[c-nsp] LDPv6 Census Check

Mark Tinka mark.tinka at seacom.mu
Fri Jun 12 03:43:34 UTC 2020

On 11/Jun/20 23:45, adamv0025 at netconsultings.com wrote:

> Right I see what you are striving to achieve is migrate from BGP in a core to a BGP free core but not leveraging 6PE or 6VPE? 

Yes sir.

> So considering you already had v4 FECs wouldn't it be simpler to do 6PE/6VPE, what do you see as drawbacks of these compared to native MPLSv6 please?

Because 6PE, for us, adds a lot more complexity in how we design the

But most importantly, it creates a dependency for the success of IPv6 on
IPv4. If my IPv4 network were to break, for whatever reason, it would
take my IPv6 network down with it.

Years back, there was a nasty bug in the ASR920 that set an upper limit
on the MPLS label space it created FEC's for. Since Juniper sometimes
uses higher label numbers than Cisco, traffic between that ASR920 and
our Juniper network was blackholed. It took weeks to troubleshoot, Cisco
sent some engineering code, I confirmed it fixed the issue, and it was
rolled out generally. During that time when the ASR920 was unavailable
on IPv4, it was still reachable on IPv6.

Other issues are also with the ASR920 and ME3600X/3800X routers, where
0/0 and ::/0 are the last routes to be programmed into FIB when you run
BGP-SD. It can be a while until those boxes can reach the rest of the
world via default. IPv6 will get there faster.

I also remember another issue, back in 2015, where a badly-written IPv4
ACL kicked one of our engineers out of the box. Thankfully, he got back
in via IPv6.

I guess what I'm saying is we don't want to fate-share. IPv4 and IPv6
can operate independently. A failure mode in one of them does not
necessarily propagate to the other, in a native, dual-stack network. You
can deploy something in your IPv6 control/data plane without impacting
IPv4, and vice versa, if you want to roll out gracefully, without
impacting the other protocol.

6PE simply has too many moving parts to setup, comparing to just adding
an IPv6 address to a router interface and updating your IGP. Slap on
LDPv6 for good measure, and you've achieved MPLSv6 forwarding without
all the 6PE faffing.

> Well my point was that if v4 FECs would be enough to carry v6 traffic then I wouldn't need SRv6 nor LDPv6, hence I'm curious to hear from you about the benefits of v6 FEC over v4 FEC (or in other words MPLSv6 vs 6PE/6VPE). 

No need for 6PE deployment and day-to-day operation complexity.

A simplified and more native tunneling for IPv6-in-MPLSv6, rather than

No inter-dependence between IPv6 and IPv4.

Easier troubleshooting if one of the protocols is misbehaving, because
then you are working on just one protocol, and not trying to figure if
IPv4 or MPLSv4 are breaking IPv6, or vice versa.

For me, those 4 simple points help me sleep well at 3AM, meaning I can
stay up longer having more wine, in peace :-).


More information about the NANOG mailing list