Routing protocol codes are abbreviations displayed in a router's routing table that identify the source or method by which a route was learned. When you execute the 'show ip route' command on a Cisco router, each route entry is preceded by a single letter or combination of letters indicating how th…Routing protocol codes are abbreviations displayed in a router's routing table that identify the source or method by which a route was learned. When you execute the 'show ip route' command on a Cisco router, each route entry is preceded by a single letter or combination of letters indicating how that particular route was added to the routing table.
The most common routing protocol codes include:
**C (Connected)** - Routes for networks that are physically attached to the router's interfaces. These routes are automatically created when an interface is configured with an IP address and brought up.
**L (Local)** - Represents the specific IP address assigned to the router's interface itself, with a /32 subnet mask.
**S (Static)** - Routes that have been manually configured by a network administrator using the 'ip route' command.
**R (RIP)** - Routes learned through the Routing Information Protocol, a distance-vector routing protocol.
**O (OSPF)** - Routes discovered via Open Shortest Path First, a link-state routing protocol commonly used in enterprise networks.
**D (EIGRP)** - Routes learned through Enhanced Interior Gateway Routing Protocol, Cisco's advanced distance-vector protocol.
**B (BGP)** - Routes received from Border Gateway Protocol, typically used for inter-domain routing between autonomous systems.
**i (IS-IS)** - Routes from Intermediate System to Intermediate System protocol.
Additional codes exist for specific route types within protocols. For example, OSPF uses 'O IA' for inter-area routes, 'O E1' and 'O E2' for external routes. EIGRP uses 'D EX' for external EIGRP routes.
Understanding these codes is essential for troubleshooting network connectivity issues, as they help administrators quickly identify how routes were learned and determine the appropriate protocol handling each network path. The routing table legend, displayed at the top of the 'show ip route' output, provides a complete reference of all codes in use.
Routing Protocol Code - CCNA Exam Guide
Why is Routing Protocol Code Important?
Understanding routing protocol codes is essential for network engineers because they appear in routing tables and help identify how routes were learned. When troubleshooting network connectivity issues, these codes tell you the source of each route, which is critical for diagnosing problems and understanding network behavior.
What is a Routing Protocol Code?
A routing protocol code is a single letter or combination of letters that appears in a router's routing table to indicate how a particular route was learned or configured. Each code represents a different routing method or protocol.
Common Routing Protocol Codes:
C - Connected - Networks that are attached to the router's interfaces L - Local - The specific IP address assigned to the router's interface S - Static - Routes manually configured by an administrator R - RIP - Routes learned via Routing Information Protocol O - OSPF - Routes learned via Open Shortest Path First D - EIGRP - Routes learned via Enhanced Interior Gateway Routing Protocol B - BGP - Routes learned via Border Gateway Protocol i - IS-IS - Routes learned via Intermediate System to Intermediate System * - Candidate default route
How Routing Protocol Codes Work:
When you execute the show ip route command, the router displays its routing table with each entry prefixed by a code. For example:
C 192.168.1.0/24 is connected, GigabitEthernet0/0 O 10.0.0.0/8 [110/20] via 192.168.1.1 S 172.16.0.0/16 [1/0] via 192.168.1.254
The letter at the beginning tells you the route's origin. Connected and Local routes have the highest trust because they represent physical connections. Static routes are trusted because an administrator configured them. Dynamic routing protocol codes indicate the route was learned through protocol exchanges with neighboring routers.
Administrative Distance Connection:
Each routing protocol code is associated with an administrative distance (AD), which determines route preference when multiple protocols advertise the same destination. Lower AD values are preferred.
Exam Tips: Answering Questions on Routing Protocol Code
1. Memorize all codes thoroughly - Know that C means Connected, S means Static, O means OSPF, D means EIGRP, and R means RIP.
2. Understand the difference between C and L - Connected (C) represents the network, while Local (L) represents the specific interface IP address with a /32 mask.
3. Practice reading routing tables - Exam questions often show partial routing table output and ask you to identify route sources.
4. Link codes to administrative distances - Questions may ask which route will be preferred, requiring you to know both the code and its associated AD value.
5. Remember less common codes - Be aware of codes like B for BGP and i for IS-IS, even if they appear less frequently.
6. Look for context clues - If a question mentions a route with [110/20], the 110 indicates OSPF administrative distance, confirming an O code.
7. Distinguish between interior and exterior protocols - OSPF, EIGRP, and RIP are interior protocols, while BGP is an exterior protocol.
8. Pay attention to the asterisk (*) - This indicates a candidate default route and may appear alongside other codes.
9. Practice show ip route command interpretation - Many simulation questions require you to analyze routing table output to troubleshoot connectivity.