EIGRP Distance Vector Route Discovery

April 27, 2014

EIGRP’s route discovery process uses a “routing by rumour” approach. Meaning that the routers in an autonomous system heard about their routes from another autonomous system/router, they did not receive the route information first hand.This essentially means the routers heard about the routes by listening to network gossip.

The information collected is stored in the following tables.

  • Neighbour Table
  • Topology Table
  • Router Table

With this EIGRP will calculate the best paths and any redundant links (if available) using the following:

  • Bandwidth
  • Delay
  • Load
  • Reliability

MTU is sometimes reffered to being used but it in fact is not, it is only used for some commands but doesn’t affect the outcome of the formula.


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.


IPv6 – The Lowdown

April 7, 2014

IPv6 – Why and When

So what is IPv6, why do we need it and what is it good for ?

It was anticipated back in the 80’s that we would one day run out of IPv4 address’s and the IANA ran out of IPv4 allocations on Jan 31st 2011. This is why IPv6 was developed, it is the successor to IPv4.

We all know that IP, a network layer (Layer 3) protocol is used for logical addressing, so without an IP address any attempt to access services or devices outside your local Broadcast Domain (Layer 2) would prove unsuccessful. With IPv6 now becoming more prevalent in todays networks and the fact Cisco now covers it in the ICND1 exams, it’s a sure sign that it’s time to move with the times. Not only is IPv6 a more efficient protocol, (as we have learnt from IPv4) is also has some cool features which are included within the protocol itself (IPsec being one).

So what’s changed ? , well the most notable is the length of the address and instead of using decimal numbers to represent the address, IPv6 makes use of Hexadecimal to represent it. Each HEX value is worth 4bits and with now 8 groups of 4 hexadecimal values we have increased from a 32Bit addressing system to 128Bit. This means that we now have 3.4×10^38 IPv6 addresses available (Goodluck pronouncing that), this means for any foreseeable future it is very unlikely that we will run out of addresses to use (But they did say who would ever need more then 4kb of storage).

IPv6 Addresses

An Example IPv6 would look like so: FE80:0003:AC33:1111:1200:ADFF:FE43:00D4 (For those keen eyed readers, yes it’s a link local address)

Like IPv4 where we use subnet masks to represent a host and network portion, the same goes for IPv6. a /64 “mask”, using the above example, would mean it is in the FE80:3:AC33:1111 – network. with the other 64bits available for host addressing.

Address Condensing Rules

If you noticed that above i wrote the second group of hex digits as :3 instead of :0003, this brings me to the following rules to condensing an IPv6 address down, cause they are just so damn long now. Anyway they are:

  • Leading Zero’s can be dropped – eg 00AB:6500 can become AB:6500
  • Double Colons can represent one or more groups of zero’s but it can only be used once – eg 1234:0000:0000:4567 becomes 1234::4567
  • To iterate the above rule, you can only do this once, as the router/computer is expecting 8 groups of hex, it will be confused if we use more then one set. As it won’t know how many groupings each double colon represents.

 

My next posts i will go into detail about the type of IPv6 addresses and something called EUI-64. Check out my twitter if you want to get a peek at what it is. @networkinggahan (Twitter)


EIGRP – Enhanced Interior Gateway Protocol – The Basics

April 1, 2014

EIGRP Notes:

  • Reffered to as a hybrid routing protocol or advanced distance vector routing protocol within some documentation
  • Supports VLSM and route summarisation
  • Cisco proprietary protocol
  • Includes Subnet mask in routing updates

 

EIGRP supports the following features.

  • IPv4 and v6
  • Classless
  • VLSM and CIDR
  • Summarisation of networks and discontiguous ones
  • Uses Reliable Transport Protocol (RTP)
  • Path selection is via Diffusing Update Algorithm (DUAL)
  • No broadcasts

The following three values must match between EIGRP enabled routers before they become neighbours and then they can share routes.

  • Reception of either a Hello or ACK
  • Autonomous System Numbers
  • Same K Values

The Hello timer by default is set to 5 seconds, the Hello message is used to establish neighbour relationships with other routers. The Hellos will alert other routers, also of new routers on the internetwork. This also means that when the Hello messages stop, the neighbour relationship will stop.

