From: Howard C. Berkowitz (hcb@gettcomm.com)
Date: Sat Feb 28 2004 - 18:12:27 GMT-3
At 2:15 PM -0600 2/28/04, Hoyle, Anthony (AL) wrote:
>You gave the answer. The external distance of BGP is 20.
>Disappearing from the database..well you are redistributing the
>connected route into ISIS. You are then redistributing isis into
>OSPF. This route will always be in the OSPF database
>regardless(unless you have a filter), however it won't be placed in
>the routing table under OSPF -if there is another protocol (like
>BGP) advertising the same route with a lower admin distance (BGP
>external routes).
>When you say disappear from the OSPF database, you mean when you do
>a "show ip route ospf" ?
>
>Did I understand your question correctly..?
>
>Anthony Hoyle
I'm also not sure that I understand your question correctly, but a
general rule of IGP design is that an IGP should not readvertise a
route that it learned from an external source. Much of redistribution
loop avoidance, as well as the path vector algorithm of BGP, is based
on this principle. It's not the best analogy in the world, but you
can think of these various loop avoidance mechanisms as a relative of
the split horizon rule inside a DV IGP.
>
>-----Original Message-----
>From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
>Of Ahmed Mustafa
>Sent: Saturday, February 28, 2004 3:22 AM
>To: ccielab@groupstudy.com
>Subject: BGP versus OSPF/ISIS
>
>
>Can someone look into this
>
>
>R6 connected to R2 via ISIS level 1
>
>R2 connected to R3 via HDLC link/ISIS Level 2
>R2 connected to R3 via Frame-Relay link/OSPF-----> R2 is also doing
>mutual redistribution
>
>I announced r6 loopback interface 132.1.6.0/24 to ISIS via
>redistribute connected.
>
>R3 learned that route obviously through Frame-Relay due to lower
>admin distance. I then changed the admin distance of ISIS to 106,
>and r3 installed this route via ISIS. So far so good
>
>Now comes BGP, and totally screwed this nice scenerio above
>
>R6 is in AS 100
>R2 and R3 are in AS 300
>
>I redistributed r6 loopback now into BGP via redistribute
>connected, not through network command.
>
>r2 will obviously install this route as a BGP route because of lower
>adminstrative distance External BGP 20 over ISIS 115.
>
>Now r3 learns that route via BGP as well with the distance 200 since
>r2 and r3 are in same AS. I thought that the route 132.1.6.0 should
>have learned through ISIS or OSPF if I removed the admin distance
>command of ISIS from 106 to default 115 because both OSPF and ISIS
>have lower admin distance than IBGP 200.
>
>My guesses:
>
>The route learned via BGP since it is in r2 routing table regardless
>of r2 should have this route in ISIS database as well as in OSPF
>database since r2 is also doing redistribution.
>
>Issues: If I shut down the BGP link between r2 and r6, bingo, the
>route will appear in OSPF database as an external route on r2, but
>when the BGP link is up between r2 and r6, the route will disappear
>from OSPF database.
>
>Any ideas !!!
>
>Sorry for the tedious email, but Group can learn from this and also advise.
>
>Regards,
>
>Ahmed
>
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