BGP - IGP-BGP redistribution best practices

DMVPN - using a default route

IP Routing table - process by which entries are matched

IP SLA - Route Flapping Problem

OSPF - default-information originate always command

Routing - default gateway


A router will look through all of its [routing](/routing) entries in the [Routing Table](/routing-table) to find the one that best matches the intended destination. If no match is found, and if the “gateway of last resort” (or the default route) is set, only then will it send the packet to the default route.

For example, take a look at this routing table:

```sql
R1#show ip route
Codes: 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
       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
       o - ODR, P - periodic downloaded static route

Gateway of last resort is 192.168.24.254 to network 0.0.0.0

C    192.168.12.0/24 is directly connected, FastEthernet0/0
S*   0.0.0.0/0 [1/0] via 192.168.24.254, FastEthernet0/1
D    192.168.23.0/24 [90/307200] via 192.168.12.2, 00:00:07, FastEthernet0/0
```

A destination [IPv4](/ipv4) address of 192.168.12.5 will match the directly connected network on FastEthernet0/0. However, a destination address of 10.10.10.5 will not match any entry, and will thus be sent via the gateway of last resort, which has a next hop of 192.168.24.254, and an exit interface of FastEthernet0/1.

[IPv6](/ipv6) operates in much the same way.

So the gateway of last resort, as its name suggests, will only be used if all other entries do not match.

Links:

https://forum.networklessons.com/t/ip-routing-explained/1472/55?u=lagapides

https://networklessons.com/cisco/ccna-routing-switching-icnd1-100-105/ip-routing-explained

IP routing table - default route

ICANN

ICMP - Mitigating Vulnerabilities

ICMP - Vulnerabilities

ICMP - response to a ping that is blocked by an ACL

ICMP codes and types

ICMP

IEEE

IETF

IGMP - Snooping Querier Election

IGMP - Snooping Querier

IGMP - access group

IGMP - fast leave

IGMP - filtering using ACLs

IGMP - ip igmp snooping querier command

IGMP - membership report

IGMP - using VLAN access maps to filter multicast traffic

IGMP Snooping internal interface

IGMP default IOS configurations

IGMP last member query interval

IGMP membership query max response time

IGMP membership reports destination

IGMP report suppression

IGMP snooping prevents report suppression

IGMP snooping source IP address for messages

IGMP snooping

IGMP version 1 how hosts stop receiving multicast traffic

IGMP

IGMPv2 - leave group message mechanism

IGRP

IOS - Configuration Archive and Rollback

IOS - Right To Use (RTU) License

IOS - ip http server

IOS - licensing

IOS - show license command

IOS - using ftp or tftp source-interface command

IOS configuration register

IOS install mode vs bundle mode

IOS key chain feature

IP - When is the TTL decremented in a router

IP Address Management (IPAM) Tools

IP Addressing on a Router

IP Fragmentation - effects on network traffic

IP Fragmentation Attack

IP SLA Parameters

IP SLA tracking syntax

IP addressing on a Switch

IP routing table - candidate default route

IP routing table - static route

IPSec - Authentication Header (AH)

IPSec - ESP vs AH

IPSec - Encapsulating Security Payload (ESP)

IPSec - Key Exchange Data

IPSec - NAT, AH, and ESP

IPSec - crypto map multiple peers

IPSec ESP Wireshark decrypt payload

IPSec NAT Transparency

IPSec how it works with NAT-T

IPSec profile operation with more than one crypto isakmp policy

IPSec

IPsec - does it support multicast

IPv4 - Internal Host Loopback Address Range

IPv4 - ToS Byte definition

IPv4 - classful addressing

IPv4 - classless addressing

IPv4 - header protocol field

IPv4 - no subnet mask information in the IP header

IPv4 - subnet mask

IPv4 Link-local address range

IPv4 Private IP address ranges

IPv4 Subnetting invalid entry causing network address of zero

IPv4 header length field

IPv4 network and broadcast addresses

IPv4 point to point addresses

IPv4

IPv6 - ACLs, RAs, and RSes

IPv6 - DHCPv6

IPv6 - IPv4-mapped IPv6 address

IPv6 - NDP Neighbor Discovery Process

IPv6 - NDP preferred and valid lifetimes

IPv6 - Network and Interface Identifiers

IPv6 - RA suppression command

IPv6 - SLAAC

IPv6 - Site Local addresses

IPv6 - Solicited Node Multicast Address

IPv6 - Unique Local addresses

IPv6 - Valid and Preferred Addresses

IPv6 - destination guard

IPv6 - ipv6 nd prefix command

IPv6 - loopback address

IPv6 - prefix length

IPv6 - understanding how to enable IPv6 functionality and routing on an IOS device

IP routing table - default route

Links to this page: