Network Engineering Asked by Eugene D. Gubenkov on January 30, 2021
I’m wondering what will happen if BGP unnumbered session is configured on a link that has multiple routers on the other side which are both IPv6-capable and will both reply on RS and produce RA messages (Router Advertisement and Router Solicitation) which auto-configuration for BGP unnumbered relies upon?
Such configuration seems inherently ambiguous, but it might, for instance, [automagically] establish BGP peering with both…
If anyone tried to set up such a configuration or knows the result in advance, can you please share your findings?
I've finally reproduced this scenario to check what will happen.
Using eve-ng network emulator (note, it's not a simulator, it runs real software images and shows what happens almost indistinguishably from real devices) I've created the following topology.
cumulus10
there (with AS 65003
) hosts vlan1
interface (which is switch virtual interface) and ports swp1
and swp2
are bridged, effectively forming a single broadcast domain (as in the problem statement).
interface bridge
bridge-ports swp1 swp2
bridge-vids 1
bridge-vlan-aware yes
interface vlan1
address 10.10.10.1/24
vlan-id 1
vlan-raw-device bridge
This system has an instruction to establish BGP unnumbered session using vlan1
interface.
router bgp 65003
neighbor vlan1 interface remote-as external
Two other devices (cumulus9
and cumulus11
) have a simple configuration for BGP unnumbered peering over swp1
and swp2
respectively.
router bgp 65004
neighbor swp1 interface remote-as external
router bgp 65005
neighbor swp2 interface remote-as external
In one of my test runs I'm going to be using as an example I see that as soon as SVI brought up AS 65003
tries to establish BGP session sequentially with both neighbors, but in the end, only a single BGP session is established, the second one always fails. I am leaving the packet capture results as seen on swp1
interface for cumulus9
and swp2
for cumulus11
which sheds the light on the internals.
The bottom line: such configuration is inherently flawed for BGP unnumbered peering. From experiments, I see that any of the three possible pairs can form a BGP session (even look at the picture in question -- it is symmetric!), and which pair will do it is a subject for undefined behavior and probabilistic.
Packet capture plus link-local addresses and MAC addresses for all 3 devices below (to make sense of the packet capture results). Raw packet capture files (two .pcapng
files, openable in Wireshark) I'm leaving for those who interested to dig here: https://www.dropbox.com/sh/bdo3w8a8tt2u8ka/AAD-Pfv2cWfuTdG2B9tFzN1va?dl=0; additionally, a quick peek at the most interesting part in form of the image is below.
| device | interface | MAC | IPv6 LLA |
|-----------+------------+-------------------+-------------------------|
| cumulus10 | vlan1 | 50:00:00:0a:00:01 | fe80::5200:ff:fe0a:1/64 |
| cumulus9 | swp1 | 50:00:00:09:00:01 | fe80::5200:ff:fe09:1/64 |
| cumulus11 | swp2 | 50:00:00:0b:00:02 | fe80::5200:ff:fe0b:2/64 |
Answered by Eugene D. Gubenkov on January 30, 2021
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