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It’s been five years since I started this blog! Time flies and a lot has happened since. Thanks for being along for the ride. What better way to celebrate than a blog post?
This post is going to be short and to the point.
Many of us run HSRP or VRRP. It is quite common to run it in a topology where you have dual routers and dual exits to the WAN and you don’t want to black hole your traffic.
One traditional way of achieving this is by tracking the interface that goes towards the WAN. There are a couple of drawbacks to this approach though:
The next option is to use IP SLA that sends ICMP Echo towards the next-hop of the WAN or some destination further into the network. Ehanced Object Tracking (EOT) can then be used to create a track object that decrements the priority of the HSRP active router when the ICMP Echo probe fails. This works better but there are still some drawbacks to this approach:
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As I’ve written about previously (The Importance of BGP NEXT_HOP in L3VPNs), the BGP NEXT_HOP attribute is key to ensuring end to end connectivity in an MPLS L3VPN. In the other article, I examine the different forwarding behavior of the network based on which of the egress PE’s IP addresses is used as the NEXT_HOP. In this article I’ll look at the subnet mask that’s associated with the NEXT_HOP and the differences in forwarding behavior when the mask is configured to different values.
There is a lot of (mis-)information on the web stating that the PE’s loopback address — which, as I explain in the previous article, should always be used as the NEXT_HOP — must have a /32 mask. This is not exactly true. I think this is an example of some information that has been passed around incorrectly, and without proper context, and is now taken as a rule. I’ll explain more about this further on in the article.
Here’s the example network:
Note that R2 and R7 are the PEs and they each have a /24 mask on their loopback0 interfaces. The PEs are peering via their loopbacks. OSPF is running between R2, Continue reading
Tom has an interesting post over at The Networking Nerd on one of my favorite areas of discussion — certifications. To give you a sense —
I think the problem exam writers face is the defensibility problem. The problem of defensibility has been so strongly pushed into my head, from my years working on the CCDE and CCAr, that I tend to apply the problem to just about everything I do any longer. To state the problem, within the certification space, as succinctly as possible —
If someone sues me because they failed this exam, what evidence can I bring forward to prove this specific person should not have passed the exam.
It’s actually not as easy of a question to answer as it might appear. Why is your cut score set to x? Continue reading
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