The routers are typically responsible for directing traffic across multiple networks. Each router maintains a list of known networks and directions in its routing table. The router performs a routing table entries lookup to find the proper interface that leads to the destination address. Each entry in a routing table is called a “route entry: or “route”.
The route identifies the destination network to which traffic can be forwarded. The destination network, in the form of an IP address and netmask, can be an IP network, subnetwork, supernet, or host. Routing table entries can originate from the following sources:
- Directly connected networks
- Dynamic routing protocols, such as EIGRP, OSPF, and RIP
- Routes imported from other routers or virtual routers
- Statically configured routes
Directly Connected Entries
When the router interface is configured with an IP address and the interface state is up and up, the directly connected route is automatically added to the router’s routing table.
Connected routes always precede static or dynamically discovered routes because they have the administrative distance value of 0 (the lowest possible value). The directly connected routes contain the following information.
- The route source is the entry from where the route has been learned. C and L are two route source codes for directly connected routes. C identifies a directly connected network automatically created when an interface is activated and configured with an IP address. L identifies the local route created whenever an interface is configured with an IP address and activated. The L entry did not appear in the routing table entries before the IOS release 15.
- Destination network– This is the address of the destination network that also shows the connectivity.
- The outgoing interface shows the exit interface for packet forwarding to the destination network.
Remote Route Source
A router stores information about both directly connected and remote routes. As with directly connected networks, the route source identifies how the route was learned. Common codes for remote networks include:
- S– This code identifies an administrator’s static route to reach a specific network.
- D– This code identifies the dynamically learned route using the EIGRP routing protocol.
- O– This code identifies the dynamically learned route using the OSPF routing protocol.
- R– This code identifies the route learned dynamically from another router using the RIP routing protocol.
Remote Network Routing Table Entries
The figure below displays an IPv4 routing table entry for the route to the remote network 192.168.0.0. We can identify the following information from this entry:
- Route source– discussed earlier in this lesson.
- Destination network– discussed earlier in this lesson.
- Administrative distance– Identifies the trustworthiness of the route.
- Metric– This is the value assigned to reach the remote network. Lower values indicate preferred routes.
- Next hop– Identifies the IPv4 address of the next router.
- Route timestamp– Identifies when the route was last heard.
- Outgoing interface– This is the outgoing interface towards the destination.
IPv6 Routing Table Entries
IPv6 routing tables function similarly to IPv4, but with global adoption in CCNA 2025. Use ‘show ipv6 route’ to view entries. Codes remain similar: C/L for connected, O for OSPFv3, etc. Metrics and AD are analogous, but IPv6 uses link-local next-hops.
R1# show ipv6 route
IPv6 Routing Table - default - 6 entries
Codes: C - Connected, L - Local, S - Static, U - Per-user Static route
B - BGP, R - RIP, H - NHRP, I1 - ISIS L1
I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary, D - EIGRP
EX - EIGRP external, ND - ND Default, NDp - ND Prefix, DCE - Destination
NDr - Redirect, RL - RPL, O - OSPF Intra, OI - OSPF Inter
OE1 - OSPF ext 1, OE2 - OSPF ext 2, ON1 - OSPF NSSA ext 1
ON2 - OSPF NSSA ext 2, l - LISP
C 2001:DB8:CAFE:1::/64 [0/0]
via GigabitEthernet0/0/0, directly connected
L 2001:DB8:CAFE:1::1/128 [0/0]
via GigabitEthernet0/0/0, receive
C 2001:DB8:CAFE:2::/64 [0/0]
via GigabitEthernet0/0/1, directly connected
L 2001:DB8:CAFE:2::2/128 [0/0]
via GigabitEthernet0/0/1, receive
S 2001:DB8:CAFE:3::/64 [1/0]
via 2001:DB8:CAFE:1::2
O 2001:DB8:CAFE:4::/64 [110/20]
via FE80::1, Serial0/1/0
FAQs
What are the key components of a Cisco routing table entry?
A routing table entries includes the route source code (e.g., D for EIGRP), destination network (e.g., 192.168.0.0/24), administrative distance (AD like 90), metric (lower preferred), next-hop IP, route timestamp, and outgoing interface (e.g., Serial0/1/0). Longest prefix match selects the best route.
How do directly connected routes appear in the Cisco routing table?
Directly connected routes have AD 0 and codes C (network) or L (local IP). They auto-add when an interface is up/up with IP configured. Example: C 192.168.1.0/24 via GigabitEthernet0/0/0. They override others due to the lowest AD.
What is the role of administrative distance and metrics in routing?
AD determines route trustworthiness (e.g., connected=0, EIGRP=90, OSPF=110); lower wins. Metrics break ties within protocols (e.g., EIGRP composite, OSPF cost). Table: Connected (0/N/A), Static (1/N/A), BGP (20/200/multiple).
How do IPv6 routing table entries differ from IPv4 in Cisco IOS?
IPv6 uses ‘show ipv6 route’ with similar codes (C/L connected, O OSPFv3). Next-hops are link-local (e.g., FE80::1). Example: C 2001:DB8:CAFE:1::/64 [0/0] via GigabitEthernet0/0/0. Essential for CCNA 2025 dual-stack networks.
