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  1. #1
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    Incoming/Outgoing BGP question

    If trace route from me TO the server is Cogent
    And trace route FROM server to me is Abovenet

    What does this mean?

    Does it mean:
    1)whoever (server or client) initiates connection, determines the transit that all incoming/outgoing packets will go through
    2)server incoming will use Cogent, server outgoing will use Abovenet? like, if the server is a gameserver, while playing, my computer will send through Cogent and recieve through Abovenet?
    3)something else?
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  2. #2
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    http://www.cisco.com/univercd/cc/td/...gp.htm#1020647

    Example configuration of inbound load balancing, and using the other path for outbound by default.
    http://www.cisco.com/warp/public/459/40.html#conf5

    Your question?
    Depends highly on the configuration choice they came up with, likely #2 in a fail safe backup way, allowing a form of load balancing.

  3. #3
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    Quote Originally Posted by TDK-Kevin
    http://www.cisco.com/univercd/cc/td/...gp.htm#1020647

    Example configuration of inbound load balancing, and using the other path for outbound by default.
    http://www.cisco.com/warp/public/459/40.html#conf5

    Your question?
    Depends highly on the configuration choice they came up with, likely #2 in a fail safe backup way, allowing a form of load balancing.
    maybe i'm just clueless, but that doesn't answer my question :|
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  4. #4
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    BGP is a tricky protocol, trust me, I've done the CCIE thing more times than necessary. BGP is all about AS hop count. There are literally hundreds of ways to determine which path is better to reach your destination but to keep things simple, let's just stick with simple AS hop counts.

    So from you to your server, your ISP determines that the "best path" is through Cogent. There could be a number of factors here (ISP selects a preferred path, they artifically weight a path, etc) but again, keeping it simple, you packet travels through fewer autonomous systems from your PC to your server via Cogent.

    Now, as far as your server, who probably sits on a different ISP (depnding on how the peering goes, what IP space it sits on, etc), the ISP that handles the initial packet determines that the Above.net path has a shorter AS hopcount than Cogent (they may not even be aware of Cogent, doubtful but again keeping it simple). The thing you are seeing is called asymetrical routing and is fairly common on the internet.

    Within an enterprise, asymetrical traffic is ugly. It presents a number of issues with firewalls, IDS/IPS and other security tools including IPSec's anti-replay window since out-of-order packets might fall outside of the replay window. It's also a doozy on VoIP. But within the wide open internet, it's fairly common especially with ISPs who use Cogent since virtually all other ISPs penalize Cogent peering paths.

    This is an overly simplified explanation and barely scratches the surface of what all possibly could be affecting this but it's large and in part accurate.

  5. #5
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    That sould have gotten you some better idea into it.
    BGP (iBGP/eBGP) is too complex, when dealing with it itself and route maps for various things.
    If you want to actually understand in depth why this or that happens via BGP, I suggest pick up a book which explains it well.

  6. #6
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    To put all of that in English, there's one of two things that I personally see happening:

    1) Your host is trying to push a majority of it's traffic over Cogent and simply splices a little bit of AboveNet in there to make it look like a higher quality product. I have nothing against Above, but they too are a low dollar product these days.

    2) AboveNet is not announcing your IP's but your provider is still egressing traffic to AboveNet, which it will accept so long as there is a BGP session established.

    Again, that's the English version. If you want to have a full blown discussion in theory about it, talk to the guys who initially responded to your post .

  7. #7
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    What I mean is, in the case where trace route from each end is different, how do you know which network actual data is travelling over?

    eg: in the case of a HTTP, is it cogent since i inititiated request? or is my request sent by cogent, and the reply given over abovenet even tho the connection was started over cogent?
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  8. #8
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    It will take the same route that the traceroutes show.
    Inbound Cogent, Outbound Abovenet
    BGP doesn't care about the layer 7 protocol in use HTTP, nor does it care if it is a udp or tcp based connection.
    Their are some possibilities of doing that with route maps, but it is uncommon.

    #2 on Jeff's list is a very good possibility
    This means that no inbound traffic will go to Abovenet because their eBGP route has no ip's listed for your AS.
    But since your AS advertises the ip's to cogent, all inbound traffic will reach you through cogent.
    And outbound traffic has a BGP attribute which says, the Abovenet AS peer has a higher weight or local preference.
    So outbound Abovenet is a more desirable path than cogent.

