IP Routing Table
Summary
This topic describe the structure of a routing table. Start learning CCNA 200-301 for free right now!!
Note: Welcome: This topic is part of Module 14 of the Cisco CCNA 2 course, for a better follow up of the course you can go to the CCNA 2 section to guide you through an order.
Table of Contents
Route Sources
How does a router know where it can send packets? It creates a routing table that is based on the network in which it is located.
A routing table contains a list of routes to known networks (prefixes and prefix lengths). The source of this information is derived from the following:
- Directly connected networks
- Static routes
- Dynamic routing protocols
In the figure, R1 and R2 are using the dynamic routing protocol OSPF to share routing information. In addition, R2 is configured with a default static route to ISP.
Click each button to see the full routing table for each router after directly connected networks, static routing, and dynamic routing is configured. The rest of this topic will demonstrate how these tables are populated.
In the routing tables for R1 and R2, notice that the sources for each route are identified by a code. The code identifies how the route was learned. For instance, common codes include the following:
- L – Identifies the address assigned to a router interface. This allows the router to efficiently determine when it receives a packet for the interface instead of being forwarded.
- C – Identifies a directly connected network.
- S – Identifies a static route created to reach a specific network.
- O – Identifies a dynamically learned network from another router using the OSPF routing protocol.
- * – This route is a candidate for a default route.
Routing Table Principles
There are three routing table principles as described in the table. These are issues that are addressed by the proper configuration of dynamic routing protocols or static routes on all the routers between the source and destination devices.
| Routing Table Principle | Example |
|---|---|
| Every router makes its decision alone, based on the information it has in its own routing table. |
|
| The information in a routing table of one router does not necessarily match the routing table of another router. | Just because R1 has route in its routing table to a network in the internet via R2, that does not mean that R2 knows about that same network. |
| Routing information about a path does not provide return routing information. | R1 receives a packet with the destination IP address of PC1 and the source IP address of PC3. Just because R1 knows to forward the packet out its G0/0/0 interface, doesn’t necessarily mean that it knows how to forward packets originating from PC1 back to the remote network of PC3. |
Routing Table Entries
As a network administrator, it is imperative to know how to interpret the content of IPv4 and IPv6 routing tables. The figure displays IPv4 and IPv6 routing table entries on R1 for the route to remote network 10.0.4.0/24 and 2001:db8:acad:4::/64. Both these routes were learned dynamically from the OSPF routing protocol.
In the figure, the numbers identify the following information:
- Route source – This identifies how the route was learned.
- Destination network (prefix and prefix length) – This identifies the address of the remote network.
- Administrative distance – This identifies the trustworthiness of the route source. Lower values indicate preferred route source.
- Metric – This identifies the value assigned to reach the remote network. Lower values indicate preferred routes.
- Next-hop – This identifies the IP address of the next router to which the packet would be forwarded.
- Route timestamp – This identifies how much time has passed since the route was learned.
- Exit interface – This identifies the egress interface to use for outgoing packets to reach their final destination.
Note: The prefix length of the destination network specifies the minimum number of far-left bits that must match between the IP address of the packet and the destination network (prefix) for this route to be used.
Directly Connected Networks
Before a router can learn about any remote networks, it must have at least one active interface configured with an IP address and subnet mask (prefix length). This is known as a directly connected network or a directly connected route. Routers add a directly connected route to its routing table when an interface is configured with an IP address and is activated.
A directly connected network is denoted by a status code of C in the routing table. The route contains a network prefix and prefix length.
The routing table also contains a local route for each of its directly connected networks, indicated by the status code of L. This is the IP address that is assigned to the interface on that directly connected network. For IPv4 local routes the prefix length is /32 and for IPv6 local routes the prefix length is /128. This means the destination IP address of the packet must match all the bits in the local route for this route to be a match. The purpose of the local route is to efficiently determine when it receives a packet for the interface instead of a packet that needs to be forwarded.
Directly connected networks and local routes are shown in the following output.
