IPv6 Router Advertisement Preference

This topic is to discuss the following lesson:

i did the configuration with 2 routers and a workstation with win 7, the comand router-preference has no impact at all.
Is there something i missed in Win7 ?

Finally the only way i found is to deactivate the nd protocol to make sure there is only one gateway in win 7.
But i loose the failover for slaac.

Two questions,
- the first RS that the host sent, what are the source/destination IPs/MACs in use in the RS packet ?
- If the gi0/1 interface on R2 was getting shutdown immediately how could the host receive the RA msg from R2 ? Is it part of port shutdown procedures to send out RA ?
- How about a router shutdown? Is RA being generated ?

Hello Ray,

In the RS, the source MAC address is the host’s MAC address, the destination is the “all routers multicast” destination FF02::2.

About the “shutdown procedure”, I just labbed this up to see how it works. Take a look at this wireshark capture:

https://www.cloudshark.org/captures/69fd7a31fe57

The first packet (length 118) is a regular router advertisement. The second packet (length 102) is sent when the interface is shut but also when you reload the router.

The difference between the two is that in the “goodbye” router advertisement, all ICMPv6 options are removed except for the prefix information. Also, the router lifetime is set to 0.

This seems to be specified in the RFC:

https://tools.ietf.org/html/rfc4861

A Lifetime of 0 indicates that the router is not a default router and SHOULD NOT appear on the default router list.

I couldn’t find anything in the RFC about withdrawing options as a requirement so that might be specific to Cisco.

Hope this helps!

Rene

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Good question.
I just keep a doubt, RA is sent periodically like BPDU or OSPF hello interval?
Regards

Hello Carlo

Yes, RAs are sent out periodically every few seconds. However, they are also sent as responses to Router Solicitations (RS) when an IPv6 device requests information during the SLAAC process. More on SLAAC can be found at this lesson:

I hope this has been helpful!

Laz

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So I see this Lifetime 1800 sec in show ipv6 routers output, I guess that´s like a hold time in OSPF, how often is hello/keepalive timer sent? in IPv6 RA

Hello Carlo

The lifetime is the amount of time that a device will consider the specific router as its default router, unless informed otherwise. By default, this value is set to 1800 seconds, or 30 minutes. The RA advertisement interval, which is how often an RA is sent, is set to 30 seconds by default. Both the lifetime and the advertisement intervals can be modified on a per interface basis using the following commands respectively:

R1(config-if)# ipv6 nd ra interval ?
    RA Interval (sec)
  msec      Interval in milliseconds

and

R1(config-if)# ipv6 nd ra lifetime ?
    RA Lifetime (seconds)

As you can see, the interval can be configured with an accuracy of milliseconds, while the lifetime with an accuracy of seconds.

A lifetime value of 0, if configured, indicates to the client that this specific router is not available as a default router.

It’s interesting to note, that if you shutdown an interface of a router that is functioning as a default router, before it actually shuts down, it will send out an RA with a lifetime of 0, indicating gracefully to all concerned hosts that it is no longer available as a default router.

I hope this has been helpful!

Laz

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really good explanation. Thank you

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