Re: Multihome with BGP using /24 prefix

donlon · ‎01-06-2005

Hello all

Am hoping to get some info and advice to sort out a possible multihoming implementation.

Scenario: Have separate 3725 with 256MB of memory,each connected to different ISPs. An E-commerce site, we're more concerned with high-availability rather than load-sharing (although both would be nice).

We can get a class C from one of the Service Providers (ISP A), my concern is the advertising via ISP B.

Assuming they advertise my class C via BGP,

how likely is a class C route to get propagated throughout the Internet without some major Providers filtering it for being too small a block? Being an E-commerce site, if our primary connection to ISP A goes down, the redundant link is of limited usefulness if say... 30% of the Internet doesn't know how to route to us (because local Providers drop our /24 prefix).

Comments?

marikakis · ‎01-07-2005

As far as I know, a /24 prefix is generally acceptable.

But you have to check if your ISPs will agree on the setup.

Will you rely on the two ISPs to advertise the /24 ?

Has ISP B agreed to advertise the /24 of ISP A ?

Has ISP A agreed to let ISP B advertise part of their blocks ?

Will ISP A advertise the /24 separately from any summarized block it has been assigned ?

Or will you be running BGP in your routers ?

M.

jgiri · ‎01-07-2005

Hi,

You can always inform the upstream service providers on the networks you own and ask them to allow the same.

Even if it is /24 it should not be a problem.

regds,

Jagadeesh Giri

donlon · ‎01-08-2005

Thanks for your input. Yes I would run BGP on the 3725s, though I think there's little need to accept anything other than a default route from upstream. I figure I could use AS path prepending to my outgoing announcements and thus adjust incoming traffic if I wish to explore load sharing (that's a secondary concern).

However the real question I have is not about whether the two Service Providers I do business with will advertise my /24 (they will), rather its other large Service Providers on the Internet. If some of them drop a /24 prefix for being too small then that partially defeats the purpose of implementing BGP to begin with (High-Availability to a E-commerce site is the goal).

In regards to ISP A, It doesn't matter if they aggregate my /24 into their large block (in fact its probably desirable as that guarantees it won't get dropped). The potential problem I see is the BGP route I advertise that comes out of ISP B (it's a /24 prefix). In the instance of my primary link to ISP A going down, I could find (unpleasantly) that a good portion of the Internet isn't finding their way to my site because my route passed out from ISP B was filtered by some Internet heavyweights for being to short.

The purpose of this post was to tap into the knowledge base out there and attempt to get some feedback on if BGP would really serve my purpose.

Once again, I'd appreciate anyones ideas on the matter.

Richard Burts · ‎01-09-2005

There is a significant effect if ISP A does summarize the address space and ISP B advertises the /24. Then your route will appear as part of a block (perhaps part of a /20 from the ISP A) and as a /24. IP routing prefers the longest prefix match, so all the traffic will flow to you via ISP B. Whether that is a problem or not is something that you will have to figure.

Given that you have said several times that your primary purpose was high availability, it may be that you do not care much whether traffic comes to you via A or B as long as it gets to you. But you will need to think about this.

One other factor to consider is the possibility (or liklihood) of assymetric paths. What would happen if someone attempted to access your site and the packet arrived via ISP B and you sent the response via ISP A? (Are there stateful firewalls inspecting traffic for which assymetric paths are a problem?)

HTH

Rick

HTH

Rick

marikakis · ‎01-10-2005

Running BGP is good for your multihoming case,

no matter if you are interested only in receiving defaults or partial routing.

I asked if ISP A will advertise the /24 separately from any

summarized block it has been assigned for the effect that Rick mentioned above.

This was for the case you wouldn't be running BGP in your routers

and ISP A would have to actually originate the /24 prefix.

In such a case, if ISP A wouldn't originate the /24 separately from

any summarized block it has been assigned ,under normal circumstances

you would get all your traffic via the link to ISP B.

But now that you will run BGP, there will be 2 advertisements originated from your AS,

plus any summarized block of ISP A originated from ISP A's AS.

This is not a problem for you, because ISP A will also leak your specific to the Internet

the same way that ISP B does (transit, and not originating AS).

Since you are running BGP, you will have no general problem with the/24.

The /24 is generally acceptable in the SP community.

If you are still not convinced,

there is actually a way to check this out to some extent.

If you proceed with a load-sharing scenario,

you will normally receive some traffic from ISP B.

Do the sum of traffic in/out via the 2 ISPs,

and see if this is roughly equal to the traffic you used to get from ISP A only.

Also, you can check from some Looking Glasses

if the /24 prefix through ISP B (and ISP A)

is reaching some indicative routers around the world.

For example,

http://etabeta.seabone.net/cgi-bin/lg.pl

http://reporter.teleglobe.net/bin/lg

http://www.linx.net/tools/index.thtml

Or choose some other LG from the following URL:

http://www.sjp.dropbear.id.au/lookingglass.html

M.

fransedano · ‎01-09-2005

I'm using a solution similar as yours; I have a /24 PI assigned and I'm using also a /24 PA from one of our ISPs. Announcing via BGP both /24 to both ISPs, no problems so far.

donlon · ‎01-10-2005

Thanks all

That sort of real world feedback was exactly what I was looking for. I don't really want to go to the effort of implementing BGP (getting ASN, ISPs buyin, etc) only to find it didn't suit my purpose.

Apparently it will, I don't particularly care which link traffic follows in and out of. They're both Top Tier Providers and we've got DS3 speeds.

Yes,I'm aware of the sortof "reverse" effect that can come about if ISP A aggregates my /24 into their /20 (assuming here that I got my /24 from ISP A). Perhaps traffic will tend to follow the /24 route that ISP B advertises. I can live with that.

Regarding asymmetric paths, don't see how that will bother the PIX. The source and destination IPs and port info stay the same between the web server and the client hitting our site no matter if it goes out one link and back in another(the site isn't physically distributed).

Here's an issue I'd like to toss out for discussion.

There's a 4705 switch in the core. I'm thinking I can run EIGRP between it and the 3725s and redistribute the default route on each router (default route to each ISP) into EIGRP. Then the switch would have equal cost default routes and could load-balance outbound traffic.

Any comments on that ?