1. RFC 4191 Router Preferences and More-Specific Routes
Speaker: Ching-Chen Chang
Date:

2. Outline Introduction Preference Value The Format of message
Conceptual Model of Host
Example
Reference

3. Introduction
An optional extension to Neighbor Discovery Router Advertisement messages for communicating default router preferences and more-specific routes from routers to hosts.
using Neighbor Discovery
Router Advertisement are an existing standard and stable protocol.
Shielding the host from having to participate in more router-to-router interactons.
It carries route lifetimes.

4. Preference Value 01
High 00
Medium 11
Low 10
Reserved

5. The Format of message(1/3)
| Type | Code | Checksum |
| Cur Hop Limit |M|O| Reserved | Router Lifetime |
| Reachable Time |
| Retrans Timer |
| Options |

6. The Format of message(2/3)
| Type | Code | Checksum |
| Cur Hop Limit |M|O|H| Prf |Resvd | Router Lifetime |
| Reachable Time |
| Retrans Timer |
| Options |
Prf => Default Router Preference

7. The Format of message(3/3)
| Type | Length | Prefix Length |Resvd|Prf|Resvd|
| Router Lifetime |
| Prefix (Variable Length) |

8. Conceptual Model of Host
Type A:
Neighbor Discovery.
Type B:
Hosts uses a Default Router List augmented with preference values, but ignore all Route Information Options.
Type C:
Hosts use a Routing Table instead of a Default Router List.

9. Example ::/0 -> router W with a Medium prefrence
2002::/16 -> router X with a Medium preference
2001:db8::/32 -> router Y with a High preference
2001:db8::/32 -> router Z with a Low preference

10. Reference RFC 4191 Default Router Preferences and More-Specific Routes
RFC 3775 Mobility Support in IPv6
RFC 2461 Neighbor Discovery for IP Version 6