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2.1 IPv4 Addressing

NSF – Non Stop Forwarding

1. What is NSF NSF is a feature that allows routers to keep on forwarding traffic (non stop forwarding) even in the event of a restart. This is done by separating the control and the data plane, having one process involved in building the routing table and another process in …

BFD – Bidirectional Forwarding Detection

1. What is BFD? BFD stands for Bidirectional Forwarding Detection and it’s a protocol that is used for rapid detection of link failures when the line-protocol is still “up”. BFD is enabled on interface and creates a BFD session with the neighboring router (BFD Peer). Routing protocols such as EIGRP, …

Tunnel Interfaces

1. Tunnel Modes A tunnel makes two distant devices appear directly connected over a logical interface. When a packet is sent out on the tunnel interface, it is encapsulated in the “carrier” protocol and sent over a physical interface.The most used carrier protocols are GRE, IP-in-IP and IPv6, and this …

IPv4 101

1. Setting an IP Address Any combination of IP-ADDR and NETMASK can be used as long as the host portion of the address is not all zeros. One exception is allowed, when using a /31 mask. By default, the router will accept combinations that result in a subnet portion of …

ARP 101

1. ARP ARP is a protocol used on broadcast networks such as Ethernet, Token Ring or FDDI that is used to map L3 Addresses (like IP) to layer 2 Addresses (like Ethernet MAC). When a host needs to send traffic to another host, it knows its Layer 3 address, but …

DNS 101

1. Defining hosts and domains locally To define a static host name to address mapping, use the following command: For hosts that are accessed without a domain name at the end, (hostname.domain-name), you can define a default domain name or a list of domain names to be used, using one …

DHCP 101

1. DHCP Server 1.1 DHCP Pools On a router, you have to create one or more pools of DHCP addresses available for lease. When a DHCP server receives a DHCP request, it will know what pool to use based on the IP address of interface that received it. If the …

FHRP 101

1. HSRP HSRP provides a virtual MAC address and a virtual IP address that is shared among a group of routers in order to have a HA infrastructure for the default gateway in a subnet. 1.1 Starting HSRP 1.2 Timers Timers are usually learned from the active router. Millisecond timers …

GRE Tunnels

1. Configuring GRE GRE tunnels appear as directly connected logical interfaces to the router even though the traffic that goes into the tunnel will actually be carried over other physical interfaces to the destination. To configure a GRE tunnel you have to set the source and the destination of the …

Backup Interfaces

In Theory A backup interface is an interface that stays inactive as long as the primary interface is in “up/up” state, but becomes active when the primary interface’s protocol status becomes “down”. When the protocol on the primary interface comes back up, the backup interface moves back in a “standby …