From: James (james@towardex.com)
Date: Wed Sep 15 2004 - 01:41:04 GMT-3
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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