R1# show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
(Output omitted)
C 10.0.1.0/24 is directly connected, GigabitEthernet0/0/0
L 10.0.1.1/32 is directly connected, GigabitEthernet0/0/0
R1#
R1# show ipv6 route
IPv6 Routing Table - default - 10 entries
Codes: C - Connected, L - Local, S - Static, U - Per-user Static route
(Output omitted)
C 2001:DB8:ACAD:1::/64 [0/0]
via GigabitEthernet0/0/0, directly connected
L 2001:DB8:ACAD:1::1/128 [0/0]
via GigabitEthernet0/0/0, receive
R1#
Static Routes
After directly connected interfaces are configured and added to the routing table, static or dynamic routing can be implemented for accessing remote networks.
Static routes are manually configured. They define an explicit path between two networking devices. Unlike a dynamic routing protocol, static routes are not automatically updated and must be manually reconfigured if the network topology changes. The benefits of using static routes include improved security and resource efficiency. Static routes use less bandwidth than dynamic routing protocols, and no CPU cycles are used to calculate and communicate routes. The main disadvantage to using static routes is the lack of automatic reconfiguration if the network topology changes.
Static routing has three primary uses:
- It provides ease of routing table maintenance in smaller networks that are not expected to grow significantly.
- It uses a single default route to represent a path to any network that does not have a more specific match with another route in the routing table. Default routes are used to send traffic to any destination beyond the next upstream router.
- It routes to and from stub networks. A stub network is a network accessed by a single route, and the router has only one neighbor.
The figure shows an example of stub networks. Notice that any network attached to R1 would only have one way to reach other destinations, whether to networks attached to R2, or to destinations beyond R2. This means that networks 10.0.1.0/24 and 10.0.2.0/24 are stub networks and R1 is a stub router.
In this example, a static route can be configured on R2 to reach the R1 networks. Additionally, because R1 has only one way to send out non-local traffic, a default static route can be configured on R1 to point to R2 as the next hop for all other networks.
Static Routes in the IP Routing Table
For demonstrating static routing, the topology in the figure is simplified to show only one LAN attached to each router. The figure shows IPv4 and IPv6 static routes configured on R1 to reach the 10.0.4.0/24 and 2001:db8:acad:4::/64 networks on R2. The configuration commands are for demonstration only and are discussed in another module.
The output shows the IPv4 and IPv6 static routing entries on R1 that can reach the 10.0.4.0/24 and 2001:db8:acad:4::/64 networks on R2. Notice that both routing entries use the status code of S indicating that the route was learned by a static route. Both entries also include the IP address of the next hop router, via ip-address. The static parameter at the end of the command displays only static routes.
R1# show ip route static
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
(output omitted)
10.0.0.0/8 is variably subnetted, 8 subnets, 2 masks
S 10.0.4.0/24 [1/0] via 10.0.3.2
R1# show ipv6 route static
IPv6 Routing Table - default - 8 entries
Codes: C - Connected, L - Local, S - Static, U - Per-user Static route
(output omitted)
S 2001:DB8:ACAD:4::/64 [1/0]
via 2001:DB8:ACAD:3::2
Dynamic Routing Protocols
Dynamic routing protocols are used by routers to automatically share information about the reachability and status of remote networks. Dynamic routing protocols perform several activities, including network discovery and maintaining routing tables.
Important advantages of dynamic routing protocols are the ability to select a best path, and the ability to automatically discover a new best path when there is a change in the topology.
Network discovery is the ability of a routing protocol to share information about the networks that it knows about with other routers that are also using the same routing protocol. Instead of depending on manually configured static routes to remote networks on every router, a dynamic routing protocol allows the routers to automatically learn about these networks from other routers. These networks, and the best path to each, are added to the routing table of the router, and identified as a network learned by a specific dynamic routing protocol.
The figure shows routers R1 and R2 using a common routing protocol to share network information.
Dynamic Routes in the IP Routing Table
A previous example used static routes to the 10.0.4.0/24 and 2001:db8:acad:4::/64 networks. These static routes are no longer configured and OSPF is now being used to dynamically learn all the networks connected to R1 and R2. The following examples show the IPv4 and IPv6 OSPF routing entries on R1 that can reach these networks on R2. Notice that both routing entries use the status code of O to indicate the route was learned by the OSPF routing protocol. Both entries also include the IP address of the next-hop router, via ip-address.
Note: IPv6 routing protocols use the link-local address of the next-hop router.
Note: OSPF routing configuration for IPv4 and IPv6 is beyond the scope of this course.
