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Purple

ISDN I hate it

here is the problems , we have a PRI that is randomly dialing itself out . According to debug dialer the addresses that are causing the problem are the dialer interfaces themselves trying to send to 255.255.255.255 which triggers the dialers . How can i tell what is actually causing the dialers to start this action . Can I use something like debug ip packet with an access list that is limited to the 4 dialer addresses to see if I can see what may be coming into the dialer interfaces making them dial ? All 4 dialers will dial out at the same time . Any ideas as to where I can head with this ?

6 REPLIES
Cisco Employee

Re: ISDN I hate it

Somehow router is considering traffic destined for 255.255.255.255 as "Interesting".

By the way you can configure router's isdn interface for not to "dialout" for a packet destined for 255.255.255.255 by denying packets for that ip address as "Interesting". Here is the way

Under the pri interface (interface serial x:23) or under the interface which dials out, enter command "dialer-group 1"

Now in global config deny destination address 255.255.255.255 as interesting traffic, like this

dialer-group 1 protocol ip list 101

access-list 101 deny ip any host 255.255.255.255

access-list 101 permit ip any any

The above access-list 101 will deny all the traffic for 255.255.255.255 to dialout but permit rest ip traffic to dialout over the pri line. You can deny the traffic which are not allowed to dialout.

By the way do you want to dialout from the router in any conditions? ..If NO...then don't configure any "dialer string" or dialer map with tel number on a router.

Do you run any routing protocol?

Purple

Re: ISDN I hate it

this was along the path I was leaning , by denying broadcast will this break anything that you can think of ? The box is running ospf and bgp . The isdn is used as backup services for remote locations .

Cisco Employee

Re: ISDN I hate it

Don't configure any telephone number on the router shouldn't be calling out. Now i don't think that by denying the broadcast will break anything related to ospf and bgp. To be safe you can allow all the ospf and bgp traffic in the access-lists and deny the rest like this.

access-list 101 permit ospf any any

access-list 101 permit bgp any any

access-list 101 deny ip any any

If the isdn is used as a backup of some primary line then isdn interface should not be sending any traffic in/out if the primary line is still up.

mm1
New Member

Re: ISDN I hate it

hi,

you probably already know - 'debug dialer' shows you the interesting traffic for opening your dial connections. if you have windows and w2k clients in your LAN, perhaps LDAP triggers the call. I used an access list which denies ports 137-139 and 389.

hth,

martin

New Member

Re: ISDN I hate it

It seems to me like either the OSPF or the BGP is triggering the call. What is common in this type of situation is to make the OPSF hellos as uninteresting traffic using an access-list. This way only interesting traffic will trigger the ISDN.

There are numerous Cisco articles on this

Hope this helps

Velimir

New Member

Re: ISDN I hate it

I agree with you velimirmkd...

It seems that OSPF's multicast hello packets are probably triggering the call.

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