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So does BGP consider the amount of time it takes to traverse each hop, or are routing tables built only based on the minimum number of hops it takes to reach ea

So does BGP consider the amount of time it takes to traverse each hop, or are routing tables built only based on the minimum number of hops it takes to reach each destination?

IamMario 9y ago
I am out for a few years and haven't done too much with BGP, but iirc it heavily depends on your BGP configuration. BGP is a highly complex routing protocol and you can tweak it to your exact needs. On default it is just like RIP and takes the shortest path, except that RIP counts each router and BGP only counts the outbound routers.
- bogomipz 9y ago
  >" BGP is a highly complex routing protocol and you can tweak it to your exact needs." BGP itself is actually a relatively simple protocol. The complexity really comes from all the available configuration options.
dsr_ 9y ago
The first thing that a BGP speaking router does is to look up the destination IP address in its table of autonomous system numbers (ASNs). An AS is a group of networks that are all under the control of the same policy maker. If you get your internet connectivity from one ISP, you fit in their AS. If you talk to multiple ISPs, you need to be your own AS (and have routable IP addresses assigned to that AS). Finally, the largest networks may be a confederation of several AS, either as a result of purchases or by deliberate policy. BGP transmits reachability information by building paths of AS that are neighbors. Then, typically, it selects the shortest path of AS hops. (There are a lot of knobs to twiddle with here, both on the advertising side and on the deciding side.) BGP doesn't care or know about numbers of router hops, just the number of AS between here and there. There are lots of "network engineer in a box" products that map out performance and then twiddle knobs on your BGP router to select for best performance (or least cost, if some of your paths are more expensive than others.)
tyingq 9y ago
BGP doesn't inherently measure time for hops. The base setup is to use the path with the lowest number of unique Autonomous Systems (AS). But, there are also concepts of weight, where you can influence the chosen path manually. And, there's also "Multi-Exit Discriminator", or MED, where you can pass a metric to a peer on what the best path to a specific AS is. Even that's an unfair simplification, though. Cisco has a pretty good page that shows how path choices are made: http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-... There are products, though, that tweak these settings to pick for best performance, or lowest bandwidth cost.
bogomipz 9y ago
The latter, BGP is simply a path metric. Your question reminded me though that Internap used to sell an appliance called FCP Flow Control Platform that would take the top 10% of destination ASNs from your edge(via Netflow)and run ping probes to them and allow you set the preference via iBGP to prefer routes with the lowest latency. It was nice in that you could route around network "brown outs" upstream. It has since turned into their Miro platform. There was also a similar appliance called Route Science. I was always surprised that there wasn't ever an open source version of something similar. Although you don't hear a lot about these appliances today. I think maybe because of the proliferation of CDN edge networks. http://www.internap.com/network-services/miro-controller/ http://www.internap.com/network-services/miro-controller/

So does BGP consider the amount of time it takes to traverse each hop, or are routing tables built only based on the minimum number of hops it takes to reach each destination?

IamMario 9y ago

I am out for a few years and haven't done too much with BGP, but iirc it heavily depends on your BGP configuration. BGP is a highly complex routing protocol and you can tweak it to your exact needs. On default it is just like RIP and takes the shortest path, except that RIP counts each router and BGP only counts the outbound routers.

bogomipz 9y ago
>" BGP is a highly complex routing protocol and you can tweak it to your exact needs." BGP itself is actually a relatively simple protocol. The complexity really comes from all the available configuration options.

dsr_ 9y ago

The first thing that a BGP speaking router does is to look up the destination IP address in its table of autonomous system numbers (ASNs). An AS is a group of networks that are all under the control of the same policy maker. If you get your internet connectivity from one ISP, you fit in their AS. If you talk to multiple ISPs, you need to be your own AS (and have routable IP addresses assigned to that AS). Finally, the largest networks may be a confederation of several AS, either as a result of purchases or by deliberate policy. BGP transmits reachability information by building paths of AS that are neighbors. Then, typically, it selects the shortest path of AS hops. (There are a lot of knobs to twiddle with here, both on the advertising side and on the deciding side.) BGP doesn't care or know about numbers of router hops, just the number of AS between here and there. There are lots of "network engineer in a box" products that map out performance and then twiddle knobs on your BGP router to select for best performance (or least cost, if some of your paths are more expensive than others.)

tyingq 9y ago

BGP doesn't inherently measure time for hops. The base setup is to use the path with the lowest number of unique Autonomous Systems (AS). But, there are also concepts of weight, where you can influence the chosen path manually. And, there's also "Multi-Exit Discriminator", or MED, where you can pass a metric to a peer on what the best path to a specific AS is. Even that's an unfair simplification, though. Cisco has a pretty good page that shows how path choices are made: http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-... There are products, though, that tweak these settings to pick for best performance, or lowest bandwidth cost.

bogomipz 9y ago

The latter, BGP is simply a path metric. Your question reminded me though that Internap used to sell an appliance called FCP Flow Control Platform that would take the top 10% of destination ASNs from your edge(via Netflow)and run ping probes to them and allow you set the preference via iBGP to prefer routes with the lowest latency. It was nice in that you could route around network "brown outs" upstream. It has since turned into their Miro platform. There was also a similar appliance called Route Science. I was always surprised that there wasn't ever an open source version of something similar. Although you don't hear a lot about these appliances today. I think maybe because of the proliferation of CDN edge networks. http://www.internap.com/network-services/miro-controller/ http://www.internap.com/network-services/miro-controller/