wrong BGP AS number transmitted

[Sven P.]

Hello,

i have a problem with BGP AS number. We have configured two BGP peers to different ISPs on a Cisco ISR4451-X Router. One peer works fine but at the second peer the state oft the connection is always idle.

When I look into the logging I see that the wrong AS number is transmitted:

I have configured the AS 202xxx but transmitted is always the number 23456.

 Log Buffer (4096 bytes):

Dec 20 12:52:13.940: BGP_Router: unhandled major event code 128, minor 0

Dec 20 12:52:19.609: BGP: xxx active went from Idle to Active

Dec 20 12:52:19.609: BGP: xxx open active, local address xxx

Dec 20 12:52:19.621: BGP: ses global xxx (0x7F33116D6630:0) act Adding topology IPv4 Unicast:base

Dec 20 12:52:19.621: BGP: ses global xxx (0x7F33116D6630:0) act Send OPEN

Dec 20 12:52:19.621: BGP: xxx active went from Active to OpenSent

Dec 20 12:52:19.621: BGP: xxx active sending OPEN, version 4, my as: 23456, holdtime 180 seconds, ID B99E6022

Dec 20 12:52:19.634: BGP: ses global xxx (0x7F33116D6630:0) act Remote close.

Dec 20 12:52:19.634: BGP: nbr_topo global xxx IPv4 Unicast:base (0x7F33116D6630:0) Not scheduling for GR processing [Peer did not advertise GR cap]

Dec 20 12:52:19.634: %BGP-5-NBR_RESET: Neighbor xxx active reset (Peer closed the session)

Dec 20 12:52:19.634: BGP: ses global xxx (0x7F33116D6630:0) act Reset (Peer closed the session).

Dec 20 12:52:19.634: BGP: nbr_topo global xxx IPv4 Unicast:base (0x7F33116D6630:0) NSF delete stale NSF not active

Dec 20 12:52:19.634: BGP: nbr_topo global xxx IPv4 Unicast:base (0x7F33116D6630:0) NSF no stale paths state is NSF not active

Dec 20 12:52:19.634: BGP: nbr_topo global xxx IPv4 Unicast:base (0x7F33116D6630:0) Resetting ALL counters.

Dec 20 12:52:19.634: BGP: xxx active closing

Dec 20 12:52:19.634: BGP: ses global xxx (0x7F33116D6630:0) act Session close and reset neighbor xxx topostate

Dec 20 12:52:19.634: BGP: nbr_topo global xxx IPv4 Unicast:base (0x7F33116D6630:0) Resetting ALL counters.

Dec 20 12:52:19.634: BGP: xxx active went from OpenSent to Idle

Dec 20 12:52:19.634: %BGP_SESSION-5-ADJCHANGE: neighbor xxx IPv4 Unicast topology base removed from session  Peer closed the session

A monitor capture output with wireshark also show the wrong AS Number and the BGP notification message show Bad Peer AS.

 Border Gateway Protocol - OPEN Message

    Marker: ffffffffffffffffffffffffffffffff

    Length: 29

    Type: OPEN Message (1)

    Version: 4

    My AS: 23456

    Hold Time: 180

    BGP Identifier: xxx

    Optional Parameters Length: 0

Border Gateway Protocol - NOTIFICATION Message

    Marker: ffffffffffffffffffffffffffffffff

    Length: 21

    Type: NOTIFICATION Message (3)

    Major error Code: OPEN Message Error (2)

    Minor error Code (Open Message): Bad Peer AS (2)

 router bgp 202xxx

 bgp router-id xxx

 bgp log-neighbor-changes

 neighbor xxx remote-as 212xx

 neighbor xxx dont-capability-negotiate

 neighbor xxx password 7 0337723xxxxxxxx

BGP router identifier xxx, local AS number 202xxx

BGP table version is 1814938, main routing table version 1814938

613561 network entries using 152163128 bytes of memory

613561 path entries using 73627320 bytes of memory

102358/96231 BGP path/bestpath attribute entries using 25384784 bytes of memory

88809 BGP AS-PATH entries using 4146064 bytes of memory

1 BGP ATTR_SET entries using 40 bytes of memory

201 BGP community entries using 7928 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

BGP using 255329224 total bytes of memory

BGP activity 1988167/1374600 prefixes, 3700228/2473531 paths, scan interval 60 secs

Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd

xxx             4           xxx    0      0        1       0   0    5d04h    Idle

xxx             4           xxx  609982   3337     1814810 0   0    1d01h    613562

xxx            4           xxx     0      0        1       0   0    never    Active

Why ist he wrong AS transmitted at this peer?

thanks in advance

Sven

[Richard Burts]

Sven

It is helpful to see the error messages. But to give you good advice we also need to see exactly what is in your config for BGP.

HTH

Rick

[Sven P.]

Hello Rick,

which other config do you need?

thanks

Sven

[paul driver]

Hello

Are you or the neighbor peering with the correct ASN

Are you or the neighbour using bgp local-as to advertise a different ASN then the root ASN of your bgp config?

res

paul

[Sven P.]

Hello

yes I and the neighbor peering with the correct ASN. I have configured my correct ASN at the router.

No I dont use bgp local-as. I cant say if the neighbor use it.

thanks Sven

[Giuseppe Larosa]

Hello Sven,

you have a 4 byte AS nunber as as you say you have 202xxx > 65535.

The second ISP has an old router that is not able to manage 4byte AS number for this cases the special AS number that you see in your captures is reserved to attempt to establish a session with the 2-byte AS number eBGP peer.

the as number that you see is reserved for backward compatibilty this explains why your router looks like to use a wrong AS number with second peer.

the session shoulg go up there is something else going wrong here between your router and the ISP router.

Contact the ISP and have them change/update their device to support 4 byte BGP AS number

see

https://tools.ietf.org/html/rfc4893

Hope to help

Giuseppe

[Sven P.]

Hello Giuseppe,

thank you for your answer. We will contact our partner to check his router if he support 4 byte BGP AS number.

I have add a wireshark capture to the post from the BGP connection, maybe you can check if you can find something in the config. The BGP connection that does not work starts with the IP 185.

thanks Sven