EIGRP and DUAL

April 19, 2014

DUAL or Diffusing Update Algorithm is responsible for the following.

 

  • Responsible for selecting and maintaing the best path to remote networks
  • DUAL also allows EIGRP to execute the following:
    • Calculate backup routes (if available)
    • Allows support of VLSM
    • Allows EIGRP to perform Dynamic Route Recovery
    • Questions EIGRP neighbours for any unknown alternate routes
    • Queries for alternate routes
    • Is the reason EIGRP can converge amazingly fast.

Reliable Transport Protocol (RTP)

April 16, 2014

This is a brief rundown of RTP and how it is used by EIGRP for reliable communication.

  • Proprietary Protocol
  • Used and relied on by EIGRP for reliable packet delivery to neighbours.
  • Manages inter EIGRP router communications

So EIGRP uses the multicast address 224.0.0.10 and each time a multicast is sent, it is recorded and awaits a reply of receipt. If and when the multicasts fails, EIGRP will switch to unicast packets and continue to await a reply. If there is no response after 16 attempts, that neighbour is declared dead.

So any information these routers send between neighbours is tracked and tagged with sequence numbers, this allows the routers to identify:

  • Old
  • Out of order
  • Redundant Information

EIGRP Metric Terminology

April 15, 2014

Feasible Distance:

One of the metrics used to calculate the best path and it is the best cost path as determined by the “Local” router

Reported Distance:

Reported or Advertised Distance is a metric that is calculated by a remote neighbour in a remote network.

 

Nb. The route with the lowed FD is the route that will populate the EIGRP routing table.

Neighbour Table:

Stored in RAM, this table is a record of all adjacent neighbours address and interfaces. Each router will use this table to track the current state information of its neighbours

Topology Table:

This table records destinations as reachable by its neighbours or simply the topology of the network. Any changes will be triggered with partial updates

Routing Table:

Using the information found in the topology table, EIGRP will place the best routes into the routing table. known as successor routes, they provide the least cost to reach the remote network.

Feasible Successor:

An inferior alternate route to the successor, which is located in both the topology and routing table, whereas the feasible successor is only recorded inside the topology table. If the successor route was to fail this would be considered the backup route and take over.

It is good to note that in IOS 15.0 code you can keep upto 32 feasible successor routes in the topology table and previous codes upto 16.

Successor Route:

This is the best route to a remote network, and as mentioned previously it is recorded in both the routing and topology tables.


Types of IPv6 Addresses

April 13, 2014

There are alot of new terms and techniques that are needed to be learnt in order to get a grasp of the IPv6 principles and as promised i will go over the types of IPv6 addresses that exist. The one that doesn’t exist anymore is the broadcast. Anyway they are:

IPv6 Multicast

  • First 8Bits are 1
  • or FF00::/8

This works just like the IPv4 brother and is used to send traffic to a subset of devices, it also functions, at times, like a broadcast. In the new IPv6 world there is no such thing but the multicast addressing does the same thing.

 

Unspecified

  • Devices use this IPv6 address to ask for an IPv6 address from a DHCP server
  • Written as ::/128

Does as described, before a device has an IP it will use the ::/128 IP as the source and when a DHCP server responds to the request an unused IPv6 address will be assigned.

 

Loopback

  • Performs the same functions as 127.0.0.1 does in IPv4
  • ::1/128

This address is used basically to troubleshoot and test the IP stack on devices.

 

Link Local

  • Every device will generate a link local address
  • Only used to communicate on the local broadcast domain, meaning it won’t pass a router or the Layer 2 link.
  • Written and identified by FE80::/10
  • Link-Local address are generated using the EUI-64 rules aka Stateless Address Auto-configuration (SLAAC)

The link-local address is quite unique and as stated above it will not be used outside the broadcast domain, so without any configuration at all you should be able to connect devices on the same broadcast domain and have connectivity using the Link Local addresses. It’s a pretty cool feature of IPv6 and something to look out for too.

 

Global

  • Can be routed via the internet and between networks
  • Reachable by routers globally
  • 2000::/3 – First 3Bits must be 001
  • AKA Global Unicast Address

Basically this is the address that your local RIR or ISP will allocate to you to use,  without going into details you would be given a further subdivided piece of the above address space to use to connect to the internet. Once this is setup and configured you will be reachable by other routers globally.

 

Anycast

  • Allow the use of the same IP addresse in different locations
  • Function like Global IPv6 Addresses.

The Anycast feature of IPv6 can be used to allow the use of identical IPv6 addresses in different locations. The reason for this is so that you can setup servers for almost anything and the requesting host will find the destination which is closer, not just via metric but this could mean physically too.


