Hey there! If you're diving into the world of networking, especially with Cisco routers, understanding how to analyze routing tables is a crucial skill. Let's break down some essential parameters you should commit to memory to make sense of routing tables and interpret forwarding decisions effectively. ## Routing Protocol Codes: Deciphering the Abbreviations When you look at a Cisco router's routing table, you'll notice a series of codes next to each route. These codes indicate how the router learned about each network. Here's a handy list of the most common routing protocol codes: - **C** — **Connected**: Directly connected network. - **S** — **Static**: Manually configured static route. - **S*** — **Static Default Route**: A default route configured by an administrator. - **R** — **RIP**: Learned through the Routing Information Protocol. - **B** — **BGP**: Learned via the Border Gateway Protocol. - **D** — **EIGRP**: Learned through the Enhanced Interior Gateway Routing Protocol. - **EX** — **EIGRP External**: Routes redistributed into EIGRP from an external source. - **O** — **OSPF**: Learned via the Open Shortest Path First protocol. - **i** — **IS-IS**: Learned through the Intermediate System to Intermediate System protocol. **Tip:** Memorizing these codes helps you quickly identify the source of each route, which is vital for troubleshooting and understanding how your network learns about different paths. ## Administrative Distance (AD): Trust Levels of Routing Information Administrative Distance is a value that routers use to rate the trustworthiness of the routing information they receive from different protocols. The lower the AD, the more trustworthy the source. Here's a list of default AD values for common routing sources: - **Connected** — **0** - **Static** — **1** - **EIGRP** — **90** - **OSPF** — **110** - **IS-IS** — **115** - **RIP** — **120** - **External EIGRP** — **170** - **Unknown** — **255** (Considered unreachable) **Why Does AD Matter?** Suppose your router learns about the same network from both OSPF and RIP. Since OSPF has an AD of **110** and RIP has an AD of **120**, the router will prefer the OSPF route because it has a lower AD, meaning it's deemed more reliable. ## How to Use This Information When analyzing a routing table: 1. **Identify the Route Source:** - Look at the routing protocol code to see how the route was learned. 2. **Check the Administrative Distance:** - If multiple routes to the same network exist, the router will prefer the one with the lowest AD. 3. **Consider the Metric:** - If routes have the same AD (e.g., learned from the same protocol), the router uses the metric to determine the best path. ## Example: Interpreting a Routing Table Entry Let's look at an example to tie it all together: ``` O 192.168.20.0/24 [110/2] via 10.0.0.2, 00:00:15, GigabitEthernet0/0 ``` Breaking it down: - **O**: Route learned via **OSPF**. - **192.168.20.0/24**: Destination network and subnet mask. - **[110/2]**: - **110**: Administrative Distance for OSPF. - **2**: OSPF metric (cost) to reach the network. - **via 10.0.0.2**: Next-hop IP address. - **00:00:15**: Time since the route was last updated. - **GigabitEthernet0/0**: Outgoing interface. **Analysis:** - Since the AD is **110**, this route is preferred over any routes to the same network learned via RIP (AD 120) or IS-IS (AD 115). - If another OSPF route to the same network exists with a higher metric, this route would still be preferred due to the lower metric of **2**. ## Practice Makes Perfect Familiarize yourself with these codes and values by examining real routing tables: - **Use Simulation Tools:** Platforms like Cisco Packet Tracer or GNS3 allow you to build networks and view routing tables. - **Compare Routes:** See how the router chooses routes when multiple options are available. - **Modify AD and Metrics:** Experiment by changing administrative distances and metrics to see how routing decisions are affected. ## Quick Reference Table ### Routing Protocol Codes | Code | Protocol | |------|----------------------------------------| | C | Connected | | S | Static | | S* | Static Default Route | | R | RIP | | B | BGP | | D | EIGRP | | EX | EIGRP External | | O | OSPF | | i | IS-IS | ### Administrative Distances | Protocol | AD Value | |-----------------|----------| | Connected | 0 | | Static | 1 | | EIGRP | 90 | | OSPF | 110 | | IS-IS | 115 | | RIP | 120 | | External EIGRP | 170 | | Unknown | 255 | ## Final Thoughts By committing these codes and administrative distances to memory, you'll be better equipped to interpret routing tables and understand how routers make forwarding decisions. This knowledge is essential for network troubleshooting and optimization. Keep practicing, and soon you'll be navigating routing tables like a pro! --- Happy networking! ## Additional resources - Introduction to Routing Dynamically: [https://www.ciscopress.com/articles/article.asp?p=2180210&seqNum=12](https://www.ciscopress.com/articles/article.asp?p=2180210&seqNum=12)