IPv6 Router Discovery with the nexus 3k/7k

Routing & Switching IPv6
1

So I have a nexus 3k and a nexus 7k. Both I’m setting up a vlan interface and/or a management interface for the RPs. The router on the network is an ASR 1001. I’m trying something rather simple that works on every other device except the 3k and 7k devices and that is router discovery over IPv6 from the ASR to the interfaces connecting. The configs are this simple:

interface TenGigabitEthernet0/1/0.198
 encapsulation dot1Q 198
 vrf forwarding relay_admin
 ipv6 address 2600:2100:ae0:13b::1/64
 ipv6 address FE80::FEED:198:1 link-local
 ipv6 enable
 ipv6 nd router-preference High

And the N3K interface look like the following:

interface Vlan198
  ipv6 address 2600:2100:ae0:13b:0:acc5:88:4/64
  no shutdown

interface Ethernet1/1
  switchport mode trunk
  switchport trunk allowed vlan 98,198,298

When I look at the routing table on the N3k, I get the following:

IPv6 Routing Table for VRF "default"
'*' denotes best ucast next-hop
'**' denotes best mcast next-hop
'[x/y]' denotes [preference/metric]

2600:2100:ae0:13b::/64, ubest/mbest: 1/0, attached
    *via 2600:2100:ae0:13b:0:acc5:88:4, Vlan198, [0/0], 2w1d, direct, 
2600:2100:ae0:13b:0:acc5:88:4/128, ubest/mbest: 1/0, attached
    *via 2600:2100:ae0:13b:0:acc5:88:4, Vlan198, [0/0], 2w1d, local

Looking at IOS/IOS-XR based switches and RPi’s, I do see the default route:

ND  ::/0 [2/0]
     via FE80::FEED:198:1, Vlan198

Debugging shows the RD’s being transmitted (also demonstrated by other devices picking them up) but none of the NXOS based devices are inserting the route in their tables. Is there something on the NXOS device that I’m missing?

2

Hello Marcos

Hmm, that’s interesting. My thoughts are that Nexus devices are configured by default not to accept ND packets. I haven’t been able to find any definitive evidence to this, but in general, within the environment of a data center, you don’t want such mechanisms to function by default, but only when you enable them, so that’s my thought. I’d suggest you try the following troubleshooting activities:

Configure the Nexus device to obtain an IPv6 address via SLAAC. Use the ipv6 address autoconfig default command. This will force the interface to obtain an IPv6 address from any configured router on the network segment AND to use its address as the default gateway for the interface.

By doing this you’ll see if you can force the device to accept a default gateway for the interface. Take a look and let us know your results?

I hope this has been helpful!

Laz

3

so unfortunately that particular command can only be done on the MGMT0 interface and not on a vlan interface… which is difficult if the mgmt interface isn’t wired up here.

tower-parasol-sw1(config)# interface Vlan198
tower-parasol-sw1(config-if)# ipv6 address autoconfig default
autoconfig and defaultRoute configs are only applicable for mgmt interface
4

Hello Marcos

Hmm, that’s interesting. I don’t have enough experience with Nexus devices and IPv6 to be able to be more definitive, however, I have found this interesting piece of info:

IPv6 neighbor discovery functions correctly only in a VLAN with the OMF feature disabled.

This is found within the Guidelines and Limitations for Configuring IPv6 section of the following document.

The truth is that Nexus devices because they are built for datacenter environments, have certain default behaviors when it comes to particular features and capabilities. These are put into place as best-practice defaults for increased security, stability, and operability. This seems to be one of those cases, but looking deeper into it can be helpful, and helps us learn!

I hope this has been helpful!

Laz

5

Looked closer at the statement you included… It references F2 modules have this issue (rather old and only on the 7000 series, not the 3000) and the command it states will disable that issue is for the F2 card only (tried it on both the 3000 and 7000, but all my cards are M2 or M3).

Still perplexed however the multicasting “forwarding function” issue is interesting to go down the path of. Unfortunately I don’t have smartnet for these two devices anymore so I can’t even get someone from Cisco to answer the question. Rick Grazaini (author of IPv6 Fundamentals) is a friend of mine so I’ll see if he can also poke around for some answers…

1 Like
6

Hello Marcos

Thanks for the update. If you get any news and resolve the issue, let us know!

Laz