IS-IS and IPv6 LLA next-hop - just Arista, or everyone?

Adam Thompson athompson at
Tue May 4 15:15:40 UTC 2021

Thank you, both!

...that kinda sucks, though.

I don't see any rationale in RFC 5308 for why the HELLO packet may only contain the LLA - does anyone know/remember why?  (I'm hoping that understanding the rationale will make this an easier pill to swallow.)  Obviously this behaviour/requirement is net-new to the IPv6 TLVs, as there's no LLA-cognate in IPv4 (APIPA doesn't count).  There is in OSI, I think, but I'm still too sane to read those docs.

It makes sense that you would not want LLAs in LSPs, only GUAs, but does that imply that you must​ use either ULAs or GUAs in order to establish IPv6 routes in IS-IS, in an IPv6 environment?  That makes about as much sense to me as forcing LLAs for next-hops.


Adam Thompson
Consultant, Infrastructure Services
100 - 135 Innovation Drive
Winnipeg, MB, R3T 6A8
(204) 977-6824 or 1-800-430-6404 (MB only)
athompson at<mailto:athompson at><>

From: NANOG < at> on behalf of Saku Ytti <saku at>
Sent: May 4, 2021 01:44
To: Mark Tinka <mark at>
Cc: nanog list <nanog at>
Subject: Re: IS-IS and IPv6 LLA next-hop - just Arista, or everyone?

On Tue, 4 May 2021 at 07:24, Mark Tinka <mark at> wrote:

> Junos:
>> 2c0f:feb0::1/128   *[IS-IS/18] 02:43:49, metric 5870
>                        to fe80::1205:caff:fe86:4ac3 via et-4/0/2.0
>                        to fe80::5287:89ff:fef3:25c3 via et-4/0/2.0
>                        to fe80::1205:caff:fe86:4b10 via et-5/0/2.0
>                     >  to fe80::5287:89ff:fef3:2610 via et-5/0/2.0
> I2  2C0F:FEB0::1/128 [115/6410]
>      via FE80::1205:CAFF:FE86:4AC3, TenGigabitEthernet1/0/0
>      via FE80::1205:CAFF:FE86:4B10, TenGigabitEthernet0/0/0
>      via FE80::5287:89FF:FEF3:25C3, TenGigabitEthernet1/0/0
>      via FE80::5287:89FF:FEF3:2610, TenGigabitEthernet0/0/0
> i L2 2c0f:feb0::1/128
>       [115/5870] via fe80::1205:caff:fe86:4b10, 02:45:22, HundredGigE0/3/0/0 (!)
>       [115/5810] via fe80::f60f:1bff:feb0:75c4, 02:45:22, HundredGigE0/2/0/1

2001:218:0:1000::1/128                        Remote  ISIS      48d22h13m  18
       fe80::42de:adff:fe98:87e4-"lag1"                             25301


For Hello PDUs, the "Interface Address" TLV MUST
   contain only the link-local IPv6 addresses assigned to the interface
   that is sending the Hello.  For LSPs, the "Interface Address" TLVs
   MUST contain only the non-link-local IPv6 addresses assigned to the

These are hello derived:
A:ytti at show router isis adjacency detail |match Neigh
IPv6 Neighbor     : fe80::42de:adff:fe98:87e4
IPv4 Neighbor     :

Vendors do not have the option to use GUA while being RFC5308 compliant.

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