how to multi home l.
Skip this Video
Loading SlideShow in 5 Seconds..
How to Multi-Home PowerPoint Presentation
Download Presentation
How to Multi-Home

Loading in 2 Seconds...

play fullscreen
1 / 33

How to Multi-Home - PowerPoint PPT Presentation

  • Uploaded on

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?.

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
Download Presentation

PowerPoint Slideshow about 'How to Multi-Home' - jaden

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
how to multi home

How to Multi-Home

Avi Freedman

VP Engineering

AboveNet Communications

what is multi homing
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
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.
a multi homed architecture
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
multi homed architecture
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.
how the internet works
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.
inbound traffic routes
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.
inbound traffic routes8
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.
inbound traffic routes9
Inbound Traffic - Routes
  • So if you advertise to AboveNet, -
    • If you’re a network peer, they only re-announce to customers (and use it internally);
    • If you’re a transit peer/customer, they announce to all of their network peers.
  • That’s how you get global *inbound* reachability.
address space issues
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.
address space issues11
Address Space Issues
  • For example, your provider has
  • You have from them.
  • You’re single-homed.
  • The only route on the ‘net for you is the route, “originated” by your provider’s ASN (and you don’t have to do anything special).
address space issues12
Address Space Issues
  • If you have your own CIDR block and are single-homed, your provider will originate it.
  • So, if you have, it’ll be visible as an announcement by your provider, originated into the BGP mesh with your provider’s ASN as the “origin”.
address space issues13
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.
address space issues14
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 if you have
  • If you’re multi-homed, your other provider will have to advertise
  • But *so will your first provider*.
  • Why?
address space issues15
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.)
address space issues16
Address Space Issues
  • So, if ISP 1 advertises only and ISP 2 advertises only, 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 route.
address space filtering
Address Space: Filtering
  • Some ISPs do or did filter on routes smaller than (more specific than) /19s in > space.
  • But it doesn’t matter as long as your two upstreams have good connectivity.
  • Why?
address space filtering18
Address Space: Filtering
  • If Sprint doesn’t see from ISP1 or ISP2, they’ll still see your provider’s route.
  • So if your connectivity to ISP1 (the owner of goes down, all will be well as long as ISP1 still sees from ISP2.
  • Sprint -> ISP1 -> ISP2
  • This is why people don’t let you take IPs...
load balancing outbound
Load-Balancing Outbound
  • You can use static default routes to control outbound packets.
    • ip route serial0/0
    • ip route serial1/0
  • If they’re equal-cost (no metric at the end), it’ll load-balance based on *destination*, by default.
load balancing outbound20
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.
how it works single homed
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.
how it works multi homed static
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.
how it works multi homed static23
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.
how it works multi homed static24
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.
how it works multi homed bgp
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).
the transition static routing
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.
the transition static routing27
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.
the transition bgp routing
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.
the transition bgp routing29
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.
bgp or no
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)
same provider or multiple
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.
  • inet-access mailing list