IP Connectivity


Interpret the components of routing table

  • Routing protocol code
  • Prefix
  • Network mask
  • Next hop
  • ¬†Administrative distance
  • Metric
  • Gateway of last resort

The routing table is a core component of any router’s operation. It stores information about paths to different network destinations. Each entry in a routing table provides the necessary details to determine the best path for packets to reach their intended destination.

This denotes the source of the route. Each routing protocol (or route source) has an associated code. Some common ones include:

  • C: Connected network
  • S: Static route
  • R: RIP (Routing Information Protocol)
  • O: OSPF (Open Shortest Path First)
  • E: EIGRP (Enhanced Interior Gateway Routing Protocol)
  • B: BGP (Border Gateway Protocol)
  • i: IS-IS
Router# show ip route

Codes: C - connected, S - static, D - EIGRP, M - mobile, B - BGP
O - OSPF, IA - OSPF inter area
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default

C is directly connected, FastEthernet0/0
S [1/0] via
D [90/30720] via, 00:00:12, FastEthernet0/1
O [110/10] via, 00:00:10, FastEthernet0/2
S* [1/0] via

A routing table in a Cisco router is typically displayed using the show ip route command.


    • 3.1.a: C indicates that this is a Connected network.
    • 3.1.b: Prefix is
    • 3.1.d: This network is directly connected to interface FastEthernet0/0.
    • 3.1.a: S indicates a Static route.
    • 3.1.b: Prefix is
    • 3.1.d: Next Hop is
    • 3.1.e: Administrative Distance is 1.
    • 3.1.f: Metric is 0.
    • 3.1.a: D represents the EIGRP routing protocol.
    • 3.1.b: Prefix is
    • 3.1.d: Next Hop is via interface FastEthernet0/1.
    • 3.1.e: Administrative Distance is 90 (default for internal EIGRP routes).
    • 3.1.f: Metric (also known as the EIGRP composite metric) is 30720.
    • 3.1.a: O represents OSPF.
    • 3.1.b: Prefix is
    • 3.1.d: Next Hop is via interface FastEthernet0/2.
    • 3.1.e: Administrative Distance is 110.
    • 3.1.f: Metric is 10.
    • This represents the default route, often called the Gateway of Last Resort.
    • 3.1.a: S* indicates it’s a Static route and is a candidate default.
    • 3.1.d: Next Hop is

By understanding the different components of a Cisco router’s routing table, you can discern the router’s knowledge about the network and its configured routes.



  • Default route
  • Network route
  • Host route
  • Floating static

Regional ISP Network

You are a network engineer for a regional Internet Service Provider (ISP). Your ISP provides internet connectivity to various small towns in the region. The network has routers deployed at key junctions. Each router has routes from various sources: static routes, OSPF (an internal routing protocol), and BGP (used for external routes).

A user from the town of “Pineville” wants to access a website whose IP address is

Router R1# show ip route

Destination Network Mask Next Hop AD Metric Protocol
--------------------------------------------------------------------- 110 50 O (OSPF) 1 - S (Static) 20 - B (BGP) 1 - S (Static)

Router R1 has the following entries in its routing table:

  1. via OSPF, Administrative Distance (AD) 110, Metric 50, Next hop:
  2. via Static Route, AD 1, Next hop:
  3. via BGP, AD 20, Next hop:
  4. (default route) via Static Route, AD 1, Next hop:

Forwarding Decision:

When the router receives a packet destined for, it goes through the following decision-making process:

  1. 3.2.a Longest Match:
    • The router checks for the longest prefix match (most specific route).
    • Here,,, and all match the destination.
    • Among them, is the longest match (most specific).
  2. 3.2.b Administrative Distance:
    • The router then checks the AD for each of these routes to decide which one is the most trustworthy.
    • The static route has the lowest AD of 1. OSPF has an AD of 110, and BGP has an AD of 20.
    • Thus, the static route ( with AD 1 is chosen.
  3. 3.2.c Routing Protocol Metric:
    • If there were multiple static routes to the same destination with the same prefix length, the router would then look at the metric to make a decision. In this scenario, there’s only one static route, so the router doesn’t need to check the metric in this case.

The router R1 forwards the packet to the next hop based on the static route.

This scenario showcases how a router uses the longest prefix match first to narrow down the potential routes. It then uses administrative distance and, if necessary, the routing protocol metric to determine the best path for packet forwarding.


Scenario: University Campus Network

You’re a network engineer for a large university. The university’s main campus has several buildings, including an administration block, engineering faculty, and a library. These buildings are interconnected with high-speed Ethernet links, and the university administration has decided to use OSPFv2 as the routing protocol within the main campus network.

You are tasked with setting up a single area OSPFv2 configuration for the university’s main campus routers.

