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]

Can I see topolgy 

MHM

[Amitabha]

lab-r1 has SDWAN tunnels to both irv-r1 and chr-r1, which learned the route from BGP on the LAN side.  Is this what you are asking for?

[MHM Cisco World]

MHM

[Amitabha]

It's not the same.  The issue is OMP is learning that route with different AS length.  Why is it showing the same AS length for both?

[Kanan Huseynli]

Hi

please share "show bgp vpnv4 unicast vrf 13 2611:6bC0:6000::/40"

[Amitabha]

BGP Table on the lab router is showing the correct path.  However, the routing table is using OMP routes.  See above.

lab-r1#sh bgp vpnv6 unicast vrf 13 2611:6bC0:6000::/40
BGP routing table entry for [1:13]2611:6bC0:6000::/40, version 5832
Paths: (1 available, best #1, table 13)
Not advertised to any peer
Refresh Epoch 1
Local
::FFFF:10.200.34.1 (via vrf 13) from 0.0.0.0 (172.29.236.1)
Origin incomplete, metric 1000, localpref 50, valid, sourced, best
Extended Community: SoO:0:14009
rx pathid: 1, tx pathid: 0x0
Updated on Dec 27 2023 21:15:33 UTC

[Kanan Huseynli]

And you have one route in BGP because it is redistribution of the best route.

Even though you have ECMP on OMP side, in BGP you have one route which is "best of the best" using tie-breakers.

But still there is question: on which devices you have enabled "propagate-as" on bgp config? And can you confirm that you have "overlay-as" in OMP configuration?

[Amitabha]

It looks like OMP doesn't take AS-PATH length into route selection process.

[MHM Cisco World]

10.131.1.1 is your vsmart 

You receive ibgp different as path from two peers

10.200.34.1 and 10.201.34.1

The show bgp show only one path not two' so it  not issue of as path the bgp not see all prefix from .201

Two points to check

1-did you receive tloc of .201 ?

2- did you use any policy in redistrubte between omp and bgp

MHM

[Kanan Huseynli · Accepted solution]

Exactly,

there is no such step in OMP bestpath selection:

https://www.cisco.com/c/en/us/td/docs/routers/sdwan/configuration/routing/ios-xe-17/routing-book-xe/m-unicast-routing.html#best-path

[Amitabha]

1. Yes.

2. Yes

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#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]

Can i see config of lab router 

MHM

[MHM Cisco World]

show omp tlocs [ detail ] [ color lte ] [ encap ipsec ] [ ip ip-address ] [ advertised ] [ received ]

use this to see if there is route for 10.201.34.1 or not 
even if the OMP dont select short path we need to see it in BGP 

but as I see there is only one path

lab-r1#sh bgp vpnv6 unicast vrf 13 2611:6bC0:6000::/40
BGP routing table entry for [1:13]2611:6bC0:6000::/40, version 5832
Paths: (1 available, best #1, table 13)
Not advertised to any peer
Refresh Epoch 1
Local
::FFFF:10.200.34.1 (via vrf 13) from 0.0.0.0 (172.29.236.1)
Origin incomplete, metric 1000, localpref 50, valid, sourced, best
Extended Community: SoO:0:14009
rx pathid: 1, tx pathid: 0x0
Updated on Dec 27 2023 21:15:33 UTC

[Kanan Huseynli]

One path in service side BGP is normal, it is redistributed route. Best is selected (using tie-breakers among ECMP routes) and one is redistributed only.