From: john matijevic (matijevi@bellsouth.net)
Date: Wed Sep 15 2004 - 13:46:02 GMT-3
Hello Jongsoo, James and Team,
The next-hop-unchanged command was released in 12.0(16)ST and 12.0(22)S,
limited lifetime release. This command was introduced, so you do not
have to worry about this command. However, as far as the best route for
the next-hop, it should appear as the lowest ip address, considering the
rest of the decision criteria is equal. Im not sure why you have i code,
the answer key does not have an i code, maybe you advertised these
networks in your igp. Which is why the bgp router prefers these routes.
The lab did not state to do so. Only said to inject the networks into
BGP via new loopback interfaces. It says also in the task you are only
permitted to perform configuration on R1 and "not by manipulating the
underlying IGP." Again follow the instructions carefully.
"12. If multipath is not enabled, prefer the route with the lowest IP
address value for the BGP router ID.
The router ID is usually the highest IP address on the router or the
loopback (virtual) address, but might be implementation-specific."
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/
fipr_c/ipcprt2/1cfbgp.htm
Sincerely,
John Matijevic, CCIE #13254, MCSE, CNE, CCEA
CEO
IgorTek Inc.
151 Crandon Blvd. #402
Key Biscayne, FL 33149
Hablo Espanol
305-321-6232
http://home.bellsouth.net/p/PWP-CCIE
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Jongsoo.Kim@Intelsat.com
Sent: Wednesday, September 15, 2004 11:44 AM
To: james@towardex.com
Cc: ziutek@mac.com; ccielab@groupstudy.com
Subject: RE: BGP reachability issues Lab 2 Cisco Press
James
BGP protocol will choose which next hop to use for a BGP route.
But still it is IGP's decision which interface to use to reach the next
hop.
Nexthop 10.90.90.1 for 2.2.2.0/29 is selected on R6 by iBGP process
learned
from R1.
However, which interface R6 to use to reach 10.90.90.1 is based on IGP
process ( OSPF in this case).
So R6 will forward any packet of 2.2.2.0/29 to R4 via F/R interface.
In the same way, R4 learned 2.2.2.0/29 from two eBGP connections from R1
and
R6.
But it chooses R6's peer address as nexthop as you said higher IP
address
(unless bgp loadbalance is enable.)
To determine which interface to use to reach 10.6.6.6, R4 will check IGP
and
forward to F/R interface going to R6.
If you make bgp nexthop unchanged on R1 so that R4 choose 10.90.90.1 as
nexthop for 2.2.2.0/29, then still R4 will rely on IGP to select the
proper
interface to forward.
Most likely, this kind of issue won't happen in real Lab test.
Regards
JK
-----Original Message-----
From: James [mailto:james@towardex.com]
Sent: Wednesday, 15 September, 2004 11:18 AM
To: Kim, Jongsoo
Cc: ziutek@mac.com; ccielab@groupstudy.com
Subject: Re: BGP reachability issues Lab 2 Cisco Press
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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