bgpd extcommunity dmzlink-bw is incompatible with Cisco

[foxnetwork-ru]

Description

FRR sends extcommunity for bw. Cisco gets it, but can't recognize.
Cisco virtual Catalyst c8000v ver 17.18.1a

Version

10.5.1

How to reproduce

FRR config (partial)
route-map test permit 10
set extcommunity bandwidth 10
exit

neighbor 192.168.0.10 route-map test out

Cisco config (partial)
neighbor 192.168.0.11 dmzlink-bw

Expected behavior

On Cisco side
Router#sho ip bg 1.1.1.1/32
BGP routing table entry for 1.1.1.1/32, version 2
Paths: (1 available, best #1, table default)
Not advertised to any peer
Refresh Epoch 1
2
192.168.0.11 from 192.168.0.11 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external, best
DMZ-Link Bw 1250 kbytes
rx pathid: 0, tx pathid: 0x0
Updated on Jan 12 2026 18:21:34 UTC

Actual behavior

On Cisco side
Router#sho ip bg 1.1.1.1/32
BGP routing table entry for 1.1.1.1/32, version 2
Paths: (1 available, best #1, table default)
Not advertised to any peer
Refresh Epoch 1
2
192.168.0.11 from 192.168.0.11 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external, best
Extended Community: 0x4:2:1234736768
DMZ-Link Bw 125000 kbytes
rx pathid: 0, tx pathid: 0x0
Updated on Jan 12 2026 18:21:34 UTC

Extended Community: 0x4:2:1234736768
This is how Cisco detects extcommunity from FRR
DMZ-Link Bw 125000 kbytes
This is default value.

Additional context

Traffic dump (BGP update message from FRR).

Checklist

• I have searched the open issues for this bug.
• I have not included sensitive information in this report.

[ton31337]

What happens if you set neighbor X disable-link-bw-encoding-ieee?

[foxnetwork-ru]

Hi Donatas,
Same difference.

FRR config (partial)

`route-map test permit 10

set extcommunity bandwidth 101

exit
!
interface ens33
ip address 10.64.1.128/24
exit
!
interface lo
ip address 1.1.1.1/32
exit
!
router bgp 128
neighbor 10.64.1.129 remote-as 129
neighbor 10.64.1.129 disable-link-bw-encoding-ieee
!
address-family ipv4 unicast
redistribute connected
neighbor 10.64.1.129 route-map test out
exit-address-family
exit
`

Cisco config (partial)

`interface GigabitEthernet1
ip address 10.64.1.129 255.255.255.0
negotiation auto

router bgp 129
bgp log-neighbor-changes
neighbor 10.64.1.128 remote-as 128
neighbor 10.64.1.128 dmzlink-bw
`

And now output from Cisco

`Router#sho ip bgp

BGP table version is 7, local router ID is 10.64.1.129
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
t secondary path, L long-lived-stale,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 1.1.1.1/32 10.64.1.128 0 0 128 ?
r> 10.64.1.0/24 10.64.1.128 0 0 128 ?
Router#sho ip bg 1.1.1.1/32
BGP routing table entry for 1.1.1.1/32, version 6
Paths: (1 available, best #1, table default)
Not advertised to any peer
Refresh Epoch 2
128
10.64.1.128 from 10.64.1.128 (1.1.1.1)
Origin incomplete, metric 0, localpref 100, valid, external, best
Extended Community: 0x4:128:12625000
DMZ-Link Bw 125000 kbytes
rx pathid: 0, tx pathid: 0x0
Updated on Jan 14 2026 08:42:09 UTC
Router#
`

[ton31337]

Isn't the issue that Cisco might be using different types of units? bytes/s versus bits/s.

[foxnetwork-ru]

As I believe, it is not because of the units mismatch. As you can see above, Cisco detects 'something' in extcomminity from FRR, but can't understand what to do with it. This is how, Cisco sees our dmzlink-bw.
Extended Community: 0x4:128:12625000

Since Cisco doesn't get any BW information included, it inserts default value.
DMZ-Link Bw 125000 kbytes

So if you change config in FRR, Cisco changes this: Extended Community: 0x4:128:12625000
But DMZ-Link Bw 125000 kbytes stays unchanged.

I guess, Cisco doesn't understand our extcommunity as anything related to bandwidth.

[ton31337]

The main problem is that the link bandwidth extended community is quite old, AND refreshed recently, and it's not clear what encoding Cisco is using. Is it Cisco-specific or IETF-compliant... What FRR side sees when Cisco sends its own extended community?

[foxnetwork-ru]

The issue is solved. The solution includes marking this extcommunity as non-transitive. Than Cisco recognized it correctly.
RP/0/RP0/CPU0:test2#sho ip bg 1.1.1.1 Mon Jan 19 10:58:19.006 UTC BGP routing table entry for 1.1.1.1/32 Versions: Process bRIB/RIB SendTblVer Speaker 14 14 Last Modified: Jan 19 10:54:53.756 for 00:03:25 Paths: (2 available, best #1) Not advertised to any peer Path #1: Received by speaker 0 Not advertised to any peer 1 192.168.0.1 from 192.168.0.1 (1.1.1.1) Origin IGP, metric 0, localpref 100, valid, external, best, group-best, multipath Received Path ID 0, Local Path ID 1, version 14 Extended community: LB:1:560 Origin-AS validity: (disabled) Path #2: Received by speaker 0 Not advertised to any peer 1 192.168.0.3 from 192.168.0.3 (3.3.3.3) Origin IGP, metric 0, localpref 100, valid, external, multipath Received Path ID 0, Local Path ID 0, version 0 Extended community: LB:1:1001000 Origin-AS validity: (disabled) RP/0/RP0/CPU0:test2#
192.168.0.1 - yet another Cisco router with IOS XR
192.168.0.3 - Ubuntu with FRR 10.5.1.
Route-map on FRR side should be as follows
route-map test permit 10 set extcommunity bandwidth 1001 non-transitive exit
Anyway, thanks for being so cooperative. The ticket can be closed. Thanks a lot!

[ton31337]

Glad you solved that.