, it is a numeric value ranging from 0 to 255. A smaller AD value is more reliable and trustworthy. Therefore, the best AD value is 0, and the worst is 255. The static route AD is 1, whereas the AD of EIGRP is 110, so the static route is more reliable and trustworthy.

When there are static and EIGRP routes to the same destination, the router chooses the static route because it has the lowest AD value.

If EIGRP and OSPF are configured to the same destination, the router will choose EIGRP because the AD of EIGRP is 90 and OSPF’s is 110. So, the router will determine the route with the lowest value, EIGRP its AD, and OSPF’s Thus,

Default Administrative Distance Values

The table below illustrates the default router administrative distance values. We can change and modify the administrative distance of a routing protocol through the distance command in the routing sub-configuration mode. However, modifying the AD value can lead to routing loops and black holes, so use caution if you change it.

Below are Cisco’s default AD values for familiar route sources:

Route SourceDefault AD
Directly Connected Interface0
Static Route1
Enhanced Interior Gateway Routing Protocol (EIGRP) Summary Route5
External Border Gateway Protocol (eBGP)20
Internal EIGRP90
Open Shortest Path First (OSPF)110
Intermediate System-to-Intermediate System (IS-IS)115
Routing Information Protocol (RIP)120
External EIGRP170
Internal BGP (iBGP)200

Key Notes:

  • Directly connected networks (AD 0) are always preferred.
  • Static routes (AD 1) override dynamically learned routes unless their AD is manually increased.
  • EIGRP has separate ADs for internal (90) and external (170) routes.

How Administrative Distance Works: Examples

Example 1: Static Route vs. OSPF

A router receives a route to 10.0.0.0/24 via:

  • A static route (AD = 1).
  • OSPF (AD = 110).

Result: The static route is preferred due to its lower AD.

Example 2: RIP vs. EIGRP

A router learns about 192.168.1.0/24 through:

  • RIP (AD = 120).
  • EIGRP (AD = 90).

Result: EIGRP’s route is chosen because of its lower AD.

Administrative Distance vs. Metric

It’s crucial to distinguish between AD and metric:

  • AD determines trustworthiness between different routing sources.
  • Metric determines the best path within the same routing protocol.

For example:

  • Two OSPF routes to the exact network use cost (metric) to decide the best path.
  • An OSPF route (AD 110) will always be preferred over a RIP route (AD 120), even if the RIP route has a better metric (e.g., fewer hops).

Configuring Administrative Distance

Administrators can modify AD values for flexibility.

Example 1: Changing Static Route AD

To configure a static route with AD 150 (instead of the default 1):

Router(config)# ip route 192.168.1.0 255.255.255.0 10.0.0.2 150

Example 2: Adjusting OSPF AD

To set OSPF’s AD to 100 globally:

Router(config)# router ospf 1
Router(config-router)# distance 100

Caution: Incorrect AD configurations can lead to routing loops or suboptimal paths.

Troubleshooting with Administrative Distance

Common scenarios where AD impacts routing decisions:

  1. Unexpected Route Selection: A static route might override a dynamic route due to lower AD.
  2. Backup Routes: Configuring a floating static route (with higher AD than dynamic protocols) ensures it’s used only if the primary route fails.

ip route 192.168.1.0 255.255.255.0 10.0.0.2 200 # Backup route (AD 200)

This route activates only if the primary OSPF route (AD 110) becomes unavailable.

Conclusion

Administrative Distance (AD) is a foundational concept in Cisco networking that ensures routers select the most reliable path when multiple routes exist. Network engineers can optimize traffic flow and maintain robust network designs by understanding default AD values, configuring custom ADs, and troubleshooting route preferences. Mastery of AD is essential for CCNA certification and real-world network management.

Key Takeaways:

  • Lower AD = More trustworthy route.
  • AD is prioritized over metrics.
  • Always verify AD values when troubleshooting routing issues.

MCQs for CCNA Exam Preparation

CCNA Administrative Distance Self-Assessment

CCNA Administrative Distance Self-Assessment

1. What is the primary purpose of Administrative Distance (AD)?

2. Which of the following has the lowest default Administrative Distance?

3. What is the default Administrative Distance for OSPF?

4. When two routes to the same destination have the same Administrative Distance, what does the router use to choose the best path?

5. A router receives a route via RIP (AD 120) and EIGRP (AD 90). Which route is preferred?

6. What is a floating static route?

7. Which statements are true about Administrative Distance and metrics? (Select all that apply)

8. Why should changing the default Administrative Distance be done cautiously?

9. Which route source has a default AD of 20?

10. What is the Administrative Distance of a directly connected network?