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I've just lodged a request with upstream to clarify some specifics of their recommended BGP configuration. Currently, I will be dual-homing with my upstream via PI addresses. Advertising via BGP with my own AS, and have requested they pass me a default route.

Their recommended configuration is to have two cross-connects, one from each of my primary/secondary router, connecting to their primary/secondary router, respectively. My primary would their peer with their primary, and my secondary with theirs. This is all fine (aside from the potential no-connectivity scenario if my primary is down, and their secondary is down).

But my confusion is this: My provider is suggesting that by taking only a default, I am going to have an active/standy configuration, and that I need a full set of routes for active/active.

My understanding was that if I'm only homed via a single ISP, then a full-feed becomes effectively pointless, since all traffic is going the same path either way. What am I missing that would cause this to be an active/standy configuration? Surely the secondary router, if traffic hits it, is going to prefer to send out it's local default gateway, as opposed to first sending traffic to the primary?

Taking it from another perspective, I can't see how full routes is going to change the behavior (ignoring possible traffic engineering scenarios), since both routers would have the same set of routes from upstream.

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Absent any non-standard configuration, it is how you say. For outbound traffic, which uplink is used depends on which router the traffic hits. For inbound traffic, it should work the same way but that depends largely on your upstream's configuration. Full routes are really only required if you intend to peer with another upstream.

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With only a single prefix (0.0.0.0/0) received on your end, you cannot distinguish between their uplink. Maybe what you requested (dual homing) implies for them to have redundancy (POP/Link/Upstream router) and that with a default route you pay for something you don't use???

As one of their router could have a better AS-Path to some prefix, you could have seen one link with better routes than the other.

If your provider has two different path to their routers in your site (ie: they're connected to different network location in their own network) and that you only want a default route, you will not see any difference between one link or another as you would if receiving their whole prefix table.

Full Route is useful if you plan to fiddle with attributes to do some decision yourself (LocalPref for outgoing traffic, AS-Path Prepend to attract incomming traffic and other fun BGP stuff)

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  • I agree with your points on traffic engineering (LocalPref, MED, etc) though I'm trying to avoid these to keep things simple for the team at the moment. I'll have to shoot upstream an email regarding if the AS-path differs between their routers (I had just assumed they would be identical). Totally valid points there. – Geekman Feb 3 '14 at 22:49
  • But the bit I don't understand is where you say "and with a default route you pay for something you don't use"... This seems to be what my upstream implies, and I don't get it? To me, either with defaults or a full feed, I need to rely on my IGP to push traffic to both routers, since I'm obviously not going to push the full feed into my IGP. And so if I have to do that anyway, surely that's automatically going to allow traffic to pass over both uplinks simultaneously (eg. active/active), it just might not be optimal if the AS path is slightly longer on one upstream link. – Geekman Feb 3 '14 at 22:50
  • Again, I can totally see this in multi-homed or traffic engineering scenarios. But short of that, in a simple dual-homed setup, I don't see how it helps (aside from if there's different AS paths on the upstream routers, as you say) – Geekman Feb 3 '14 at 22:53
  • My point is that "Maybe" you are paying for something you're not using. If you only have default routes, both uplink looks the same from your standpoint (you only receive prefix 0.0.0.0/0 with a single AS in the path) and it COULD be that by asking your provider to be dual homed they engineered a setup that is truly multi-path (connected to different points, with different physical path & logical path in their network) This is only a possibility, you need to check what their "dual homed" service entails and why they commented about active/passive. – Remi Letourneau Feb 4 '14 at 13:27
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If you take the default routes inject them into your IGP and you have a L3 switch between your firewalls and your ISP routers then you will CEF load balance (src-dest-port). If you don't have an L3 switch then running GLBP if your routers are on the same subnet with your firewalls will distribute your outbound traffic. If you cant do that (or only have 1 firewall) then you have to content yourself that most of the time, inbound traffic is heaver than outbound traffic. Now as to inbound since it seems you said you have a single provider it is likely that they will use only 1 link for inbound because at some point BGP only passes 1 path (bestpath). Taking in the full routing table can help you with outbound as you will take the best path out BUT, since its the same ISP, I don't think you will see a lot of difference in the routes each side sends you.

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