Distance Vector and Routing Loop Preventions

April 2, 2014

Distance vector routing protocols are considered the most efficient routing protocols when it comes to the use of the device resources. Also unlike Link State protocols, who will share the entire routing “map” with every other adjacent device, Link State protocols prefer to share only the bare essentials and advertise only the best route.

To compare, Link Sate will advertise all routes to a destination but in Distance Vector they will only advertise the “best”. This helps keep the routing tables small and like i said at the  start, the router operates more efficiently and uses less device resources.

Distance Vector also has some route looping mitigation techniques in operation, to help stop Routing Loops from forming.

They are:

  • Triggered Updates
  • Route Poisoning
  • Maximum Distance (Hops)
  • Split Horizon
  • Hold Down Timers

These are all on and running by default and usually they won’t need to be turned off. There are some exceptions though where these preventions impair the operation of certain network designs. eg.. Frame relay network setup in a hub-spoke topology, due to things like split horizon certain routes won’t be advertised to adjacent routers, i’ll explain that later.


Open Shortest Path First (OSPF)

December 12, 2013

Moving forwards the next bunch of topics on the list look like so:

Configure and Verify OSPF

  • Neighbour Adjacencies
  • OSPF States
  • Discuss Multi-area
  • Configure OSPFv2 and v3
  • Router ID
  • LSA Types

So lets first get a little background on OSPF before moving any further.

OSPF or Open Shortest Path First is, as the name suggest, an open standard link state routing protocol. The AD of OSPF is 110 and OSPF uses a metric of the outgoing interface Cost to determine the best route. It’s a fast converging and flexible routing protocol which also enables us to separate the topology into areas.

The Topology data for the OSPF network is contained within Link State Advertisements and all routers in the same area share the exact same topology information.

I won’t delve any further into OSPF at this point, i just simply want to introduce the upcoming topics and as you can see there are a few things to get through. So in the upcoming days i will describe the purpose of the LSA’s the RID , configuration and some helpful troubleshooting commands.

Don’t stress too hard this stuff is pretty easy.


Routing Metrics

December 10, 2013

On our journey through the Cisco ICND2 objectives, we are expected to know a thing or two about “Metrics”

So what is a metric you ask ?

Well a routing metric is used by all routing protocols to decide on the best path to any given network. Not to be confused with Administrative Distance (AD). Which tell the router which path to believe when a router learns a route from more then one routing protocol.

Below is an example of what a metric is, with the use of RIPv2. As you can see R1 has two paths to reach R2. One is out FA0/1 directly to R2 (Which in the real world would be a connected route with a lower AD then RIP) and out FA0/2 through R3.

The metric with RIP is calculated using hop count, so the path with the least hops is the best path. The path highlighted in red would be used in this example.

RIP Metric


Split Horizon

December 9, 2013

The next routing protocol feature to talk about is Split Horizon.

A feature of Distance Vector that prevents routing loops from forming. Split horizon rules stop a router from sending routing updates back out the interface from which it learnt them from.

Split horizon is used with RIPv2 and EIGRP and yup you guessed it they are Distance Vector routing protocols.

The example below gives you some understanding of what Split Horizon is actually doing. The RIP update going out R1’s FA0/1 interface only includes the subnet 192.168.0.0/24 (Route highlighted in red). As you can see from the routing table below, R1 learnt this network from an update off FA0/3, so Split Horizon Rules allow this network to be sent in an update out FA0/1.

The routes highlighted in black, are learnt from FA0/1 so Split horizon rules will not allow these routes to be sent in an update, over FA0/1

Split_Horizon


Default Administrative Distances

December 1, 2013

The ICND2 exam asks for knowledge about Routing protocols and their Default Administrative distances. The following table below should be burned into your brain before taking the exam i would imagine.

An Administrative Distance is used so a router can make a decision on how believable the route is as reported by the corresponding routing protocols. Learn this and in the coming days we will use this to configure and Verify both flavours of OSPF and EIGRP.

Route Administrative Distance
Connected 0
Static 1
BGP (External Routes) 20
EIGRP (Internal Routes) 90
IGRP 100
OSPF 110
IS-IS 115
RIP 120
EIGRP (External Routes) 170
BGP (Internal Routes) 200
Unusable 255

Power On Self Test (POST)

November 28, 2013

The next item on the agenda for studying towards the ICND2 is the Power On Self Test or POST.

POST is the first thing to run when you first turn on a Cisco Router, stored and run from ROM, the POST tests the serviceability of the routers components. One example that is often described is that POST checks to see if the interfaces are fully functional.

Once everything checks out, the router will then continue with loading the bootstrap.