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Linux - Networking This forum is for any issue related to networks or networking.
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Old 04-07-2010, 04:40 AM   #1
Registered: Dec 2006
Location: London, UK
Distribution: Centos, Fedora
Posts: 53

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why accept routes in bgp when you have default gateway from ISP?

I am realtively new to BGP. I use BGP in my network to advertise my /20 subnet. What i would like to know is what is the point of accepting routes from your neighbouring AS(ISP)?

My ISP has given me a default gateway, and no matter what I want to reach on the internet I have to go through that default gateway, so why populate the routing table with soo many routes?

also, as i understand it, there are 3 kinds of routes that you can accept, those are full routes, directly connected routes and default routes. What is the diffrence between them and again how do they affect routing since my network only has the one default gateway?

Maybe I am totally unaware of the mechanics of BGP and am asking a stupid question, but any insight or links to url that will help me understand will be appreciated.

Last edited by saiyen2002; 04-07-2010 at 05:05 AM.
Old 04-10-2010, 08:37 AM   #2
Registered: Mar 2009
Posts: 249

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It's only useful if you have more than one Internet connection.

Full routes - an explicit route to every prefix advertised into the default free zone using BGP, it allows your router to choose the best route out of you network, but the router must be able to cope with a large (> 300,000 entries) route table.
Default routes - a default gateway is configured using BGP, the advantage over static configuration is that broken link or router is detected. Large route tables are not needed, but your router can't pick the best route.
Directly connected routes - I'm not sure, it could either refer to networks directly connected to yours, or to your ISP.
Old 04-12-2010, 04:32 AM   #3
Registered: Dec 2006
Location: London, UK
Distribution: Centos, Fedora
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Originally Posted by TimothyEBaldwin View Post
the advantage over static configuration is that broken link or router is detected.
can the problem of static route being broken be overcome by the use of administrative distance. If i set up 2 static routes of default gateways to 2 ISP and setup administrative distances, then if one route falls off the other will kick in?

Last edited by saiyen2002; 04-12-2010 at 04:41 AM.


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