ASR1001-X traceroute EIGRP ASN anomaly

[Brandon Matthee]

Good Day Fellow Members

I am reaching out in hopes to find some clarity regarding potential mismatch with EIGRP ASN, we see incorrect AS numbers in traceroute outputs. We see the ISP ASN in hops that are devices not directly connected to the ISP.

We see this on two pairs (four devices) of Cisco ASR 1001-X running this software version: asr1001x-universalk9.17.03.03.SPA.bin

The ISP ASN is AS 7474

sample output

ME1 Datacentre
################
AU-ME1-CORE-RTR01#sh invent
NAME: "Chassis", DESCR: "Cisco ASR1001-X Chassis"
PID: ASR1001-X , VID: V06 ,

!

AU-ME1-CORE-RTR01#traceroute 8.8.8.8 source loopback 0
Type escape sequence to abort.
Tracing the route to 8.8.8.8
VRF info: (vrf in name/id, vrf out name/id)
1 10.53.65.26 [AS 7474] 1 msec 0 msec 1 msec [Device ASN: 65473]
2 10.53.65.9 [AS 7474] 1 msec 1 msec 0 msec [Device ASN: 65474]
3 164.63.202.5 [AS 7474] 0 msec 1 msec 1 msec [Device ASN: 55504]
4 202.139.7.113 [AS 7474] 2 msec 2 msec 2 msec [ISP ASN: 7474]
5 * * *
6 124.19.61.213 [AS 7474] 3 msec
124.19.61.215 [AS 7474] 3 msec 3 msec
7 72.14.209.126 [AS 7474] 3 msec 3 msec 4 msec [GOOGLE]
8 * * *
9 8.8.8.8 [AS 7474] 3 msec 3 msec 2 msec [GOOGLE]
AU-ME1-CORE-RTR01#

!

AU-ME1-CORE-RTR01#sh ip bgp 0.0.0.0 bestp
BGP routing table entry for 0.0.0.0/0, version 989130
Paths: (3 available, best #2, table default)
Advertised to update-groups:
34 71
Refresh Epoch 1
65473 65474 55504 7474
10.53.65.27 from 10.53.65.27 (10.53.64.2)
Origin IGP, localpref 100, valid, external, best
Community: 55504:2300
rx pathid: 0, tx pathid: 0x0
Updated on Nov 17 2021 15:58:34 UTC
AU-ME1-CORE-RTR01#

Is this normal? Any advice? 

Thanks in advance

[Georg Pauwen]

Hello,

I assume you mean BGP and not EIGRP ASN ?

Either way, ISPs deploy several mechanisms such as Carrier Grade NAT (where the same IP address is shared with different customers). There also might be a divergence of the control path and the traceroute path.

Have a look at the discussion below...very interesting.

https://www.researchgate.net/publication/229023870_Traceroute_and_BGP_AS_Path_Incongruities

Ask your provider (Sing Tel Optus I guess), maybe they are willing to share what mechanisms they use...

[Harold Ritter]

Hi @Brandon Matthee ,

Yes, this is normal behavior. If the only BGP route that you have is the default and that this default route is known from ASN 7474, all traceroute entries will show that ASN. This would be different if you had more specific routes for the intermediate hops and these routes were learnt from other ASNs.

Regards,

[Brandon Matthee]

@Georg Pauwen @Harold Ritter 

Thank you very much for the affirmation and for the information provided. I will definitely take a deeper look into this

Thanks once again