GNS3, Dynamips, PuTTY and HSRP

Posted: November 6, 2010 in Infrastructure Management and Disaster Recovery
I chose to use PuTTY for my console access because it’s the one I’m most familiar with. Here’s how I set it up:

Just to prove that GNS3 and Dynamips is working correctly I put together a really simple network with dynamic routes using OSPF. Got it talking within minutes, which is great news. However…

…Dynamips is too resource-hungry for my old PC. It uses all my CPU all the time, making it impossible to build a sizeable network in my environment.
Today I will undertake a simple experiment to achieve something that’s impossible in Packet Tracer: prove that HSRP works. (Packet Tracer 5.3 doesn’t support HSRP.) This means that R4 and R5 are equivalent to each other and one can take over is the other fails. They have their own IPs but share a HSRP IP address. It is this address which is used by the rest of the network, meaning that other devices don’t need to be configured in the event of an R4/5 failure.

Simple network where R4 and R5 should be in the HSRP pool, and R6 wants to communicate with one of them
R6 has the higher priority so gets all the load while it is available. The above screenshot shows each router’s config. I have proven complete communication between all three.
IP sits in front of and
Experiment by switching off R4: R5 takes over.
Switch off R5: no service
Switch R4 back on again: we get a response again. Success!
Simple, but it shows how easy it is to build in resilience, assuming you have the money to double up your devices. Other things to think about are the cables. There’s (may be) no goo building resilience into your devices unless you have resilient power and communication lines. The old classic ‘man in digger breaks your cables’ scenario would take out everything to the left of the switch unless you had an alternative physical route. Same is true for power: if R4 and R5 use the same power source and that goes down, what benefit did you gain by having resilient devices?
Another issue would is configuration management: R4 and R5 need to be kept in sync at all times for failover to work seamlessly.
This is a good example of how to build simple resilience in a small network, but on a large scale you’ll be thinking about GLBP or dedicated hardware load balancer devices.
Tomorrow I will hook up Dynamips to my local network and show how my VM devices can communicate with each other via my virtual Dynamips devices. But I will leave GNS3 after this because it’s just too slow on my computer.



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