May
10
Update: Congrats to Mark, our winner of 100 rack rental tokens for the first correct answer, that XR2 is missing a BGP router-id.  In regular IOS, a router-id is chosen based on the highest Loopback interface.  If there is no Loopback interface the highest IP address of all up/up interfaces is chosen.  In the case of IOS XR however, the router-id will not be chosen from a physical link.  It will only be chosen from the highest Loopback interface, or from the manual router-id command.  Per the... Read More
Oct
12
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
Nov
22
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
Aug
21
One of the common questions that comes up is "Why does the router choose THAT route? We all know, (or at least after reading the list below, we will know), that BGP uses the following order, to determine the "best" path. So now for the question.   Take a look at the partial output of the show command below: Read More
Aug
16
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
May
25
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 1.1.1.1 and 3.3.3.3 respectively, with those 2 networks, being carried by BGP.  R2 is performing NAT.    The topology diagram looks like this: The... Read More
Apr
08
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
Apr
06
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
Jan
30
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
Jan
17
Abstract: Inter-AS Multicast VPN solution introduces some challenges in cases where peering systems implement BGP-free core. This post illustrates a known solution to this problem, implemented in Cisco IOS software. The solution involves the use of special MP-BGP and PIM extensions. The reader is assumed to have understanding of basic Cisco's mVPN implementation, PIM-protocol and Multi-Protocol BGP extensions. Abbreviations used mVPN – Multicast VPN MSDP – Multicast Source Discovery Protocol PE... Read More

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