Routing Table

The routing table is a construct within as Layer 3 network device that is used to determine the egress interface of a received packet during the process of routing. The routing table is created using one or more of the following methods:

  1. Directly connected routes
  2. Statically configured routes
  3. Dynamically learned routes via routing protocols such as EIGRP, RIP, OSPF, IS-IS, and BGP.

The routing table entries consist of prefixes that correspond to next-hop IPs, exit interfaces, or both.

An example of an IPv4 routing table found within a Cisco router is the following:

PE1#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 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, m - OMP
       n - NAT, Ni - NAT inside, No - NAT outside, Nd - NAT DIA
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       H - NHRP, G - NHRP registered, g - NHRP registration summary
       o - ODR, P - periodic downloaded static route, l - LISP
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR
       & - replicated local route overrides by connected

Gateway of last resort is not set

      1.0.0.0/32 is subnetted, 1 subnets
i L2     1.1.1.1 [115/20] via 192.168.12.1, 19:48:33, GigabitEthernet1
      2.0.0.0/32 is subnetted, 1 subnets
C        2.2.2.2 is directly connected, Loopback0
      3.0.0.0/32 is subnetted, 1 subnets
i L2     3.3.3.3 [115/20] via 192.168.23.3, 19:48:33, GigabitEthernet2
      4.0.0.0/32 is subnetted, 1 subnets
i L2     4.4.4.4 [115/20] via 192.168.24.4, 19:48:33, GigabitEthernet3
      5.0.0.0/32 is subnetted, 1 subnets
i L2     5.5.5.5 [115/30] via 192.168.24.4, 19:48:33, GigabitEthernet3
      6.0.0.0/32 is subnetted, 1 subnets
S        6.6.6.6 is directly connected, Tunnel1
      7.0.0.0/32 is subnetted, 1 subnets
i L2     7.7.7.7 [115/40] via 192.168.23.3, 19:48:33, GigabitEthernet2
      192.168.12.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.12.0/24 is directly connected, GigabitEthernet1
L        192.168.12.2/32 is directly connected, GigabitEthernet1
      192.168.23.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.23.0/24 is directly connected, GigabitEthernet2
L        192.168.23.2/32 is directly connected, GigabitEthernet2
      192.168.24.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.24.0/24 is directly connected, GigabitEthernet3
L        192.168.24.2/32 is directly connected, GigabitEthernet3
i L2  192.168.36.0/24 [115/20] via 192.168.23.3, 19:48:33, GigabitEthernet2
i L2  192.168.45.0/24 [115/20] via 192.168.24.4, 19:48:33, GigabitEthernet3
i L2  192.168.56.0/24 [115/30] via 192.168.24.4, 19:48:33, GigabitEthernet3
                      [115/30] via 192.168.23.3, 19:48:33, GigabitEthernet2
i L2  192.168.67.0/24 [115/30] via 192.168.23.3, 19:48:33, GigabitEthernet2

The above routing table shows routes learned from various sources.

The following is an example of an IPv6 routing table found within a Cisco router:

R2#show ipv6 route   
IPv6 Routing Table - default - 8 entries
Codes: C - Connected, L - Local, S - Static, U - Per-user Static route
       B - BGP, HA - Home Agent, MR - Mobile Router, R - RIP
       H - NHRP, I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea
       IS - ISIS summary, D - EIGRP, EX - EIGRP external, NM - NEMO
       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
       la - LISP alt, lr - LISP site-registrations, ld - LISP dyn-eid
       lA - LISP away, a - Application
R   2001::1/128 [120/2]
     via FE80::5054:FF:FE15:A1FE, GigabitEthernet0/1
LC  2001::2/128 [0/0]
     via Loopback0, receive
R   2001::3/128 [120/2]
     via FE80::5054:FF:FE16:BB44, GigabitEthernet0/2
R   2001::33/128 [120/2]
     via FE80::5054:FF:FE16:BB44, GigabitEthernet0/2
R   2001::41/128 [120/2]
     via FE80::5054:FF:FE16:BB44, GigabitEthernet0/2
R   2001::543/128 [120/2]
     via FE80::5054:FF:FE16:BB44, GigabitEthernet0/2
R   2001:DB8:3:3::3/128 [120/2]
     via FE80::5054:FF:FE16:BB44, GigabitEthernet0/2
L   FF00::/8 [0/0]
     via Null0, receive

Links:

https://networklessons.com/cisco/ccna-200-301/introduction-to-routers-and-routing

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