BGP ISDN flapping

Unanswered Question
Mar 6th, 2004
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R1----ISDN-----R2


only ISDN between R1 and R2


R1

Loop0 192.168.1.1/32

bri0 192.168.21.1/24

R2

Loop1 192.168.2.2/32

bri0 192.168.21.2/24


Only run eBGP between R1 and R2,


I set ISDN dialer idle-timeout 30 and did not change the default keep-alive(60s) for BGP.


now, the ISDN is flapping. I tried to modify BGP timer and deny tcp port 179, but still flapping.


any ideas?


Thanks


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seilsz Sat, 03/06/2004 - 23:10
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I'm assuming since you blocked tcp/179 that you don't want BGP to initiate an ISDN call?


You can determine what is causing the ISDN to dial by using the debug command 'debug isdn events'.


~Zach

ruwhite Sun, 03/07/2004 - 07:45
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I'm assuming he means he blocked it from triggering the interesting traffic. If he blocks tcp completely across the port, then how's he going to run BGP to get routing information? You'll have to block BGP out of both ends, though, and make it not only traffic sourced from 179, but also destined to 179. One end of any given BGP connection is always on an ephermal port.


:-)


Russ.W

seilsz Sun, 03/07/2004 - 08:40
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Yep ... I was referring to the dialer-list when I mentioned blocking.


~Zach

samsam_wang Sun, 03/07/2004 - 11:16
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I guess i asked a stupid question, i think no way to solve the flapping.

ruwhite Thu, 03/11/2004 - 07:11
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No, we're all just trying to figure out what you mean by "flapping." Is the link dialing, and you don't want it to? Or is the link shutting down, and you don't want it to? Or is the link staying up, and the BGP session is failing and coming back?


:-)


Russ.W

samsam_wang Thu, 03/11/2004 - 09:18
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Thanks for your reply.


R1---ISDN---R2


only run OSPF between R1 and R2, we can use "ip ospf demand-circuit" to avoid the ISDN flapping, because the Hello was suppressed.


But, if run BGP between R1 and R2, and BGP send "keep alive" every 60 seconds, and the ISDN will be up because the "keep alive", after that, ISDN will be down because "no traffic",(idle-timeout 30), and ISDN will be up again, because the next "keep alive" coming and will be down again without traffic.


Right?


so I think, no way to solve the problem, cisco does not provide command like "ip bgp demand-circuit".



I have the question because I was thinking in a large enterprise network, run IGP and BGP, the ISDN just for backup and no flapping for IGP. but maybe the BGP will keep the ISDN flapping and we have to pay for the flapping.




seilsz Thu, 03/11/2004 - 15:08
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You can make the BGP keepalive packets not initiate the ISDN call by denying them in an access-list referenced by the 'dialer-list' command. However, that would require you to use some other type of routing to get the intial user data to initiate the ISDN call. A floating static route may work ...


Can you share more information about your architecture?


~Zach

horst.wagner Thu, 03/11/2004 - 23:30
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Hi,

if you want to use isdn for Backup purposes you can use for example loopback interfaces as neighboring ip´s. Don´t forget bgp multihop and neighbor .... source interface and so on.

So you can force bgp via static routes or igp to use the primary line for peering to the neighbor if availabel and if the primary line goes down to trigger isdn.

So you configure normal Backup strategies as "backup-interface" "dialer-watch" "opsf demand" or "floating static routes" and overlay BGP-Peering.


regards Horst

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