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:
- Directly connected routes
- Statically configured routes
- 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
Links to this page:
- ACL - Applying in a VRF environment
- ARP - Static ARP entry for own IP address
- BGP - Leaking more specific routes
- BGP - Next Hop Address Tracking vs Route Dampening
- BGP - Synchronization Rule
- BGP - The BGP table vs the Routing table
- BGP - auto-summary best practices
- BGP - network vs redistribute commands
- BGP - routing table entry without exit interface
- BGP Advertising a prefix
- BGP Advertising a route using the Null0 exit interface
- BGP ISPs prefixes advertised to enterprise edge routers
- BGP Next Hop Address Tracking
- BGP RIB failure
- BGP advertising a default route
- BGP hold timer
- BGP installing a BGP-learned route into the routing table
- BGP paths indicated in the show ip bgp command
- BGP table status codes
- DHCPv6 - determining the IPv6 prefix assigned to an interface
- DMVPN - Phase 2 and summarization
- DMVPN - spoke redundancy
- EIGRP - redistribute connected and static routes
- EIGRP Refining the behavior of load balancing with traffic-share
- EIGRP rib-scale
- EIGRP what happens when a feasible successor fails
- FlexVPN Hub and Spoke backup routes
- GRE - Keepalives and VRF-aware tunnels
- ICMP - Vulnerabilities
- IGRP
- IOS - using ftp or tftp source-interface command
- IP Routing table - process by which entries are matched
- IP SLA - Route Flapping Problem
- IP routing table - 'L' and 'C' entries
- IP routing table - candidate default route
- IP routing table - default route
- IP routing table - static route
- IPv4 - no subnet mask information in the IP header
- IPv4 point to point addresses
- IPv6 - understanding how to enable IPv6 functionality and routing on an IOS device
- IS-IS - route filtering
- IS-IS
- LISP
- MPLS - L3VPN BGP EIGRP redistribution
- MPLS - L3VPN BGP OSPF Redistribution
- MPLS Layer 3 VPN communication between CE routers
- MPLS Troubleshooting
- MPLS addresses bound to peer LDP Ident output
- Memory - CAM and TCAM
- Memory - TCAM Lookups
- Multicast - PIM
- Multicast - reading the multicast routing table
- Multicast Anycast RP
- Multicast PIM-assert forwarder election
- NAT - add-route keyword
- Network - Example of communication, encapsulation, and decapsulation, between hosts
- Network - Flapping
- OSPF - default-information originate always command
- OSPF - distribute-list filtering
- OSPF LSA Persistence with DNA flag
- OSPF NSSA ABR advertises default route
- OSPF Summarization Restrictions Within an Area
- OSPF Why it is not suitable for use on the Internet
- OSPF extent of reconvergence process
- OSPFv3 communication between routers and next hop addresses
- Ping - troubleshooting concepts
- Policy Based Routing acts on incoming traffic
- Protocol Independent Multicast (PIM)
- RIP timers
- Routing - How the routing table is populated
- Routing - IP event dampening
- Routing - Injecting a Static Route
- Routing - The Null0 Interface
- Routing - Using the Null0 interface to prevent routing loops
- Routing - fully specified route
- Routing - ip default-network command
- Routing - redistribution and the routing table
- Routing - what is recursive routing
- Routing what if the administrative distance is the same
- Routing
- Serial Interface default broadcast address
- Summarization
- Supernetting
- Troubleshooting high CPU and memory usage on a switch
- Unicast Reverse Path Forwarding (uRPF)
- VPN - default gateway for site to site VPN
- VRF use cases
- VRF