BGP Dynamic Neighbors are a way to bring up BGP neighbors without specifically defining the neighbors remote IP address. Using the BGP Listen Range command you specify a range of IP addresses typically on your Hub site (maybe in a DMVPN environment) that you trust to become BGP neigbors with you. When a TCP request […]
BGP Weight Attribute – Configuration Tutorial
The BGP weight attribute is the first on the list in the BGP best path algorithm Prefer the path with the highest WEIGHT. Note: WEIGHT is a Cisco-specific parameter. It is local to the router on which it is configured. So if you want to influence your BGP routing outbound on a cisco router you […]
Peering with Route Servers
As your network grows bigger and your internet traffic grows, it starts to make sense to peer directly with other networks rather than simply pay an ISP to deliver all that traffic. Peering is one of the areas in life where the Pareto principle, also known as the 80-20 rule applies: 20% of your potential […]
BGP VPNv4 Troubleshooting Commands
When working with MPLs Layer 3 VPN a lot of people get stuck with the verification, simply because they don’t know the bgp vpnv4 troubleshooting commands. This post will step through some of the verification you can use to verify the routes end to end through a simple MPLS Layer 3 vpn topology. The topology […]
BGP Peer in wrong AS
When configuring BGP with a remote peer you might get the error message BGP peer in wrong AS *Apr 18 08:39:15.455: %BGP-3-NOTIFICATION: received from neighbor 10.0.12.2 passive 2/2 (peer in wrong AS) 2 bytes 0002 This means that you have mis-matched AS numbers in your BGP configuration. You can phone up the remote end and […]
BGP Local Preference Explained
So, what is local preference in BGP? The BGP Local Preference attribute or local-pref is used to influence the outbound routing decision of a router running BGP. The local preference has to be set inbound on routes being received to influence the outbound routing behaviour. A higher local preference is preferred and the default is […]
BGP Confederation vs Route Reflector
To achieve a full mesh of iBGP peerings between more than a few devices becomes quite an administrative challenge. To overcome this you have two tools at your disposal. BGP Confederation and Route Reflectors. For the purpose of this tutorial I will be using the simple topology below. For the first configuration I will be […]
What is BGP Syncronization?
A BGP router with synchronization enabled does not install iBGP learned routes into its routing table if it is not able to validate those routes in its IGP. The BGP syncronization rule states that if your AS passes traffic from another AS to a third AS, BGP should not advertise a route before all the […]
BGP Update Source
If you have started to look at BGP you will very soon come across the BGP update-source command. This command tells BGP which interface to source the BGP packets out of. The short demo below will highlight the problem. I have a very simple setup with 2 routers connected with a serial interface and an […]
Where is the BGP Best Path Selection Algorithm
If you are trying to find the BGP best path selection algorithm on the DOC CD it has been removed from the 12.4 documentation for some crazy reason! You will need to go to this link https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html Let’s examine each step in detail, you will need to master this document so print it off and […]