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shutdown before bgp configuration

Posted on 2012-03-26
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Last Modified: 2012-04-10
I have an existing BGP connection ISP1 and I am in the process to add another BGP connection with ISP2. It will be a multihomed failover with single router. My question is do I have to shut down my interfaces to ISP1 and ISP2 before configuring the multihomed failover? Thx
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Question by:biggynet
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by:Garry Glendown
ID: 37766031
One question comes to mind - why failover, why not dual homed with load balancing over both ISPs? If it's a price or performance issue, you could always just do some incoming or outgoing prepends on the more expensive (or less performant) link ...
As for the shutdown, I would not see a reason to shut the existing connection down. Though, do make sure you know what you're doing and have suffcient filters in place to avoid announcing routes learned from one ISP to the other and vice versa (though I suppose any decent ISP should have filters in place on their end, apart from a max prefix setting). Once your second session is up, make sure the anouncements sent look correct ("show ip bgp nei x.x.x. advertised" on Cisco)
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by:biggynet
ID: 37766116
"why failover" because that is my customer's choice. So if I understand correctly, there is not need to shut down the interfaces. Correct?
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Robert Sutton Jr earned 1000 total points
ID: 37766141
What type of device are you using and what IOS/Firmware version are you using? Does it support "soft reset"?
Either way fwiw:
If you subsequently change a BGP filter, weight, distance, version, or timer, or make a similar configuration change, you MUST reset BGP connections for the configuration change to take effect.

Hope this helps.
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by:biggynet
ID: 37766186
Cisco 3825
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by:Garry Glendown
Garry Glendown earned 1000 total points
ID: 37766860
No, shutdown is not necessary - otherwise, how would you expect ISPs to work when they have dozens or hundreds of BGP sessions on a router and need to add one?
Will you be receiving just a default route from both ISPs or a full table? I guess the 3825 would be pretty hard pushed receiving two full feeds (though it can handle it)... do not attempt with anything less than 768 megs of memory ;)

How do you plan to run the failover? Prepends, MED, ... ?
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by:biggynet
ID: 37767029
Failover = prepend.
Please tell me more about full routing table and default route from the ISP perspective. Let say I want a full table from the ISP, how would they advetise that? With many many NETWORK commands? Thx
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Expert Comment

by:Garry Glendown
ID: 37767191
No ... prefixes learned through BGP are usually automatically advertised to downstream BGPs ...

If your customer only wants backup, not load balancing/optimization, you might as well let both ISPs just advertise a default route, less strain on the router, and you won't lose much functionality either ... OTOH, taking full prefixes, albeit with prepends on the less desired link, will allow you to reach destinations that for some reasons are not available through the primary link ...
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