Wrong BGP AS-PATH in the routing table

[Amitabha]

Hello Experts,

I have been looking into this long enough and need help.  See below.  chr-r1 is advertising shorter AS-PATH.  However, the lab router is showing 2611:6bC0:6000::/40 as the same length.

Any idea?

lab-r1#sh ipv route vrf 13 2611:6bC0:6000::/40 pro

Routing entry for 2611:6bC0:6000::/40

  Known via "omp", distance 251, metric 0, type overlay

  Redistributing via bgp 395191

  Route count is 2/2, share count 0

  Routing paths:

    10.200.34.1%default, Sdwan-system-intf%default

      From ::FFFF:10.200.34.1

      opaque_ptr 0x7F530E63F330

        pdb 0x7F5307CE0580, route 0x7F530BC849A8, adj 0x7F530EA92158

        OMP attribute 0x7F530E63F330, ref 5

        aspath 0x7F530D2D5758, ref 2, length 18, value 13979 65051 65051 65051

        Total OMP attr count 7, aspath 7, community 0

      Last updated 00:00:50 ago

    10.201.34.1%default, Sdwan-system-intf%default

      From ::FFFF:10.201.34.1

      opaque_ptr 0x7F530E63F330

        pdb 0x7F5307CE0580, route 0x7F530BC849A8, adj 0x7F530EA95CA8

        OMP attribute 0x7F530E63F330, ref 5

        aspath 0x7F530D2D5758, ref 2, length 18, value 13979 65051 65051 65051

        Total OMP attr count 7, aspath 7, community 0

      Last updated 00:00:50 ago

lab-r1##

irv-r1#sh ipv route vrf 13 2611:6bC0:6000::/40 pro

Routing entry for 2611:6bC0:6000::/40

  Known via "bgp 395191", distance 200, metric 0

  Tag 13979, type internal

  Redistributing via omp

  Route count is 1/1, share count 0

  Routing paths:

    2605:97C0:2070:3:13::1

      MPLS label: nolabel

      MPLS Flags: NSF

      From 2605:97C0:2070:3:13::1

      opaque_ptr 0x7F3515B72C78

        pdb 0x7F35150A4888, route 0x7F350F7BB800, adj 0x7F350F7BF118

        BGP attribute 0x7F3515B72C78, ref 5

        aspath 0x7F3515FE4048, ref 2, length 18, value 13979 65051 65051 65051

      Last updated 01:33:22 ago

chr-r1#sh ipv route vrf 13 2611:6bC0:6000::/40 pro

Routing entry for 2611:6bC0:6000::/40

  Known via "bgp 395191", distance 200, metric 0

  Tag 65000, type internal

  Redistributing via omp

  Route count is 1/1, share count 0

  Routing paths:

    2605:97C0:2071:3:13::1

      MPLS label: nolabel

      MPLS Flags: NSF

      From 2605:97C0:2071:3:13::1

      opaque_ptr 0x7FE64B0EEB38

        pdb 0x7FE64A087F30, route 0x7FE6449197A8, adj 0x7FE644921F30

        BGP attribute 0x7FE64B0EEB38, ref 5

        aspath 0x7FE64AD0D998, ref 2, length 14, value 65000 19852 19852

      Last updated 00:17:42 ago

lab-r1#sh sdwan omp ipv6-routes 2611:6bC0:6000::/40 

---------------------------------------------------

omp route entries for vpn 13 route 2611:6bC0:6000::/40

---------------------------------------------------

            RECEIVED FROM:                   

peer            10.131.1.1

path-id         17356

label           1005

status          C,I,R

loss-reason     not set

lost-to-peer    not set

lost-to-path-id not set

    Attributes:

     originator       10.200.34.1

     type             installed

     tloc             10.200.34.1, private1, ipsec

     ultimate-tloc    not set

     domain-id        not set

     overlay-id        1

     site-id          200

     preference       not set

     tag              not set

     origin-proto     iBGP

     origin-metric    0

     as-path          "13979 65051 65051 65051"

     community        not set

     unknown-attr-len not set

            RECEIVED FROM:                   

peer            10.131.1.1

path-id         17360

label           1003

status          C,I,R

loss-reason     not set

lost-to-peer    not set

lost-to-path-id not set

    Attributes:

     originator       10.201.34.1

     type             installed

     tloc             10.201.34.1, private1, ipsec

     ultimate-tloc    not set

     domain-id        not set

     overlay-id        1

     site-id          201

     preference       not set

     tag              not set

     origin-proto     iBGP

     origin-metric    0

     as-path          "65000 19852 19852"

     community        not set

     unknown-attr-len not set

[MHM Cisco World]

I analysis issue one by one, 

1. Route Resolvability

Next-hop TLOC is Reachable (data plane BFD session is here)
first we need to check if .201 is reachable 

2. Prefer highest route preference

3. Prefer highest TLOC preference

4. Prefer best origin code (Connected, Static, eBGP, EIGRP Internal, OSPF Intra, OSPF Inter, OSPF External, EIGRP External iBGP, Unknown/Unset
it same iBGP

6. Route Source Preference

On vSmart: prefer Edge router-sourced route over vSmart-sourced route

it same since both learn from vsmart 

5. Prefer OMP Route with lowest origin metric
here other check point, the metric one is 200 and other is 201, so 200 is win

7. Prefer route received from lowest System-IP
again 200 is win 

8. Prefer route from highest Private TLOC IP-address originated from the same site-id
not from same site-id

9. outbound control policy

10. send-path-limit

[Kanan Huseynli]

Per outputs, the origin-metric in both OMP route is 0.

[Amitabha]

@Kanan Huseynli  

I came to the same conclusion as you do.  OMP does not take AS-PATH in the route selection process, so it will load balance.

[MHM Cisco World]

If you make .200 not advertise prefix do you see it in lab?

MHM

[Amitabha]

Yes. I see one route in the lab when either 200 or 201 stop advertising.

[MHM Cisco World]

Now change metric to vedge inject the prefix and see the result.

I see two metric make .201 lower than .200

Then make metric same value.

For originate IP i will check how we can override it to make omp redistrubte both into bgp.

MHM

[Kanan Huseynli]

Exactly, there is no comparison using as-path in OMP. It is BGP copied attribute and can be used on service side for BGP speakers, if propogate-as is enabled.

[Amitabha]

To accomplish this, I need to configure the preference value in the SDWAN control policy.