[j-nsp] bgp to ospf

[Jeff Wheeler]

On Thu, Jun 16, 2011 at 12:48 PM, Payam Chychi <pchychi at gmail.com> wrote:
> Were you able to figure this out?

I don't have lab gear to test this correctly, so please take my post with a
grain of salt.  Please excuse the use of gmail "Rich Text" to get a
fixed-width typeface in my composer:

  /----------------\
  |                |
ASBR1----ABR5---ASBR3

ABR5 redistributes 192.0.2.0/24 into iBGP, but not into OSPF.
ASBR3 redistributes same /24 from BGP into OSPF as an E2 with "set next-hop"
to ABR5 loopback in the routing policy.
ASBR1 learns the E2 route and its RIB (and FIB) show a next-hop directly to
ABR5

However, in ASBR1 "show ospf database external" CLI output, the Fwd Adr is
0, not the ABR5 next-hop.

I am really not surprised by this, because the purpose of OSPF Forwarding
Address is, as expressly documented in relevant RFCs, for situations where
several routers are using a multi-access or broadcast media (frame-relay,
Ethernet, etc.) to reach an external neighbor, yet not all of these routers
have routing protocol sessions to same neighbor.  For example:

       AS16631
          |
  ==================
    |            |
  ASBR1        ASBR2
    |            |
   AR3          AR4

In this stick-figure, === might be Ethernet, ATM, Frame, smoke signals,
whatever.  What matters is you will have eBGP from ASBR1 to the external
neighbor AS16631, but not to ASBR2.  However, if you want ASBR2 to be
capable of routing traffic directly to AS16631 without sending it through
ASBR1, you can use OSPF External routes with Fwd Adr set to the next-hop
address of AS16631 (imagine that ASBR2 just doesn't have the capability of
speaking BGP.)

In fact, on Cisco IOS, the router will not let you accidentally send Fwd Adr
if you send these External routes also to AR3.  It will omit Fwd Adr and so
AR3 will utilize ASBR1 to reach the neighbor.  So Fwd Adr is only set on
LSAs flooded to the === interface.  Further, I do not believe ASBR2 would
preserve Fwd Adr when sending LSA to AR4.

As I mentioned, take my post with a grain of salt.  I may be incorrect here
about the actual functioning, as I have never, ever had reason to utilize
this in a practical network.  But if you do some reading, the intended
purpose of Fwd Adr is expressed above.  The original poster should not be
using it for what he wants to do (I don't think it will work), and should
instead utilize BGP or change his topology.

-- 
Jeff S Wheeler <jsw at inconcepts.biz>
Sr Network Operator  /  Innovative Network Concepts