11-23-2007 11:46 AM - edited 03-03-2019 07:39 PM
If you have multiple connections to the internet with multiple ISP's (ISP1 10mb, ISP2 T1, ISP3 T1) which BGP metric would you use to advertise to all internet bgp routers the path with the most bandwidth? I know you can set the MED but doesnt that only advertise to the neighbor AS? In this case the 3 ISP are all in different AS's and I dont think MED would be the right one. Anybody have any suggestions?
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11-26-2007 03:03 AM
Hi,
ask your provider a question if he is permitting you to prepend your AS#.
Sometimes ISPs apply as-path lists which allow your AS# only once ocuuring.
BR,
Milan
11-23-2007 01:20 PM
You need to use AS path Prepending for route selection into your AS
eg
route-map preferred permit 10
set as-path prepend
router BGP
neighbor
neighbor
you can also include an access-list or prefix-list in the route map pointing to specific subnets
It is quite possible that even after prepending, the return path is preferable via a particular ISP as it totally depends upon the ISP's connection to the NAP
HTH
Narayan
11-23-2007 01:45 PM
I have tried prepending the routes for the two slower ISP's and it worked succesfully on 1 ISP but when I prepended the route of the other it would not populate BGP routes anymore. (I have two different internet AS's so I can check what is happening in the routing tables of my other AS internet bgp router). I saw the one I prepended working correctly but the other one as soon as I added the prepend it stopped populating routes. Could this be the ISP? or something else?
11-23-2007 01:52 PM
Yes it could be.
How many times you prepended your AS? Sometimes ISPs do filter based on the AS length but it is very rare
can you post the configuration btw
Narayan
11-23-2007 02:12 PM
I only prepended it once....here is a look at what Im doing without posting too much info.
Router 1 (HSRP Primary)
router bgp 1234
no synchronization
bgp log-neighbor-changes
network z.z.z.z mask 255.255.255.0
network v.v.v.v mask 255.255.255.0
network n.n.n.n mask 255.255.255.0
neighbor x.x.x.x remote-as 1234 <---Secondary Router
neighbor 1.1.1.1 remote-as 1111
neighbor 1.1.1.1 soft-reconfiguration inbound
neighbor 1.1.1.1 filter-list 1 out
neighbor 2.2.2.2 remote-as 2222
neighbor 2.2.2.2 soft-reconfiguration inbound
neighbor 2.2.2.2 route-map SETASPATH out
neighbor 2.2.2.2 filter-list 1 out
no auto-summary
route-map SETASPATH permit 10
set as-path prepend 1234
ip as-path access-list 1 permit ^$
Router 2 (HSRP Secondary)
router bgp 1234
no synchronization
bgp log-neighbor-changes
network z.z.z.z mask 255.255.255.0
network v.v.v.v mask 255.255.255.0
network n.n.n.n mask 255.255.255.0
neighbor x.x.x.x remote-as 1234 <---Primary Router
neighbor 3.3.3.3 remote-as 3333
neighbor 3.3.3.3 soft-reconfiguration inbound
neighbor 3.3.3.3 route-map SETASPATH out
neighbor 3.3.3.3 filter-list 1 out
no auto-summary
route-map SETASPATH permit 10
set as-path prepend 1234
ip as-path access-list 1 permit ^$
The secondary router prepended and populated the routes correctly to the internet but the primary router did not advertise the prepended route to the internet after I implemented this. ***UPDATED*** To be more clear it only advertised the route the was not prepended and the other route was removed from other bgp internet routing tables.
11-23-2007 02:20 PM
you should be prepending your own AS i.e AS 1234 and not the remote neighbor AS.
HTH
Narayan
11-23-2007 02:22 PM
that was just a typo on my part...i am prepending my own AS number.
11-23-2007 02:27 PM
Can you use a match statement and prepend only your local subnets and not others?
network topology would be helpful
Narayan
11-23-2007 02:36 PM
I just want to prepend the route so ISP 1 is used to come into our AS. I have a default route already to use ISP 1 for all internal routes...that works fine for uploads. But when I download the traffic is coming in the other ISPs. This is why I was tryin to prepend the routes...
11-26-2007 03:03 AM
Hi,
ask your provider a question if he is permitting you to prepend your AS#.
Sometimes ISPs apply as-path lists which allow your AS# only once ocuuring.
BR,
Milan
11-26-2007 06:27 AM
I called...that was the issue, I knew I was doing all the configuration correctly. Thanks for the tip.
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