James wrote:
So BGP-4 is still the only solution to multi-homed networks.....?
Here's one treatise on the subject:
http://www.ietf.org/internet-drafts/draft-nagami-mip6-nemo-multihome-fixed-network-03.txt
BGP is really your only option, but your providers are not going to give
you the option. Let say we actually get bgp setup and it looks like this.
your router
int fe0/0
description comcast
ip address 24.24.24.124/24
int fe1/0
description tmobile
ip address 64.64.64.164/24
router bgp 65555
network 24.24.24.124/32
network 64.64.64.164/32
neighbor 24.24.24.1 remote-as $comcast-as#
neighbot 64.64.64.1 remote-as $tmobile
So at this point you're announcing to 24.24.24.124/32 and
64.64.64.164/32 to both providers. They accept these routes and also
announce 0.0.0.0/0 back to your. Your route table will look like this
CON 24.24.24.124/24 [1/4] is directly connected
CON 64.64.64.164/24 [2/4} is directly connected
BGP 0.0.0.0/0 [3/4] via 24.24.24.1
BGP 0.0.0.0/0 [4/4] via 64.64.64.1
If int fe0/0 fails that BGP session will drop and that route will be
withdrawn from the routing table. That the basic way BGP works for a
multi-homed system.
So far so good. However your providers will not announce the /32 routes
you are announcing to them to their peers. Ever. Because those routes
are too small and the memory requiremets to do that aren't feasible.
These days with massive filter you can get a full BGP table down to
140-160k routes. If you allowed routes more specific than a /24 to be
announced we'd see a million plus routes and the internet would start
breaking. You could say pay your providers enough and they might
actually announce the /32 routes, but their peers would filter anything
smaller than a /24 route and some peers have been known to filter
anything smaller than /22.
I want to connect to 24.24.24.124 from my DSL account. The routers
between us route my packet to you via the comcast route because BGP on
those routers says "24.24.0.0/16 --> Comcast". I never see a
"24.24.24.124 --> Comcast or Tmobile" so when Comcast goes down I can't
get to you through tmobile because there is no route for that in the
routing tables.
This is why most important server go to a data center. The data center
has a nice big chunk of IP space say 209.247.4.0/22 which they can
announce to their multiple providers who is turn pass the route on to
the their peers. So the route table might look like this:
my DSL -> Level 3 -> XO -> datacenter
my DSL -> PSI -> MCI -> datacenter
and so on.
If this is something you're interested in learning more about the most
useful book I found back in my Wan Engineer days was Internet Routing
Architectures by Sam Halabi and Danny McPherson. It's a bit Cisco
centric, but very readable and has a number of good real world examples.
kashani
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