Initial Configuration:

  1. Router Names and Functions:
    • AdminRouter: Located in the administration block.
    • EngRouter: Located in the engineering faculty.
    • LibraryRouter: Positioned in the library.
  2. IP Addresses: Each router interface facing the campus network has IP addresses in the 10.0.x.0/24 range, where x is a unique number for each link.

Configuration Steps:

1. Configure Basic OSPF Settings:

On each router:

Router> enable
Router# config terminal
Router(config)# router ospf 1
network 10.0.x.0 area 0

Here, x should be replaced with the actual number for each interface’s network. The area 0 command specifies that these routers belong to OSPF Area 0, which is the backbone area in OSPF.

2. Point-to-Point Link Configuration:

Suppose there’s a point-to-point link between AdminRouter and EngRouter. On both routers, the link’s OSPF network type needs to be specified as point-to-point.

On both AdminRouter and EngRouter:

Router(config-if)# ip ospf network point-to-point

3. Broadcast Network & DR/BDR Selection:

The link between EngRouter and LibraryRouter is a broadcast Ethernet network. On an OSPF broadcast network, one router will be elected as the Designated Router (DR) and another as the Backup Designated Router (BDR).

By default, routers will automatically participate in DR/BDR elections on broadcast networks. After some time, you can verify the DR/BDR election results:

Router# show ip ospf neighbor

This command will show OSPF neighbor adjacencies and will also indicate which router is the DR and which one is the BDR for each broadcast network.

4. Setting the Router ID:

The router ID is a unique identifier for each OSPF router. Even though OSPF will automatically select a Router ID based on the highest IP address of any of the router’s active interfaces, it’s often a best practice to manually set this value to avoid unpredictable behavior.

On AdminRouter:

Router(config-router)# router-id

On EngRouter:

Router(config-router)# router-id

On LibraryRouter:

Router(config-router)# router-id


After setting up the above configurations, you should verify OSPF operations.

To check OSPF neighbor adjacencies:

Router# show ip ospf neighbor

To review the OSPF routing table:

Router# show ip route ospf

This scenario provides an overview of setting up a single area OSPFv2. Specific configurations might vary based on network designs, goals, and router models or OS versions.


First Hop Redundancy Protocols (FHRPs) are network protocols that provide redundancy for the “first hop” in an IP network, ensuring that the local network’s default gateway (typically a router or switch’s IP address) is available even if the primary physical device becomes unavailable. Essentially, FHRPs provide fault tolerance for the default gateway of devices in a LAN.

Purpose of FHRPs:

  1. Redundancy: Ensure continuous availability of the default gateway.
  2. Failover: Provide a mechanism to switch to a backup gateway in case of primary gateway failure, ensuring uninterrupted network operations.
  3. Load Sharing: Some FHRPs can distribute client traffic between multiple gateways, leveraging all available network resources.

  1. Virtual IP Address: FHRPs create a virtual IP address (VIP) that clients use as their default gateway. This VIP remains consistent, regardless of which physical device currently owns or responds to it.
  2. Election Process: In case multiple devices can act as the default gateway, FHRPs have an election process to determine which device will actively use the VIP.
  3. Health Monitoring: FHRPs continuously monitor the availability of the devices participating in the protocol, ensuring immediate action if one becomes unavailable.
  4. Failback: If the primary device becomes available again after a failure, FHRPs can be configured to transition the VIP back to the primary device.

  1. Active/Standby: One device is actively using the VIP while the other(s) remain in standby mode, ready to take over if needed.
  2. Preemption: Determines if a higher priority device will reclaim the role of the active device when it comes back online after a failure.
  3. Priority Values: Used to determine which device becomes active or standby. Higher priority values typically indicate a preferred device.
  4. Hello & Hold Timers: Determine how often health-check messages (Hello) are sent and how long to wait after missing messages before declaring a peer as failed.
  5. Virtual MAC Address: Along with the VIP, a virtual MAC address is also typically created so that ARP requests for the VIP get the virtual MAC. This ensures seamless failover.

  1. HSRP (Hot Standby Router Protocol): A Cisco proprietary protocol that provides network redundancy for IP networks, ensuring the availability of a network default gateway.
  2. VRRP (Virtual Router Redundancy Protocol): An open standard similar to HSRP. Different vendors support VRRP, making it suitable for multi-vendor environments.
  3. GLBP (Gateway Load Balancing Protocol): Another Cisco proprietary protocol. Unlike HSRP and VRRP (which are mostly active/standby), GLBP allows load balancing across multiple gateways.

In conclusion, FHRPs are vital in ensuring high availability in network architectures, ensuring that end devices can always reach external networks even if their primary default gateway fails.