Grave Of A Network Engineer.... :-)

Deepak Arora

Auto-Negotiation ON vs OFF - Welcome Back To Ethernet World

Auto negotiation is one of those thing which comes back to every network engineer at some point. There is always so much talk about idea  that - if we should keep auto-negotiation "ON" or should we turn it "OFF" and hard-cord the "Duplex" & "Speed" settings.

There are always two schools of thoughts about this and you may choose which one you like more. 

So idea boils down to which school you prefer. 

But...

Here are some of drawbacks or I should say list of things which you won't be able to figure out easily or will not work at all if you turn off auto-negotiation over interface:

1. Performance Issue
2. Link failures
3. Pause Frames or Flow Control
4. Bad Cable Problem
5. Link Partner Capabilities
6. Auto-MDIX will not work

For more:

http://en.wikipedia.org/wiki/Autonegotiation 

http://etherealmind.com/ethernet-autonegotiation-works-why-how-standard-should-be-set/

http://www.sun.com/blueprints/0704/817-7526.pdf

http://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1923.html

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00800a7af0.shtml 

http://blog.ine.com/2008/07/08/802-3x-flow-control/ 
 
HTH...
Deepak  Arora

EIGRP Variance & Unequal Cost Load Balancing Demystified - With Little GNS Bug

It's been a month since I posted my EIGRP challenge lab:


http://deepakarora1984.blogspot.com/2010/10/eigrp-challenge-lab-1.html


So today it's time to show you how to do it. I won't be spending much time today in explaining theory behind all this as so many people have already written about it but of-course that doesn't mean they covered it from all aspects.


But still I'll provide some reference links in the end of this post to make sure you don't have any gaps regarding concepts.


So lets load the initial configurations which I provided in original post and lets get started :


Note: Click Images To Enlarge

so far we can see the route is in routing table of R4, but we don't see redundant path as yet because of it's higher cost.


Lets calculate variance to install redundant route in routing table:


For those who don't understand variance yet here is the link:


http://www.networkworld.com/community/node/29966


Lets first examine the difference between two metrics that belong to both links for same route:

Now to calculate the required variance use the equation: Higher FD / Lower FD, and result will be:

As variance can't be fractional, so we need to take higher value, which will be 4


Lets's use variance value 4 and take a look at routing table:

Looks like both routes now have made it to routing table. So first objective is achieved. Now lets take a look at Traffic Share Count for both links for same route:

so as you can see that current traffic share count is 21:80. Traffic share count is inversely proportional to metric difference between both links.


So next step is to somehow make it as 1:1 :-)


As you might be thinking that Mr. Garcia (Creator Of EIGRP) should be around :-)

so lets take a look at first how much difference is there between both metrics:

Now lets subtract the lower metric from higher metric: By Metric I mean FD AKA RD:

So the difference comes as - 30000128


Now in next step lets add this metric to link with lower metric using "offset-list".

Here offset List calling ACL "0" means matching all routes... just for your sake :-)


Lets check again different EIGRP tables and see if it works:

Duhhh....looks like GNS is creating pain in butt again.... tired of GNS bugs :-(

Actually to over come this issue I had to jump to real equipments with same configuration and here are the results:

Though IOS ver "15.x" was not liking somehow idea of offset-list 0, so I had to create a specific ACL for this route:

So all functional and we have traffic share count of 1:1 between both links though they have different bandwidth.


Some more interesting reads on EIGRP or I should say "Must" read documents on EIGRP Load Balancing:


http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094823.shtml

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094820.shtml

http://www.cisco.com/en/US/tech/tk827/tk831/technologies_white_paper09186a00800a62d9.shtml

http://www.cisco.com/en/US/products/hw/modules/ps2033/prod_technical_reference09186a00800afeb7.html/

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009437d.shtml

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c2d96.shtml

HTH...
Deepak Arora

Cisco ISP Essentials - E-Book (Free)...No Piracy :-)

While going through some web pages yesterday I came across this download link from Cisco Engineering Team. Using this link you can actually download pdf copy of this great book in free without getting into piracy issues :-)


ftp://ftp-eng.cisco.com/cons/isp/documents/IOSEssentialsPDF.zip


HTH...
Deepak Arora

Difference Between Routing Table & RIB (Routing Information Base) - Short Post

It's been long time since I wanted to talk over my blog about this. I have seen many people and books claiming that RIB is just another name of I should say fancy term to describe our old friend Routing Table. But In my personal opinion there is a significant difference between both.


RIB or Routing Information Base is local view about information from Routing Protocol Prospective. For example each routing protocol including BGP generates BGPtables in General. Yeah I know there are some more details as far how BGP builds its BGP Table :-). But for now lets talk about BGP table in short.


BGP table is local view of routing information received and generated locally. If you have ever run BGP than you must be aware about situations where you find some routing information which is in BGP table but when you run "show ip route" command , you don't see this information there. 


There are many possible reason for that in BGP like "Un-Reachable Next Hop" Address.


So what actually we see with "show ip bgp" is RIB built by BGP. On the other hand what we see with "show ip route" is Routing table which is derived from that RIB information. Or in other words best and valid information picked by router from BGP Table and placed it into routing table.

so next idea comes into mind is that how we check RIB for each protocol ?


And here is answer :-)


RIP -  "sh ip rip database"


OSPF - Older way > "sh ip ospf database"
              Modern Way > "sh ip ospf rib detail"


EIGRP - "sh ip eigrp topology all-links"


BGP - "sh ip bgp"


IS-IS - "sh isis database"


With the "sh ip eigrp topology all-links" command you can actually see routes that failed the feasibility check.


Also one of the reason that support idea of RIB being different from Routing Table is - In OSPF for example if Forwarding Address (FA) for LSA 5 is not reachable, in such case you will see entries for those LSAs in "OSPF database" with command " sh ip ospf database" but on the other hand you won't find those routes in Routing table when you run command "sh ip route"


HTH...
Deepak Arora