Cisco Systems Monitoring line status where interfaces won't go down

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Subject Author Date
Monitoring line status where interfaces won't go down Gr!scha S+egemann 06-06-08
Posted by Gr!scha S+egemann on June 6, 2008, 5:47 am
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Hi all

consider the following situation:

Customer edge Router (e.g. Cisco 28xx) with a primary link to the
provider edge router via an Etherconnect line, i.e. a connection where
the carrier provides a modem which is conected to the router.
A backup link is established over a steady DSL connection. On both
interfaces we have /30 transfer networks an run ospf with the provider
edge routers on the other ends.

Now we want to monitor the primary connection from the provider
backbone. Usually we do not use ospf and simply ping the customer edge
IP adresses of the two transfer networks in order to test their
reachability.
Now in case of the running ospf this is not an option since both IP
addresses would be always reachable as long as one connection is up.

An option would be to monitor the interface status via snmp. That's good
as long as a failing connection implies an interface status change to
down. Unfortunately this is not the case since the connection to the
carrier's modem would always be up.

Well, I've read a lot about tracking objects and routing policies but
I'm not sure whether or not (and if yes, how exactly) this would be of
any help in this case.

Any suggestions?

Thanks a lot in advance,

Grischa

Posted by Paul Matthews on June 7, 2008, 11:28 am
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Gr!scha S+egemann wrote:
>Any suggestions?

BFD?
--
Paul Matthews CCIE #4063
Please post questions to the NG, NOT by e-mail.

Posted by Stephen on June 7, 2008, 7:06 pm
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On Fri, 06 Jun 2008 11:47:00 +0200, "Gr!scha S+egemann"

>Hi all
>
>consider the following situation:
>
>Customer edge Router (e.g. Cisco 28xx) with a primary link to the
>provider edge router via an Etherconnect line, i.e. a connection where
>the carrier provides a modem which is conected to the router.
>A backup link is established over a steady DSL connection. On both
>interfaces we have /30 transfer networks an run ospf with the provider
>edge routers on the other ends.
>
>Now we want to monitor the primary connection from the provider
>backbone. Usually we do not use ospf and simply ping the customer edge
>IP adresses of the two transfer networks in order to test their
>reachability.

i think you are ruling out the best way to do this - routing protocols
are there to check reachability (as well as other things) - so why not
use it?

>Now in case of the running ospf this is not an option since both IP
>addresses would be always reachable as long as one connection is up.

Nope - far end address across the link is not "up" from your
perspective unless the link works and you have 2 way packet exchange.

Near end may stay up though - in which case you are pinging the wrong
object.
>
>An option would be to monitor the interface status via snmp. That's good
>as long as a failing connection implies an interface status change to
>down. Unfortunately this is not the case since the connection to the
>carrier's modem would always be up.
>
>Well, I've read a lot about tracking objects and routing policies but
>I'm not sure whether or not (and if yes, how exactly) this would be of
>any help in this case.

BFD may work as another suggested - but you need modern devices / code
at both ends since this is fairly recent.

cisco have a way of linking "reachability" into statics using SAA /
RTR or whatever it is called now
http://www.cisco.com/en/US/docs/ios/12_3t/12_3t4/feature/guide/gtpbrtrk.html
>
>Any suggestions?
>
>Thanks a lot in advance,
>
>Grischa
--
Regards

stephen_hope@xyzworld.com - replace xyz with ntl

Posted by Andre Wisniewski on June 9, 2008, 2:36 am
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Stephen wrote:

>
> Nope - far end address across the link is not "up" from your
> perspective unless the link works and you have 2 way packet exchange.
>
> Near end may stay up though - in which case you are pinging the wrong
> object.

I think there's a misunderstanding.

Imagine following. R1 and R2 are on one LAN, R3 and R4 at remote LAN. Running
OSPF R3 will be reachable and all of it's interfaces as long as at least one LAN
link is working.

How to discover failing LAN link A when all interfaces are still up using SNMP?
R1's and R3's interfaces will stay up because of the modems.

R1 --- Modem --------- LAN link A --------- Modem --- R3
R2 --- Modem --------- LAN link B --------- Modem --- R4

Posted by Andrey Tarasov on June 9, 2008, 4:46 am
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Andre Wisniewski wrote:

>> Nope - far end address across the link is not "up" from your
>> perspective unless the link works and you have 2 way packet exchange.
>>
>> Near end may stay up though - in which case you are pinging the wrong
>> object.
>
> I think there's a misunderstanding.
>
> Imagine following. R1 and R2 are on one LAN, R3 and R4 at remote LAN.
> Running OSPF R3 will be reachable and all of it's interfaces as long as
> at least one LAN link is working.
>
> How to discover failing LAN link A when all interfaces are still up
> using SNMP? R1's and R3's interfaces will stay up because of the modems.
>
> R1 --- Modem --------- LAN link A --------- Modem --- R3
> R2 --- Modem --------- LAN link B --------- Modem --- R4

I think you should be able to use IP SLA to monitor status of the links.
You will have to put some simple ACL in place to prevent ICMP traffic
between R1/R3 pair to travel over Link B and vice versa. That way when
link will go down so does IP SLA monitor.

Andrey.

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