    I am not a 'expert' when it comes to BGP, but I am sure what I have said is quite valid.
    Still learning myself it is a very interesting EGP. hope this has cleared it up for you somewhat.

  9. #9
    Quote Originally Posted by ewhost
    What I mean is, in the case where trace route from each end is different, how do you know which network actual data is travelling over?

    eg: in the case of a HTTP, is it cogent since i inititiated request? or is my request sent by cogent, and the reply given over abovenet even tho the connection was started over cogent?
    short answer: sure.

    tracert is an 'announce timeout' tool within ICMP with is part of the IP network layer, and specifically the 'trasport protocol' part.
    (huh? think: Please Do Not Throw Sausage Pizza Away - hell, it helped me through my CCNA a few years back LOL)

    7- application
    6- presentaion
    5- session
    4- transport
    3- network
    2- datalink
    1- physical

    so, you've verified that's the case (in one way, back the other) up to layer 3 (ICMP is part of layer 3)
    since HTTP relies on IP (tcp/ip, layer 5-3) there's no reason to think that the session layer (which builds on top of layers 1/2/3/4) which is responsible for QoS and 'host to host' type of communication would be able to steer a different WAY to or from - that's the job of layer 3 :-) Same for the Presentation layer: think data formatting, encoding/encryption- again away from the 'steering' of which way this happens.

    I have a full sized one of these in my home office, helps when i'm tired...

    http://www.javvin.com/pics/map2004-medium.gif

    (ask for one for <insert-your-holiday-of-choice>)

  10. #10
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    Quote Originally Posted by ewhost
    What I mean is, in the case where trace route from each end is different, how do you know which network actual data is travelling over?

    eg: in the case of a HTTP, is it cogent since i inititiated request? or is my request sent by cogent, and the reply given over abovenet even tho the connection was started over cogent?
    Assuming my #2 is correct (and expanding on what Kevin is saying) your traffic is actually going in a huge loop between two providers rather than just traveling on one provider. This is most likely killing your latency.

  11. #11
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    Quote Originally Posted by ewhost
    If trace route from me TO the server is Cogent
    And trace route FROM server to me is Abovenet

    What does this mean?

    Does it mean:
    1)whoever (server or client) initiates connection, determines the transit that all incoming/outgoing packets will go through
    I'm not sure the original poster understands most or any of the technical jargon that has been spewed thus far. Without touching too much on the subject of routing from the OP to his server (which might be above the OP's head), let me just say this. The autonomous system (a network that has the ability to make its own [possibly unique] routing policy) that your server sits behind can *ALWAYS* choose which path to send your packets down. For example, if the provider has Cogent and Abovenet, the provider can pick either carrier to send your bits to (although they have little to no control where the bits go from there, again, this is still a loaded subject).

    Whether or not they do so intelligently is an entirely different subject. You have companies that do default routing only (mine is one, HiVelocity's unmetered offering is another that comes to mind), where packets take *only* one route (AS path) going out, and one or many routes coming in. There are also companies that allow multiple paths coming in, and select the best route going out (GNAX is one that comes to mind). By "best", I don't mean the fewest number of autonomous systems in the path, but "best" in terms of certain metrics like latency, jitter, etc.

    As mentioned, the subject of routing packets from you to your server is influenced by many factors that are far beyond the scope of this thread. You could easily spend days enumerating these things, and many more finding out what they actually mean.

  12. #12
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    I am going to expand slightly on tical's post where he says that the host can depict exactly how the packets are sent from their network or on which part they are received.

    http://www.ev1.net/stats/207.218.223.9_24.html
    http://www.ev1.net/stats/207.218.223.10_36.html

    If you look at the first graph you will see that it has gone steady for some time.
    But the second graph shows something different.
    EV1 has not gotten much traffic on the 3rd Verio link for quite a long time.
    But recently they are now receiving a part of the load inbound on Verio #3. (108.2 Mbps Currently)
    This just goes to show, traffic for the to/from the autonomous system, can be controlled by the autonomous system.
    But like tical said, once the traffic is sent to the next autonomous system, such as Abovenet.
    It is under the control of Abovenet, I will try to stop posting confusing things now.
    Last edited by GFuse|Kevin; 12-08-2005 at 01:08 AM.

  13. #13
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    Ok, I think I get it now...

    So in my HTTP request, I send my packets over Cogent, and recieve my packets over Abovenet?
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