direction of SOO tagging

I'm finding conflicting information about the SoO tagging process.  Do the tags go from EIGRP to BGP for the use of mBGP or the other way around?  I'm going to try to use deductive reasoning and see if I hear otherwise.

The SoO tagging is an extcommunity tag.  The extcommunity tag is TLV infomation in the VPN route information.  Therefor the SoO tagging is applied in the direction of redistribution of EIGRP INTO mBGP correct?  Doesn't mBGP use the SoO tag as a loop prevention mechanism?  There isn't any way for non-IPV4 EIGRP running through the back-door to use SoO extcommunity tags.  Right?

many thanks.

Comments

  • HI,

    it's maybe too late, but as I'm in the middle of SoO config, I could try to answer you.

    I found a good document at http://blog.ine.com/2010/04/29/understanding-eigrp-soo-and-bgp-cost-community/

     

    It's said that:

    ========

    The EIGRP SoO community applies on per-interface basis, using the sitemap construct. All EIGRP prefixes learned via this interface will carry the SoO extended community value. The EIGRP prefixes redistributed into BGP keep their SoO value in the BGP extended community. Conversely, all BGP routes redistributed into EIGRP and sent over the interface will have the SoO applied. There are two main rules for processing the SoO communities:
    1. If the route is sent or received on the interface has the same SoO value as configured on the interface the route is discarded.
    2.  If the route is sent or received on the interface has an SoO value but it does not match the one configured for the interface, the value is preserved.

    ========

     

    So it seems that setting up the SoO value operates when eigrp route (wihtout SoO value) is entering the interface where the route-map is applied. Then this attribute is preserved at the redistribution step (Eigrp into M-BGP) and also when routes are exchanged between two Eigrp peer if they have SoO route-maps configured (backdoor link). This happens if SoO are different.

    if SoO values are identical, the remote EIGRP peer will discard the route update.

     

    Let me know if I'm wrong,

    Olivier

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