[isp-bgp] Re: Difference in Bgp origin attribute

[Peter van Oene <pvo@...]

Some comments inline,

At 01:28 PM 4/23/2003 +0530, Sakthivadivu.S.S wrote:
> Hi,
>
>    I am observing the difference in origin attribute when i use the
> following topologies
>
>  Topology1:
> ---------------
>      R1 -----
> ebgp -------Cisco1---------ibgp--------Cisco2-------ebgp-----R2
>
>     AS No of R1 = 100
>     AS No of Cisco1 and Cisco2 = 200
>     AS No of R2 = 300
>
>    R1 has ebgp connection with Cisco 1. Cisco1 has ibgp connection with
> Cisco2. Cisco2 has ebgp connection with R2.
>
>   When i import static route Rx in R1,  Cisco receives Rx from R1  with
> origin as incomplete. Cisco1 sends Rx with incomplete origin to Cisco2 and
> cisco2 sends to R2 with same origin.So finally the Rx has incomplete origin
> at router R2.
Cisco's typical behavior is to set origin to incomplete when routes are 
redistributed into BGP.

>   Topology2:
> -----------------
>
>     R1-------ebgp------Cisco1-------- IGP--------Cisco2--------ebgp-----R2
>
>     AS No of R1 = 100
>     AS No of Cisco1 and Cisco2 = 200
>    AS No of R2 = 300
>
>     BGP and Ospf are enabled in Cisco1 and Cisco 2. Also redistribution from
> bgp to ospf is enabled in Cisco 1 and  redistribution from ospf to bgp is
> enabled at cisco2.
>
>    When i import static route Rx in R1, Cisco1 receives Rx with imcomplete
> origin. Cisco1 redistributes with the TAG value as 0x90000064 . The same TAG
> is sent to CISCO2 ospf. When Rx is redistributed from ospf to bgp in cisco2,
> the origin attribute is set as EGP (as per RFC1745). So R2 receives Rx with
> origin as EGP from Cisco 2.
According to the now historic 1745, this is normal behavior (the setting of 
origin to EGP in this case)  Cisco did implement 1745 though I doubt it 
ever saw use in real networks.

>   So there is a difference in Origin attribute with Topology1 (incomplete)
> and Topology 2 (EGP) at Cisco2 and R2.
>
>    So anyone can please explain why this difference?
The difference seems to be that Cisco uses origin incomplete when 
redistributed static to BGP and EGP when redistributing ospf routes tagged 
with 1745 defined tags.

Most folks that actually use redistribution to put routes into BGP from 
some other means (static/igp etc)  on Cisco platforms use route-maps to 
constrain the redistribution and control attribute selection which should 
mitigate any issues this might be causing.  Of course, this topology is 
pretty impractical anyway :-)

Pete