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18 November 2019

Tips for a Multihoming BGP network



I’ve written about Same BGP Autonomous System Number in two Datacenters recently because I wanted to connect two datacenters through Internet with the same AS Number. However, I have a new task for these days. I have to add more than one ISP to the datacenter. Therefore, I will have to configure BGP properly for several ISPs, which is an interesting and amazing task. It’s also an advanced routing task because a configuration mistake can shut down Internet to customers. I really love networking.

Connecting to Two or more ISPs

Firstly, it’s important to know how Internet routers view our networks. There are lots of websites on the net which are useful to know how other routers can send packets to our networks. There are even routers where we can access freely with read-only access to run commands, which are also useful to see how Internet routes learn our networks. For instance, there are lots of free access routers in the www.routeviews.org website where we can choose a router, access with the Telnet application and finally run commands.

Route Views Project
 
Once we have accessed to a router, we can run commands. It’s important to know what commands we have to execute as well as we have to understand the output. The show ip bgp command is the best one to know how other routers view our networks. For instance, we can run show ip bgp + network IP address to know how many paths there are from one router to our network. We’ll see all paths and the best path, which is the active path. This command is useful to see the BGP topology database.

BGP Topology Database
 
The BGP topology database or BGP forwarding database is a table where all paths are store. This database is updated by the BGP process and we can search how routers can send packets to every network in this database as well as we’ll know attributes such as metric, local preference, weight, etc. It’s important to highlight BGP topology database is different to the routing table because the routing table only has the best path instead of all path as the BGP topology database does.

Routing Table
 
There is an interesting website I’m used to visiting. The Réseaux IP Européens Network Coordination Centre has lots of resources about IPv4 addresses, IPv6 addresses and autonomous system numbers. For instance, we can see easily from the www.ripe.net website how different routers from Internet see our network. Therefore, it’s easy to see from this website if our network is advertised through several ISPs.

RIPE NCC
 
Finally, if you work as a network engineer, you’ll have to understand dynamic routing protocols such as BGP properly. If you are going to advertise your own routes to Internet, you’ll have to know how the BGP protocol works. If you are not sure what you are doing, the best option is to contact with professional services.

Best regards my friends. Keep learning! Drop a line with the first thing you are thinking!

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