[eng/rtg] changing loopbacks

Sun Oct 2 20:16:30 UTC 2005

<thegameiam at yahoo.com> wrote:

> eek!  There are a couple of downsides to having the
> router-ID divorced from a physical address:
>
> 1) you get an additional number which you have to have
> to track to ensure uniqueness.
>
> 2) you lose the benefit of being able to double check
> reachability (ping/ssh to router ID)

No doubt, but the OP was trying to fend off OSPF adjacency teardowns when 
renumbering loopbacks.

> 3) RFC 1403 says that the BGP router identifier must
> be the same as the OSPF router ID, and do you really
> want your BGP to reflect an unreachable ID?

Wait a second...
<RFC 1403>

3.  BGP Identifier and OSPF router ID

   The BGP identifier MUST be the same as the OSPF router id at all
   times that the router is up.

   This characteristic is required for two reasons.

     i    Synchronisation between OSPF and BGP

          Consider the scenario in which 3 ASBRs, RT1, RT2, and RT3,
          belong to the same autonomous system.

                                     +-----+
                                     | RT3 |
                                     +-----+
                                        |

                          Autonomous System running OSPF

                                 /               \
                             +-----+          +-----+
                             | RT1 |          | RT2 |
                             +-----+          +-----+

          Both RT1 and RT2 have routes to an external network X and
          import it into the OSPF routing domain.  RT3 is advertising
          the route to network X to other external BGP speakers.  RT3

          must use the OSPF router ID to determine whether it is using
          RT1 or RT2 to forward packets to network X and hence build the
          correct AS_PATH to advertise to other external speakers.

          More precisely, RT3 must determine which ASBR it is using to
          reach network X by matching the OSPF router ID for its route
          to network X with the BGP Identifier of one of the ASBRs, and
          use the corresponding route for further advertisement to
          external BGP peers.

</RFC 1403>

Can someone explain that? Why would RT3 care about the BPG identifiers of 
the other ASBR's? Why would the ASBR's even have BGP identifiers? What BGP 
attribute has anything to do with this?

Austin

--On Sunday, October 02, 2005 12:55 PM -0700 David Barak 
<thegameiam at yahoo.com> wrote:

> eek!  There are a couple of downsides to having the
> router-ID divorced from a physical address:
>
> 1) you get an additional number which you have to have
> to track to ensure uniqueness.
>
> 2) you lose the benefit of being able to double check
> reachability (ping/ssh to router ID)
>
> 3) RFC 1403 says that the BGP router identifier must
> be the same as the OSPF router ID, and do you really
> want your BGP to reflect an unreachable ID?
>
> I've had a customer who used unreachable router IDs,
> and it made their NOC work quite a bit harder than
> they otherwise would have had to...
>
> -David