This brings me onto the Hold Timer, this timer is used to tell the router the time frame it should wait to receive a Hello from a neighbour before it declares that router dead. Once that happens it will be removed from the neighbour table and any routes which used that now dead router will have their paths recalculated.


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.


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

Last Chapter

November 22, 2013

I’m pretty much on track to completing the 200-101 ICND2 course by the end of this month. I have notes upto chapter 10, from memory i think that was OSPF.

Anyway once i’m done i will start reviewing everything with you guys online.

Check out the cisco incd2 course subjects because that’s exactly what i’ll be covering.


Study Update

November 13, 2013

Hi all, just a quick update to let you know what’s happening.

Well i’ve almost almost completed reading the ICND2 book , the new 200-101 exam and it is quite different to it’s predecessor so i highly recommend you purchase the new one if you haven’t already. The next thing i have to do is also finish up my notes and get those ios commands stuck in my head. I had initially planned to obtain the CCNA by the end of November but due to some hurdles in the last couple of weeks i will be happy with finishing up all the notes before then.

Also the networkdictionary.wikispaces.com wiki which i have created is coming along nicely. Plenty of Cisco related acronyms in there now, so don’t forget it’s free for anyone and to join also, add whatever is missing (that’s alot)

So keep at it and when the times right i will be covering the rest of the ICND2 exam soon. I’m thinking Identifying WAN technologies next, it’s my favourite.


Etherchannels

October 20, 2013

To finish up the first topic under LAN Switching Technologies, identifying enhanced switching technologies. I will give you an overview of Etherchannels.

What they are, what they do and how to configure one for yourself.

Firstly an Etherchannel is a configuration that allows a switch to treat two parallel links as one physical link. This also allows STP to treat the entire channel as one and either block or forward traffic over this link as per vlan.

As a result of this, the switch can now load balance traffic over the channel. Without Etherchannel one of the links would be blocked by STP, as is shown in the example below.

Before Etherchannel, STP blocks Fa0/2 from SW1.

In the above example, SW0 and SW1 are connected via two links. Fa0/1 to Fa0/1 and Fa0/2 to Fa0/2, but due to STP it has blacked Fa0/2 on SW1 from forwarding frames. STP at this point see’s these two links as they are, separate from one another.

02_spanning-tree_onSW1When we run a show spanning-tree command on SW1 we discover that, port Fa0/1 is in a forwarding state as the RP, Port Fa0/2 is blocked and Fa0/3 is the DP. Running the show spanning-tree command on SW0 would show us that it is the Root Bridge and all three ports are DP’s or Designated Ports.

To make STP use both the Fa0/1 and Fa0/2 links on SW0 and SW1 we need to configure the interfaces in the same Channel-group or Etherchannel. Doing so will change how STP see’s those two links. From two separate links to one link which can be used to load balance. We are not actually making one physical link but like a VLAN where we can make many Virtual LAN’s we are creating one Virtual Link using 2 parallel links.

For this network the configuration is virtually the same, just remember you will need to add the correct interfaces in the same channel group for an Etherchannel to become active.

The following image shows you how to correctly configure an Etherchannel, using the command channel-group 1 mode on, you can configure upto 8 parallel links on the one Etherchannel.

03_Channel-group

Below you can see the new show spanning-tree output displayed on SW1 after the Etherchannel has been configured. Notice the Po1, that indicates that all the configuration parameters checked and that is the new channel group 1, which was configured earlier.

04_newSTPonSW1

The new diagram, with the two links now bundled in the new Etherchannel. Notice how Fa0/2 has come out of blocking mode to become a RP

05_Etherchannel


Problems avoided using STP

October 15, 2013

If you are going to setup a LAN with redundant paths, it is important that STP is running. If not you will be opening yourself up to these few problems

  • Broadcast Storms
  • MAC Table instability
  • Multiple frame transmission

While the issue of broadcast storms may seem obvious, the other two may not be the things you think about at all. In the case of MAC table instability your switches will experience frequent updates to their MAC tables with wrong entires, from the looped frames which are being sent around the LAN.

The second issue of Multiple frame transmission can be somewhat, a disastrous side affect of not running STP. Duplicate copies of the looping frame can end up at the end host, in turn completely confusing it, resulting in discarded frames.

Luckily though on CISCO switches STP is on by DEFAULT