R1# show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
(output omitted for brevity)
O 10.0.4.0/24 [110/50] via 10.0.3.2, 00:24:22, Serial0/1/1
O 10.0.5.0/24 [110/50] via 10.0.3.2, 00:24:15, Serial0/1/1
R1# show ipv6 route
IPv6 Routing Table - default - 10 entries
(Output omitted)
NDr - Redirect, RL - RPL, O - OSPF Intra, OI - OSPF Inter
O 2001:DB8:ACAD:4::/64 [110/50]
via FE80::2:C, Serial0/1/1
O 2001:DB8:ACAD:5::/64 [110/50]
via FE80::2:C, Serial0/1/1
Default Route
A default route is similar to a default gateway on a host. The default route specifies a next-hop router to use when the routing table does not contain a specific route that matches the destination IP address.
A default route can be either a static route or learned automatically from a dynamic routing protocol. A default route has an IPv4 route entry of 0.0.0.0/0 or an IPv6 route entry of ::/0. This means that zero or no bits need to match between the destination IP address and the default route.
Most enterprise routers have a default route in their routing table. This is to reduce the number of routes in a routing table.
A router, such as a home or small office router that only has one LAN, may reach all its remote networks through a default route. This is useful when the router has only directly connected networks and one exit point to a service provider router.
In the figure, routers R1 and R2 are using OSPF to share routing information about their own networks (10.0.x.x/24 and 2001:db8:acad:x::/64 networks). R2 has a static default route to the ISP router. R2 will forward any packets with a destination IP address that does not specifically match one of the networks in its routing table to the ISP router. This would include all packets destined for the internet.
R2 has shared its default route with R1 using OSPF. R1 will now have a default route in its routing table that it learned dynamically from OSPF. R1 will also forward any packets with a destination IP address that does not specifically match one of the networks in its routing table to R2.
The following examples show the IPv4 and IPv6 routing table entries for the static default routes configured on R2.
R2# show ip route
(Output omitted)
S* 0.0.0.0/0 [1/0] via 209.165.200.226
R2#
R2# show ipv6 route
(Output omitted)
S ::/0 [1/0]
via 2001:DB8:FEED:224::2
R2#
Structure of an IPv4 Routing Table
IPv4 was standardized in the early 1980s using the now obsolete classful addressing architecture. The IPv4 routing table is organized using this same classful structure. In the show ip route output, notice that some route entries are left justified while others are indented. This is based on how the routing process searches the IPv4 routing table for the longest match. This was all because of classful addressing. Although the lookup process no longer uses classes, the structure of the IPv4 routing table still retains in this format.
Router# show ip route
(Output omitted)
192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.1.0/24 is directly connected, GigabitEthernet0/0
L 192.168.1.1/32 is directly connected, GigabitEthernet0/0
O 192.168.2.0/24 [110/65] via 192.168.12.2, 00:32:33, Serial0/0/0
O 192.168.3.0/24 [110/65] via 192.168.13.2, 00:31:48, Serial0/0/1
192.168.12.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.12.0/30 is directly connected, Serial0/0/0
L 192.168.12.1/32 is directly connected, Serial0/0/0
192.168.13.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.13.0/30 is directly connected, Serial0/0/1
L 192.168.13.1/32 is directly connected, Serial0/0/1
192.168.23.0/30 is subnetted, 1 subnets
O 192.168.23.0/30 [110/128] via 192.168.12.2, 00:31:38, Serial0/0/0
Router#
Note: The IPv4 routing table in the example is not from any router in the topology used in this module.
Although the details of the structure are beyond the scope of this module, it is helpful to recognize the structure of the table. An indented entry is known as a child route. A route entry is indented if it is the subnet of a classful address (class A, B or C network). Directly connected networks will always be indented (child routes) because the local address of the interface is always entered in the routing table as a /32. The child route will include the route source and all the forwarding information such as the next-hop address. The classful network address of this subnet will be shown above the route entry, less indented, and without a source code. That route is known as a parent route.
Note: This is just a brief introduction to the structure of an IPv4 routing table and does not cover details or specifics of this architecture.
The next example shows the IPv4 routing table of R1 in the topology. Notice that all of the networks in the topology are subnets, which are child routes, of the class A network and parent route10.0.0.0/8.
