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How to Multi-Home. Avi Freedman VP Engineering AboveNet Communications. What is Multi-Homing?. Multi-homing is the process of selecting, provisioning, and installing a redundant connection to the Internet. Could be the same provider, or a different provider. Why Multi-Home?.

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How to multi home l.jpg

How to Multi-Home

Avi Freedman

VP Engineering

AboveNet Communications


What is multi homing l.jpg
What is Multi-Homing?

  • Multi-homing is the process of selecting, provisioning, and installing a redundant connection to the Internet.

  • Could be the same provider, or a different provider.


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Why Multi-Home?

  • Slow is 1,000,000% better than dead.

  • You may be out of bandwidth.

  • And

    • Telco circuits die.

    • Routers die.

    • Providers’ networks fail.

    • Different networks have better performance to different sites.


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A Multi-Homed Architecture

  • Ideally, take advantage of the opportunity to multi-home to remove all single points of failure in your network.

  • Use -

    • Multiple providers, unless your current provider will let you have cheap backup

    • Multiple routers

    • Multiple telco vendors


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Multi-Homed Architecture

  • Two routers, each with a different WAN connection from a different telco vendor.

  • Use HSRP or VRRP internally to make both routers look like one “virtual” router.

  • Eventually, multiple providers.

  • Upcoming Boardwatch article with configs.


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How the Internet Works

  • Well, it breaks more than it works but when it does work -

  • The Internet is a network of networks.

  • Each network (called Autonomous System) on the Internet announces “routes”, which are lists of the IP addresses of the boxes on their network.

  • You need to be able to send packets *to*, and get packets *from*, everywhere.


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Inbound Traffic - Routes

  • Routes are announced via BGP4 (the Border Gateway Protocol)

  • Routers are announced to BGP peers.

  • Each “BGP peer” can be a “network peer” or a “transit peer”.

  • Network peers exchange just lists of customer routes.

  • Each route is tagged by the ASNs it passes through.


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Inbound Traffic - Routes

  • So when AboveNet and UUNET peer, only AboveNet and UUNET routes are exchanged. No Sprint, PSI, etc...

  • Transit peers -

    • Announce to their customers all of the routes on the ‘net (AboveNet, UUNET, Sprint, PSI, and the 60,000+ routes on the ‘net).

    • Announce to their peers all routes heard via transit.


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Inbound Traffic - Routes

  • So if you advertise 207.106.96.0/19 to AboveNet, -

    • If you’re a network peer, they only re-announce 207.106.96.0/19 to customers (and use it internally);

    • If you’re a transit peer/customer, they announce 207.106.96.0/19 to all of their network peers.

  • That’s how you get global *inbound* reachability.


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Address Space Issues

  • Noone wants to hear a route for you unless -

    • You are multi-homed (even then, some people don’t want to hear routers), or

    • You have your own direct IP space allocation from ARIN, RIPE, or APNIC.

  • So, when you’re single-homed without your own space, your IPs are reachable because they’re part of your provider’s “aggregate” block.


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Address Space Issues

  • For example, your provider has 207.8.128.0/17.

  • You have 27.8.197.0/24 from them.

  • You’re single-homed.

  • The only route on the ‘net for you is the 207.8.128.0/17 route, “originated” by your provider’s ASN (and you don’t have to do anything special).


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Address Space Issues

  • If you have your own CIDR block and are single-homed, your provider will originate it.

  • So, if you have 219.190.64.0/19, it’ll be visible as an announcement by your provider, originated into the BGP mesh with your provider’s ASN as the “origin”.


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Address Space Issues

  • If you have your own IP space and want to multi-home, addressing issues are simple.

  • Your other provider will start also originating your IP blocks.

  • Or you’ll start speaking BGP, originate your IP blocks, and your providers will re-advertise them to the world.


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Address Space Issues

  • If you don’t have your own IP space, it’s a bit more complicated.

  • So, normally your ISP will only be advertising 207.8.128.0/17 if you have 207.8.200.0/23.

  • If you’re multi-homed, your other provider will have to advertise 207.8.200.0/23.

  • But *so will your first provider*.

  • Why?


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Address Space Issues

  • Routes are chosen first by specificity.

  • That is, to how many IP addresses they refer.

  • The route “covering” the fewest IP is the most specific, and wins.

  • (Otherwise default would always win and nothing would work.)


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Address Space Issues

  • So, if ISP 1 advertises only 207.8.128.0/17 and ISP 2 advertises only 207.8.200.0/23, all inbound traffic from the ‘net will come in on ISP2.

  • So, ISP 1 needs to “blow a hole in their filters” to “leak” the more specific 207.8.200.0/23 route.


