From: Scott Morris (swm@emanon.com)
Date: Tue Aug 23 2005 - 14:26:53 GMT-3
It would likely be better to watch your areas more appropriately and try to
look at exactly what OSPF is thinking when it makes the decision it does.
That'll help everyone get a much better grasp of the way the OSPF "thought
process" works!
It seems like you all have the basic cause down, which is good. Now we just
need to look at "why"!
Scott
PS. Look at the decision process of how OSPF selects a route. Then look at
what is being done to the routers. Think like the router does!
-----Original Message-----
From: Swaroop Potdar [mailto:swarooppotdar@hotmail.com]
Sent: Tuesday, August 23, 2005 1:24 PM
To: swm@emanon.com; aarumuga@hotmail.com; tmitchell@allianttech.com;
thomwin_chen@yahoo.com; ChoonHo.Loi@getronics.com; ccielab@groupstudy.com
Subject: RE: OSPF Virtual-Link & GRE
guys as Scott rightfully said if you do run across this problem then you may
not use static routes ..
may be u can try filtering the the tunnel destination routes being received
through the tunnel by a distribute list into the tunnel end point routers.
Or may be you can set the cost of tunnel above par of your maximum cost of
any prefix in your OSPF domain to be safe.
>From: "Scott Morris" <swm@emanon.com>
>Reply-To: "Scott Morris" <swm@emanon.com>
>To: "'Arun Arumuganainar'" <aarumuga@hotmail.com>, "'Mitchell, TJ'"
><tmitchell@allianttech.com>, "'Thomwin Chen'" <thomwin_chen@yahoo.com>,
> "'Loi, Choon Ho'" <ChoonHo.Loi@getronics.com>, <ccielab@groupstudy.com>
>Subject: RE: OSPF Virtual-Link & GRE
>Date: Tue, 23 Aug 2005 13:01:56 -0400
>
>And here I thought static routes weren't allowed on the exam!
>
>The reason static routes fix it is because the AD is lower than OSPF is!
>:)
>So you'll likely want to figure out another reason/thought on how to fix
>that in case you run into something like this on the lab!
>
>Scott
>
>-----Original Message-----
>From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
>Arun
>Arumuganainar
>Sent: Tuesday, August 23, 2005 9:37 AM
>To: Mitchell, TJ; Thomwin Chen; Loi, Choon Ho; ccielab@groupstudy.com
>Subject: Re: OSPF Virtual-Link & GRE
>
>Yes ... You are right on top !!!
>
>Static route to tunnel destination is a sure solution for this problem !!!
>I
>have seen similar problem before !!! Just wanted to share with you as it is
>relevant to our discussion .
>
> se0/0---se0/0 fa1/0--fa1/0
>Router1------------router2-------router3
> |<----------GRE Tunnel------------->|
>
>In the above topology ... Do the Basic configuration as follows . This is a
>simple single area topology !!!
>
>1) Configure OSPF on R1 , R2 and R3
>2) Bring up GRE Tunnel between R1 and R3
>3) Turn on OSPF on GRE tunnel as well .
>
>Now the Fun Starts .... Do this peace of configuration over the base
>topology.
>
>ON R1 go to se0/0 increase the OSPF cost to very high value say for ex :
>1000
>
>Now you would see interface flapping .
>
>Scenario Note :
>~~~~~~~~~~
>
>Playing with OSPF cost is simple way to channelise all traffic via GRE
>Tunnel . However in doing so if we are not very care ful even tunnel
>destination will go via GRE and this would result in Flaps .
>
>So Best Practice Recommendation : We are free to tweak the cost but ensure
>that tunnel destination does not take the tunnel route . We can just rely
>on
>static route for tunnel destination in the respective tunnel end-points !!!
>
>Thanks and Regards
>Arun
>
>----- Original Message -----
>From: "Mitchell, TJ" <tmitchell@allianttech.com>
>To: "Arun Arumuganainar" <aarumuga@hotmail.com>; "Thomwin Chen"
><thomwin_chen@yahoo.com>; "Loi, Choon Ho" <ChoonHo.Loi@getronics.com>;
><ccielab@groupstudy.com>
>Sent: Tuesday, August 23, 2005 6:29 PM
>Subject: RE: OSPF Virtual-Link & GRE
>
>
>I know I'm new here, but I have an idea on what is going on.
>
>On router R3 you are originally learning the route 172.16.25.x through the
>virtual-link of area 1 which is fine it works. Now you are putting the
>tunnel on through the virtual-link back to the backbone area 0.
>
>The tunnel comes up and things are working great.
>
>Now guess what happens you start learning the 172.16.25.x route through the
>tunnel instead of the virtual link due to the nature of OSPF. Well when
>making tunnels with OSPF you need to make sure you don't start learning
>your
>destination networks through the tunnel otherwise the tunnel will drop
>because it will lose the route because the virtual link is now gone. Then
>the tunnel drops OSPF reconverges and you learn the link again through the
>virtual-link, tunnels comes back up because it has the route to the
>destination network, OSPF reconverges again you lose the route and the
>tunnel drops again.
>
>You see what I'm saying. Get the tunnel up and running then quickly check
>the routing tables while you can still ping. You should see that the route
>is learned from the tunnel and not the interface.
>
>Like I said I'm new here and this is my thought on the deal, I would like
>to
>know your thoughts.
