In our R&S CCIE Mock Lab 2 there is a BGP task that relates to having a particular router prefer an iBGP route as the preferred path to exit its local AS over an eBGP learned path. This seems like a very simple task and it is if you are very thorough with your verification but it ends up being the most commonly missed task in this particular mock lab. Lets start by going over the task and the solution most commonly implemented by students. In the lab R1, R2 and SW2 are in AS 300. R1 and R2 each... Read More
The BGP MED attribute, commonly referred to as the BGP metric, provides a means to convey to a neighboring Autonomous System (AS) a preferred entry point into the local AS.  BGP MED is a non-transitive optional attribute and thus the receiving AS cannot propagate it across its AS borders.  However, the receiving AS may reset the metric value upon receipt, if it so desires. Previous versions of BGP (v2 and v3) defined this attribute as the inter-AS metric (INTER_AS_METRIC) but in BGPv4 it is... Read More
Introduction BGP (see [0]) is the de-facto protocol used for Inter-AS connectivity nowadays. Even though it is commonly accepted that BGP protocol design is far from being ideal and there have been attempts to develop a better replacement for BGP, none of them has been successful. To further add to BGP's widespread adoption, MP-BGP extension allows BGP transporting almost any kind of control-plane information, e.g. to providing auto-discovery functions or control-plane interworking for MPLS/BGP... Read More
Our BGP class is coming up!  This class is for learners who are pursuing the CCIP track, or simply want to really master BGP.  I have been working through the slides, examples  and demos that we'll use in class, and it is going to be excellent.  :) If you can't make the live event, we are recording it, so it will be available as a class on demand, after the live event.    More information, can be found by clicking here. One of the common questions that comes up is "Why does the router choose... Read More
Last week we wrapped up the MPLS bootcamp, and it was a blast!   A big shout out to all the students who attended,  as well as to many of the INE staff who stopped by (you know who you are :)).    Thank you all. Here is the topology we used for the class, as we built the network, step by step. The class was organized and delivered in 30 specific lessons. Here is the "overview" slide from class: Read More
Can you solve this puzzle? R2, R3 and R4 create the service provider network, with MPLS on all three routers, and iBGP at the PE routers.  R1 and R5 are the CE routers. R2, prefers the BGP next hop of for network (R5 loopback). R4, at is an iBGP neighbor. R2#show ip route vrf v | inc B [200/409600] via, 00:06:47 Is R2 preferring an iBGP learned route, which has an AD of 200, over a EIGRP route, which would have an AD of 90? Can you identify why... Read More
It isn't my fault, they configured it that way before I got here! That was the entry level technician's story Monday morning, and he was sticking to it.  :) Here is the rest of the story.   Over the weekend, some testing had been done regarding a proposed BGP configuration.   The objective was simple, R1 and R3 needed to ping each others loobacks at and respectively, with those 2 networks, being carried by BGP.  R2 is performing NAT.    The topology diagram looks like this: The... Read More
One of our students in the INE RS bootcamp today, asked about an OSPF sham-link. I thought it would make a beneficial addition to our blog, and here it is.  Thanks for the request Christian! Reader's Digest version: MPLS networks aren't free. If a customers is using OSPF to peer between the CE and PE routers, and also has an OSPF CE to CE neighborship, the CE's will prefer the Intra-Area CE to CE routes (sometimes called the "backdoor" route in this situation), instead of using the Inter-Area... Read More
Having a blast in Chicago with the RS bootcamp students.    Thanks for all the hard work you are doing this week! A student from a past Reno class, named Michal, asked if I would create a blog post regarding BGP proportional load balancing based on the bandwidth of the links to EBGP peers. It has been on my list of things to do, and here it is. Thanks for the request Michal. The secret to this trick is to pay attention to the links between directly connected external BGP neighbors, (in this... Read More
Introduction In this series of posts, we are going to review some interesting topics illustrating unexpected behavior of the BGP routing protocol. It may seem that BGP is a robust and stable protocol, however the way it was designed inherently presents some anomalies in optimal route selection. The main reason for this is the fact that BGP is a path-vector protocol, much like a distance-vector protocol with optimal route selection based on policies, rather than simple additive metrics. The fact... Read More

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