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Address Space: Filtering

  • Some ISPs do or did filter on routes smaller than (more specific than) /19s in > 205.0.0.0 space.

  • But it doesn’t matter as long as your two upstreams have good connectivity.

  • Why?


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Address Space: Filtering

  • If Sprint doesn’t see 207.8.200.0/23 from ISP1 or ISP2, they’ll still see your provider’s 207.8.128.0/17 route.

  • So if your connectivity to ISP1 (the owner of 207.8.128.0/17) goes down, all will be well as long as ISP1 still sees 207.8.200.0/23 from ISP2.

  • Sprint -> ISP1 -> ISP2

  • This is why people don’t let you take IPs...


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Load-Balancing Outbound

  • You can use static default routes to control outbound packets.

    • ip route 0.0.0.0 0.0.0.0 serial0/0

    • ip route 0.0.0.0 0.0.0.0 serial1/0

  • If they’re equal-cost (no metric at the end), it’ll load-balance based on *destination*, by default.


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Load-Balancing Outbound

  • Why load-balance based on destination?

  • For internal networking, sometimes per-packet-load balancing makes sense.

  • But if you’re trying to talk to England and one provider has a 60ms path and the other has a 150ms path, packets will arrive out of order and TCP and UDP apps get unhappy and slow.


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How it works, Single-Homed

  • Outbound (easy):

    • Use a default route to your provider.

  • Inbound:

    • Your provider originates a large (aggregate) BGP route, and gives you some space from inside it; and/or

    • Your provider originates BGP routes for your ARIN/RIPE/APNIC CIDR blocks as well.


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How it Works, Multi-Homed, Static

  • Outbound (easy):

    • Load-balance default routes to deal with outbound packets.

  • Inbound:

    • Your providers both originate BGP routes for just the address space you’re using, even if it’s out of one provider’s space; and/or

    • Your providers both originate BGP routes for your ARIN/RIPE/APNIC CIDR blocks as well.


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How it Works, Multi-Homed, Static

  • Special note:

    • When providers configure BGP for single-homed customers, they will generally “nail up” your routes (even your directly-issued) CIDR blocks, so that if your connection goes down and up and down and ..., they don’t have to flap that route out to the whole Internet. This is a good thing.


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How it Works, Multi-Homed, Static

  • Special note (ctd):

    • But you NEED to make sure, when you’re multi-homed, that the providers are NOT nailing your routes up.

    • Why?

    • Because if they do, when one T1 goes down, that provider will still advertise you to the world, thus “blackholing” you.


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How it Works, Multi-Homed, BGP

  • Topic of next talk.

  • You either load-balance outbound with statics, or take full routes from your providers (if you can).

  • You originate advertisements under your ASN for your directly-issued CIDR blocks, AND for the parts of your providers’ space that you’re using (with their permission).


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The Transition: Static Routing

  • To transition:

    • Turn up the other T1/T3/Ethernet.

    • Put IPs on the interface.

    • Run tests end-end.

    • Start load-balancing default to the new T1.

    • Then, in the middle of the night, have the new provider start advertising your IP space. Make sure you have reachability to every other ISP you can think of afterwards.


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The Transition: Static Routing

  • To transition (ctd):

    • After testing it live, turn off your other transit pipes and make sure that, after a few minutes, you still have connectivity.


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The Transition: BGP Routing

  • To transition:

    • Turn up the other T1/T3/Ethernet.

    • Put IPs on the interface.

    • Run tests end-end.

    • Start load-balancing default to the new T1.

    • Then, undo that and bring up a BGP session that permits no routes either way.

    • Then start taking routes, and watch outbound traffic.


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The Transition: BGP Routing

  • To transition (ctd):

    • Then, start announcing your routes.

    • Then, in the middle of the night, have your ISP take out the static route and BGP announcement they were making.

    • Make sure your route is propagating.

    • Test reachability.

    • Turn off your other pipes.

    • Test reachability.


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BGP or no?

  • Advantages of doing static -

    • Cheaper/smaller routers (less true nowadays)

    • Simpler to configure

  • Advantages of doing BGP -

    • More control of your destiny (have providers stop announcing you)

    • Faster/more intelligent selection of where to send outbound packets.

    • Better debugging of net problems (you can see the Internet topology now)


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Same Provider or Multiple?

  • If your provider is reliable and fast, and affordably, and offers good tech-support, you may want to multi-home initially to them via Frame, SMDS, or some backup path (slow is 1,000,000% better than dead).

  • Eventually you’ll want t multi-home to different providers, to avoid failure modes due to one provider’s architecture decisions.


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Questions?

  • [email protected]

  • inet-access mailing list



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