From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Mon Jul 18 2005 - 16:10:16 GMT-3
If you are a transit AS it is generally bad practice to default
route, however in the lab exam that would be one option. Another option
as you mentioned is to redistribute BGP into IGP. A third option is to
run a VPN across the network using a GRE/IPIP tunnel. This is the same
principle as MPLS VPNs, in which devices in the transit network only
need to know the tunnel endpoints, and hence do not need to run BGP.
Take the situation below:
R1 R5
| |
R2 R4
\ /
R3
R1 = AS 1
R2/R4 = AS 2
R5 = AS 3
R1 peers with R2 via EBGP, R2 peers with R4 via iBGP, and R4
peers with R5 via EBGP. Assuming that BGP synchronization is disabled
routes learned from R1 will be advertised to R2, to R4, and finally to
R5. However when R5 tries to send traffic to the destination the
non-BGP speaking router, R3, will drop the traffic. Running a GRE
tunnel between R2 and R4 which the peering session is established over
can resolve this issue, because R3 in the transit path with only see GRE
traffic that it needs to transport between R2 and R4. Assuming that IGP
reachability is there and the next-hop processing is correct, traffic
between these BGP AS's will be transparent from the perspective of R3.
This situation is covered several times in our CCIE Routing &
Switching Lab Workbook volumes I and II.
HTH,
Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987 x 705
Outside US: 775-826-4344 x 705
24/7 Support: http://forum.internetworkexpert.com
Live Chat: http://www.internetworkexpert.com/chat/
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> ccie2004@excite.com
> Sent: Monday, July 18, 2005 1:01 PM
> To: ccielab@groupstudy.com
> Subject: BGP synchronization
>
> Hi All, I just came up with a question on BGP which I have been
asking
> myself but just can't seem to get a handle on. It has to do with BGP
> Synchronization. I know this has been beaten to death and newer IOS
> versions have it disabled however my question is a combination of the
> underlying issue that Synch is supposed to address and Best Practises.
> Synch is supposed to address the issue of a router in the middle of
your
> network which is not running IBGP and hence does not know how to get
to a
> particular network that your IBGP routers are aware of. BGP Best
practises
> say that never redistribute your EBGP learnt routes into your interior
> routing protocol. Thinking along those lines and if I am right how
exactly
> would you get reachability across your network. Would you use default
> routes on your non-BGP speaking routers or are there any other design
type
> fixes that I am missing. thx
>
> _______________________________________________
> Join Excite! - http://www.excite.com
> The most personalized portal on the Web!
>
>
This archive was generated by hypermail 2.1.4 : Sun Sep 04 2005 - 17:00:30 GMT-3