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New Member

ISDN link causing OSPF routing table to reset

I have 2 Core routers running OSPF to various branch sites.

One Core router has ISDN connecte sites. These sites are static routed and are Passive for OSPF. When a call is established the whole routing table seems to be refreshed as all the times for every route are reset to zero. The link state database shows various ages for the links. All routing tables throughout the OSPF network are being refreshed???

Can anyone help please.

Many Thanks

6 REPLIES
VIP Purple

Re: ISDN link causing OSPF routing table to reset

Hello,

have you tried to configure one of the ISDN links as an OSPF Demand-Circuit ? For some reason the passive-interface doesn't seem to work. Can you post the configs for the core router and one of the remote site routers ?

Regards,

GP

Gold

Re: ISDN link causing OSPF routing table to reset

The timer in the routing table is refreshed whenever the routes are "touched" in any way, not just when OSPF changes all of its routes. If the ages on the LSAs are showing the correct times, then you are probably just seeing an SPF run from the ISDN link going up and going down, which causes all the routes to be touched, even though the routes really aren't changed (since the interface is passive, the router with the ISDN link is still going to originate a new router LSA when the link goes up or comes down, etc).

I would check the spf runs (show ip ospf stat), etc. to see if this is the case.

Russ.W

New Member

Re: ISDN link causing OSPF routing table to reset

If the routes in the table are touched each time.

Why for normal FR links do only the OSPF routes for that link change its timers? Why is this only for ISDN?

Also not all the ISDN links seem to do this. Unfortunatly I was unable to test any of the links today as the network remained operational and teh customer didnt want to lose his service. I shall look at this in more detail on Monday.

Gold

Re: ISDN link causing OSPF routing table to reset

If a full SPF is run by OSPF, all the route timers would most likely change, even if those routes aren't impacted by the change in any way. I'm pretty certain OSPF installs all routes when it runs SPF, no matter whether or not they have changed. Incremental SPF, and the local OSPF database would both change this so the routes in the table are touched only when the route actually changes.

I personally consider the timers in the routing table pretty useless; if you want to know what's going on (what sorts of changes are really happening in your network), you need to look at the protocol information, pretty much.

Russ.W

New Member

Re: ISDN link causing OSPF routing table to reset

Russ,

This is the output from show ip ospf stst

CHICIS12#sho ip ospf stat

Area 0: SPF algorithm executed 16039 times

SPF calculation time

Delta T Intra D-Intra Summ D-Summ Ext D-Ext Total Reason

00:52:20 0 0 0 0 4 0 4 R,

00:35:51 0 0 0 0 4 0 4 R,

00:35:41 0 0 0 0 4 0 4 R,

00:34:51 0 0 0 0 4 0 4 R,

00:18:53 0 0 0 0 8 0 8 R,

00:18:43 0 0 0 0 4 0 4 R,

00:17:53 0 0 0 0 4 0 4 R,

00:01:28 0 0 0 0 4 0 4 R,

00:01:18 0 0 0 0 4 0 4 R,

00:00:28 0 0 0 0 4 0 4 R,

Gold

Re: ISDN link causing OSPF routing table to reset

Note how often SPF is running here, about once a minute. If each SPF run is causing all the routes in the table to have their timers touched, then it would look like the full table is begin updated every minute or two. See if these numbers seem to correlate with the timers on the routes in the routing table.

:-)

Russ.W

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