>
>Thanks
>
>T.J. Mitchell
>-----Original Message-----
>From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
>Arun
>Arumuganainar
>Sent: Tuesday, August 23, 2005 8:47 AM
>To: Thomwin Chen; Loi, Choon Ho; ccielab@groupstudy.com
>Subject: Re: OSPF Virtual-Link & GRE
>
>Cool ... Pretty interesting problem though ... But I would need some more
>information to confirm my suspicion .
>
>Pls. provide me with show ip route <tunnel destination > ... taken at two
>different times .
>
>a)When ping packets are going through fine
>b) when ping are not going through .
>
>If my suspicion is right then your tunnel protocol state in the "show
>interface tunnel 0" command should toggle between UP and DOWN state !!!
>at
>regular intervals.
>
>Only work around for this problem add a following static route in tunnel
>end
>points .
>
>R2 .
>
>ip route 172.16.35.0 255.255.255.252 serial 1/1
>
>R3
>
>ip route 172.16.25.0 255.255.255.252 serial 1/1
>
>This kind of problems are bound to happen especially when you enable
>routing
>protocols on a tunnel interface . Some times routers could have learned a
>better route for tunnel destination via tunnel interface themselves .
>This
>creates a loop . i.e Tunnel recurses over Tunnel destination and tunnel
>destination reclurse over tunnel !!! Under this scenario there will be
>flapping of tunnel interface at regular intervals !!!
>
>Best way to address it , is to have static route for tunnel destination
>alone via an interface other than Tunnel .
>
>Pls. Note : Static route is always preferred . Hence accidental loops will
>not be introduced through Dynamic Routing protocols .
>
>Pls. do let me know if this solves the problem .
>
>Thanks and Regards
>Arun
>----- Original Message -----
>From: "Thomwin Chen" <thomwin_chen@yahoo.com>
>To: "Loi, Choon Ho" <ChoonHo.Loi@getronics.com>; <ccielab@groupstudy.com>
>Sent: Tuesday, August 23, 2005 3:30 PM
>Subject: Re: OSPF Virtual-Link & GRE
>
>
> > Choon,
> >
> > Have you try to put the tunnel0 interface in R2 and R3 into area 0 ?
> > I saw your config, the tunnel is in area 3.
> >
> > Rgds,
> > Thomwin
> >
> >
> > "Loi, Choon Ho" <ChoonHo.Loi@getronics.com> wrote:
> > Hi,
> >
> > I lab-up the following Scenario for OSPF:
> >
> > Area 0 | R2 -----Area 1------- R5 ------Area 2--------R3--------| Area
>3
> >
> > Everything works fine... but, my ping drop intermittently when I try
>to
> > ping from Area3 to Area0.
> > Anything wrong with my configuration?
> >
> > Thanks.
> >
> >
> > Following is my configuration:
> >
> > hostname r2
> > !
> >
> > !
> > interface Loopback0
> > ip address 192.2.2.2 255.255.255.0
> > !
> > interface Tunnel0
> > ip address 23.23.23.2 255.255.255.248
> > tunnel source Serial1/1
> > tunnel destination 172.16.35.1
> > !
> > interface Serial1/1
> > ip address 172.16.25.1 255.255.255.252 encapsulation ppp
> >
> > router ospf 1
> > router-id 2.0.0.2
> > log-adjacency-changes
> > area 1 virtual-link 5.0.0.5
> > network 23.23.23.0 0.0.0.7 area 3
> > network 172.16.25.0 0.0.0.3 area 1
> > network 192.2.2.0 0.0.0.255 area 0
> >
>------------------------------------------------------------------------
> > ---
> > !
> > hostname r5
> >
> > interface Loopback0
> > ip address 100.100.100.100 255.255.255.0 !
> > interface Loopback1
> > ip address 5.0.0.5 255.255.255.255
> >
> > interface Serial0/0
> > ip address 172.16.25.2 255.255.255.252 encapsulation ppp no fair-queue
> > !
> > interface Serial0/1
> > ip address 172.16.35.2 255.255.255.252 encapsulation ppp !
> > router ospf 1
> > router-id 5.0.0.5
> > log-adjacency-changes
> > area 1 virtual-link 2.0.0.2
> > network 172.16.25.0 0.0.0.3 area 1
> > network 172.16.35.0 0.0.0.3 area 2
> > neighbor 5.5.5.2
> > !
> >
> >
> >
>------------------------------------------------------------------------
> > ------
> > !
> > hostname r3
> >
> > interface Loopback0
> > ip address 10.10.10.3 255.255.255.255
> > !
> > interface Tunnel0
> > ip address 23.23.23.3 255.255.255.248
> > tunnel source Serial1/1
> > tunnel destination 172.16.25.1
> >
> > interface Serial1/1
> > ip address 172.16.35.1 255.255.255.252 encapsulation ppp serial
> > restart_delay 0 clockrate 64000
> >
> > router ospf 1
> > router-id 3.0.0.3
> > log-adjacency-changes
> > network 10.10.10.0 0.0.0.255 area 3
> > network 23.23.23.0 0.0.0.7 area 3
> > network 172.16.35.0 0.0.0.3 area 2
> >
>------------------------------------------------------------------------
> > -------------------------------------
> >
> >
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Regards,
Swaroop.
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