From: James (james@towardex.com)
Date: Wed Sep 15 2004 - 12:18:11 GMT-3
Hi Jongsoo,
On Wed, Sep 15, 2004 at 10:17:58AM -0400, Jongsoo.Kim@Intelsat.com wrote:
> James
>
>
> As I didn't do any routing protocol in this lab, I am not sure why R6
> chooses R4 for next hop 10.90.90.1.
Because 10.90 is greator than 10.6? :)
> In my quick glance, OSPF topology among R1,R4, and R6 seems like the source
> of this routing loop.
No.. It is BGP routing loop. IGP is fine, as long as you have it configured
correctly in the OSPF, et al sections prior.
HTH,
-J
> R1 and R6 don't have direct connection but it is connected via R4.
> So if 10.90.90.0/28 network is redistributed from EIGRP 30 to OSPF, R6 will
> learn it from R4, which is reason why R6 chooses R4 for 10.90.90.1.
>
> I agree with your work-around solution.
>
>
> JK
>
>
>
> -----Original Message-----
> From: James [mailto:james@towardex.com]
> Sent: Wednesday, 15 September, 2004 12:41 AM
> To: Joseph Rothstein
> Cc: ccielab@groupstudy.com
> Subject: Re: BGP reachability issues Lab 2 Cisco Press
>
>
> On Wed, Sep 15, 2004 at 05:19:03AM +0200, Joseph Rothstein wrote:
> > Greetings to all.
> >
> > Has anyone else had reachability issues with the BGP part of Lab 2 int eh
> new Cisco Press book? I ahve a strange situation on R4 which prevents
> pinging the loopback 2.2.2.2 on R2:
>
> The problem is because R4 thinks to get to 2.2.2.0/29, R6 is the best path.
> Now unfortunately, R6 obviously says R4 is the path, aka routing loop.
> Reading forward..
>
> >
> > R4#sipb
> > BGP table version is 5, local router ID is 10.4.4.4
> > Status codes: s suppressed, d damped, h history, * valid, > best, i -
> internal,
> > r RIB-failure, S Stale
> > Origin codes: i - IGP, e - EGP, ? - incomplete
> >
> > Network Next Hop Metric LocPrf Weight Path
> > *> 2.2.2.0/29 10.6.6.6 0 61555 62555 i
> > * 10.1.1.1 0 61555 62555 i
> > *> 4.4.4.0/24 0.0.0.0 0 32768 i
> > *> 5.5.5.0/27 10.6.6.6 0 61555 64555 i
> > * 10.1.1.1 0 61555 64555 i
> > *> 8.8.8.0/28 10.6.6.6 0 61555 63555 i
> > * 10.1.1.1 0 61555 63555 i
> >
> > This basically causes packets to bounce back and forth between R6 and R4
> since R6's BGP table uses 10.90.90.1 as it's next hop for 2.2.2.2, which
> goes through R4. So once the packet gets to R4 on its way to 10.90.90.1, it
> just goes right back to R6.
> >
> > This seems like a pretty big problem, although the text does not state
> that IP addresses under BGP have to be reachable.
>
> Yep. Unfortunately, the labs 1 thru 3 are deliberately made to be very
> vague,
> assuming the student to understand the gotcha's. Since anything is a fair
> game
> in the real lab, vauge questions like these could potentially show up in
> real
> lab too I think... But anyway..
>
> In terms of fixing your problem here. The reason why R4 prefers R6 is
> because
> the nexthop address in the prefix arriving from R6 is higher.
>
> So in order to get around this, on the R1 router: go to bgp routing
> configuration area, then go to neighbor statement for peering with R4.
> Then type 'next-hop-unchanged' on peering configuration to R4.
>
> This will make R1 to pass nexthop to R4 unchanged, sending the prefix with
> 10.90.90.1 as the nexthop, which is higher than 10.6.6.6 delieverd from
> R6. So R4 now prefers R1 as its best path, eliminating the routing loop.
>
> Of course, as obvious as it may sound, if your IGP is broken and 10.90.90.x
> is not in your IGP table, then recursive route failure will occur and you
> may see RIB-failure error or otherwise BGP route not being installed due
> to martian next-hop.
>
> HTH,
> -J
>
>
> --
> James Jun TowardEX Technologies,
> Inc.
> Technical Lead Network Design, Consulting, IT
> Outsourcing
> james@towardex.com Boston-based Colocation & Bandwidth
> Services
> cell: 1(978)-394-2867 web: http://www.towardex.com , noc:
> www.twdx.net
>
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-- James Jun TowardEX Technologies, Inc. Technical Lead Network Design, Consulting, IT Outsourcing james@towardex.com Boston-based Colocation & Bandwidth Services cell: 1(978)-394-2867 web: http://www.towardex.com , noc: www.twdx.net
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