R1# show ip route
(output omitted for brevity)
O*E2 0.0.0.0/0 [110/1] via 10.0.3.2, 00:51:34, Serial0/1/1
10.0.0.0/8 is variably subnetted, 8 subnets, 2 masks
C 10.0.1.0/24 is directly connected, GigabitEthernet0/0/0
L 10.0.1.1/32 is directly connected, GigabitEthernet0/0/0
C 10.0.2.0/24 is directly connected, GigabitEthernet0/0/1
L 10.0.2.1/32 is directly connected, GigabitEthernet0/0/1
C 10.0.3.0/24 is directly connected, Serial0/1/1
L 10.0.3.1/32 is directly connected, Serial0/1/1
O 10.0.4.0/24 [110/50] via 10.0.3.2, 00:24:22, Serial0/1/1
O 10.0.5.0/24 [110/50] via 10.0.3.2, 00:24:15, Serial0/1/1
R1#
Structure of an IPv6 Routing Table
The concept of classful addressing was never part of IPv6, so the structure of an IPv6 routing table is very straight forward. Every IPv6 route entry is formatted and aligned the same way.
R1# show ipv6 route
(output omitted for brevity)
OE2 ::/0 [110/1], tag 2
via FE80::2:C, Serial0/0/1
C 2001:DB8:ACAD:1::/64 [0/0]
via GigabitEthernet0/0/0, directly connected
L 2001:DB8:ACAD:1::1/128 [0/0]
via GigabitEthernet0/0/0, receive
C 2001:DB8:ACAD:2::/64 [0/0]
via GigabitEthernet0/0/1, directly connected
L 2001:DB8:ACAD:2::1/128 [0/0]
via GigabitEthernet0/0/1, receive
C 2001:DB8:ACAD:3::/64 [0/0]
via Serial0/1/1, directly connected
L 2001:DB8:ACAD:3::1/128 [0/0]
via Serial0/1/1, receive
O 2001:DB8:ACAD:4::/64 [110/50]
via FE80::2:C, Serial0/1/1
O 2001:DB8:ACAD:5::/64 [110/50]
via FE80::2:C, Serial0/1/1
L FF00::/8 [0/0]
via Null0, receive
R1#
Administrative Distance
A route entry for a specific network address (prefix and prefix length) can only appear once in the routing table. However, it is possible that the routing table learns about the same network address from more than one routing source.
Except for very specific circumstances, only one dynamic routing protocol should be implemented on a router. However, it is possible to configure both OSPF and EIGRP on a router, and both routing protocols may learn of the same destination network. Each routing protocol may decide on a different path to reach the destination based on the metric of that routing protocol.
This raises a few questions, such as the following:
- How does the router know which source to use?
- Which route should it install in the routing table? The route learned from OSPF, or the route learned from EIGRP?
Cisco IOS uses what is known as the administrative distance (AD) to determine the route to install into the IP routing table. The AD represents the “trustworthiness” of the route. The lower the AD, the more trustworthy the route source. Because EIGRP has an AD of 90 and OSPF has an AD of 110, the EIGRP route entry would be installed in the routing table.
Note: The AD does not necessarily represent which dynamic routing protocol is best.
A more common example is a router learning the same network address from a static route and a dynamic routing protocol, such as OSPF. A static route has an AD of 1, whereas an OSPF-discovered route has an AD of 110. Given two separate route sources to the same destination, the router chooses to install the route with the lowest AD. When a router has the choice of a static route and an OSPF route, the static route takes precedence.
Note: Directly connected networks have the lowest AD of 0. Only a directly connected network can have an AD of 0.
The table lists various routing protocols and their associated ADs.
| Route Source | Administrative Distance |
|---|---|
| Directly connected | 0 |
| Static route | 1 |
| EIGRP summary route | 5 |
| External BGP | 20 |
| Internal EIGRP | 90 |
| OSPF | 110 |
| IS-IS | 115 |
| RIP | 120 |
| External EIGRP | 170 |
| Internal BGP | 200 |
Glossary: If you have doubts about any special term, you can consult this computer network dictionary.
Ready to go! Keep visiting our networking course blog, give Like to our fanpage; and you will find more tools and concepts that will make you a networking professional.
