Cisco Support Community
Community Member

NDP and autoconfiguration for Routers

Hi everyone,

While I research NDP functions both hosts and routers in IPV6 I can`t understand some issues about NDP behavior on nodes. I saw conceptual data structures and caches for host in lots of documents in addition to RFC 4861.

My first question is,

1- Are these structures, destination cache - default router lists - prefix lists etc... , used in a router for next hop determination or used for any other fuction? I think routers don`t need to have and use these structures, is it true?  For example when an upper layer of a router send a TCP packet from its interface does it use these caches (default router list and destination cashes) or FIB tables that implemented with routing protocols.

2-  Does autoconfiguration process applies only to hosts and not routers ? What about CPE ? RFC 6204 says that `The IPv6 CE router MUST support Stateless Address Autoconfiguration (SLAAC)`. I understand from these sentences that routers can receive an RA on the its interface and configure itself with options that included to RA. Is it really true? How can a router behave when it recieve a RA from an interface with option autoconfiguration flag set. Can we disable or enable autoconfiguration for an interface.

3- Does routers use DAD when an address is assigned to its interface by manually, DHCPv6 or autoconfiguration?  


Cisco Employee

NDP and autoconfiguration for Routers

1. These are conceptual data structures. On-link determination and next-hop determination is done using the RIB/FIB tables in IOS. IOS has a neighbor cache (equivalent to the ARP table) / adjacency database.

2. Autoconfiguration (SLAAC) has a distinct router function and a host function. The "address assignment" part only applies to hosts. A CPE according to RFC6204 behaves both as a host and a router (depending on context) on its WAN interface. An IOS router can behave as a host for the purpose of SLAAC with the ipv6 address autoconfig command.

3. Yes. DAD can be turned off with ipv6 nd dad attempts 0.

Community Member

NDP and autoconfiguration for Routers

Thank you for your response Troan.

I understood from your reply that a router can recive RA message from its interface with address autoconfig command and configure its address.

1- My question is, if RA messages includes other parameters like MTU, how receiving router (with address autoconfig command) use these parameters.Does it discard ?

2- RFC 4861 say that routers recieve RA message and verify its consistency. Can a router recieve a RA message from other interfaces without address autoconfig command and use parameters of RA for other configurations?


Cisco Employee

NDP and autoconfiguration for Routers

1. It only does a consistency check on the MTU parameter. For e.g. the prefix list it installs connected routes.

    And it does router discovery.

2. What did you have in mind?

    It does the consistency check and logs a message regardless of the configuration.



Community Member

NDP and autoconfiguration for Routers


Actually I want to take confirmation whether a router can receive and evaluate RA without any command. 

You say receiving RA messages used for autoconfiguration and consistency check by routers. But you also say routers does router discovery with receiving RA. I can understand that a host can does router discovery. But why a router does?

What do you mean with router discovery? Is it used for forwarding decisions?

On-link determination and next-hop determination is done using the RIB/FIB tables in IOS you say. It really confuse me...

CreatePlease to create content