What BGP routes require -Multihoming

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Nov 17th, 2007
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I am hosting a website which will be accessed by the clients across the globle .I have router 3725 and links from diffrenet ISPs what kind of routes should I take from these ISPs so that My site will be accissible all the times if any one ISP fails...Should I take default routes from both ISPs? Thanks in advance

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Edison Ortiz Sat, 11/17/2007 - 11:58
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The default route from the ISP should be sufficient for multi-homing.

The important thing for multi-homing is having both ISPs announcing your public segment. One ISP will function as primary while having the other ISP as secondary.

It's up to the ISP how they want to manage the routing preference.

ghanshyam.saini Sun, 11/18/2007 - 07:56
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Thanks for your reply, So

1> I can take default routes from both the ISPs.

2> What things i need to keep in mind when configuring my router.

I have /24 network.


Edison Ortiz Sun, 11/18/2007 - 10:37
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saimbt Sun, 11/18/2007 - 21:13
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Since you have a /24 network, load sharing is not possible and the secondary link would be lying idle.

To avoid asymmetric routing Local Preference for outgoing traffic and AS-prepend for incoming.


ghanshyam.saini Sun, 11/18/2007 - 21:19
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Why with /24 network load sharing not possible.

And i din get your second line in your reply

saimbt Tue, 11/20/2007 - 00:25
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If you have a /24 range, usually ISP will not advertise anything higher than a /24 range.

You can do outbound load SHARING using PBR, but for incoming traffic you will do a AS prepend for the entire /24 range and BGP will always select a single path.


saimbt Tue, 11/20/2007 - 00:39
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for incoming you must atleast have 2 /24 networks.

A single /24 will not be enough

ghanshyam.saini Tue, 11/20/2007 - 00:43
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Can I have any link thats provides info about the config BGP with 2 /24

saimbt Tue, 11/20/2007 - 01:23
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I am not aware of any link, but i can run you through the logic,

consider the following scenario

a/24 b/24 ISP1 ISP2

for a/24 pri would be ISP1 sec would be ISP2

for b/24 pri would be ISP2 sec would be ISP1

Rate if this helps.

ghanshyam.saini Tue, 11/20/2007 - 01:36
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In this case from which block I will use the IP address in router's inside Interface, FWs etc..will I use diffrenet matrics for this and what does it make difference for ISP other than they will publish my two networks


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