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2.2 IPv4 Routing

How CEF works

1. Process Switching 1.1 How it works Network interface detects a new packet on the wire. The interface will receive the packet and will place it in the I/O memory. It will then send a “receive interrupt” to the processor to indicate that a new packet needs to be switched. …

IS-IS Mechanics – CLNP

1. ISO OSI Terminology ISO OSI term TCP/IP Equivalent End System Host Intermediate System Router Circuit Interface Area Area Domain Autonomous System IS-IS = Intermediate System to Intermediate System CLNP = Connection-Less Network Protocol = Layer 3 network protocol that is used to communicate between ESes. CLNP offers a CLNS …

IS-IS 101

1. Starting the routing process Starting IS-IS process requires a 2 step configuration: 1. In the global config 2. On the interfaces that will be enabled for IS-IS 1.2 Passive interface The passive interface command in IS-IS has a basically an opposite meaning to what it means in the other …

PfR 101 – Perfromance Routing

1. PfR Technology PfR stands for Performance Routing, but the feature was first called OER (Optimized Edge Routing). This is why most commands still start with the oer keyword. The idea behind PfR is to have a controlling entity (Master Controller) that takes over routing decisions for one or more …

Routing Order of Operations

The original information was taken from Cisco article on NAT Order of Operations. However, this order helps understand other features, like WCCP. 1. Routing Order of Operations If IPSec then check input access list decryption – for CET (Cisco Encryption Technology) or IPSec check input access list check URPF (Unicast …

More BGP

1. Route Dampening It is used to stop unstable routes from being forwarded throughout the network. When a route flaps, a penalty is assigned to the route (Default: 1000 per flap). A timer called Half-Life is used to reduce the penalty value to half (Default: 15 min). If the penalty …

Route Redistribution

1. Route Redistribution You can redistribute routes from one routing process to another using the redistribute command inside the destination routing process: When a routing protocol starts, it automatically redistributes connected routes that are matched by the network command. This also happens for static routes that point to an interface …

BGP Attributes

1. BGP Data Structures 1.1 Neighbor Table The address in the bgp summary table shows the IP used in the peering, not the Router ID. 1.2 BGP Table Lists all prefixes learned from all peers If no routes towards a destination show the “>” code, you should investigate why no …

BGP 101

1. Starting the Routing Process 1.1 Define the routing process Only one BGP process can run on a router and it can be started using: The AS Number used to be a 16 bits number ranging from 0 to 65535. According to RFC 4893, the AS number can have 32 …

OSPF Mechanics

1. OSPF Router ID Each router selects an OSPF Router ID when the OSPF process starts. The Router ID is a 32 bit number, usually written in dotted decimal format. The selection process is: Manually configured Router ID Highest Loopback IP address Highest non-Loopback “up/up” IP address The interface used …