Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 116 Neighbor Discovery Protocol (NDP) and ICMPv6 The neighbor discovery protocol (NDP) provides a multitude of services for IPv6 enabled devices, including: • Automatic address configuration, and prefix discovery • Duplicate address detection • MTU discovery • Router discovery • Address resolution NDP replaces many IPv4 specific protocols, such as DHCP and ARP. NDP utilizes ICMPv6 to provide the above services. Periodically, IPv6 routers send out Router Advertisements (RA’s) to both announce their presence on a link, and to provide auto-configuration information for hosts. This RA (ICMP packet type 134) is sourced from the link-local address of the sending router, and sent to the link-scope all-nodes multicast group. The sending router sets a hop limit of 255 on a RA; however, the RA packet must not be forwarded outside the local link. Hosts use RA’s to configure themselves, and add the router to its local default router list. A host can request an RA by sending out a Router Solicitation (RS, ICMP packet type 133) to the link-local all-routers multicast address. A RS is usually sent when a host is not currently configured with an IP address. The RA messages contain the following information for hosts: • The router’s link-layer address (to be added to the host’s default router list) • One or more network prefixes • A lifetime (measured in seconds) for the prefix(es) • The link MTU Routers send Redirect messages to hosts, indicating a better route to a destination. Hosts can have multiple routers in its default router list, but one is chosen as the true default router. If this default router deems that another router has a better route to the destination, it forwards the Redirect message to the sending host. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 117 Neighbor Discovery Protocol (NDP) and ICMPv6 (continued) Neighbor Solicitations (NS’s, ICMP packet type 135) are sent by hosts to identify the link-layer address of a neighbor, and ensure its reachability. A NS message’s source address is the link-local address of the sending host, and the destination is the solicited-node multicast address of the destination host. A neighbor will reply to a NS with a Neighbor Advertisement (NA, ICMP packet type 136). This process replaces the Address Resolution Protocol (ARP) used by IPv4, and provides a far more efficient means to learn neighbor address information. Hosts additionally use the NS messages to detect duplicate addresses. Before a host assigns itself an IPv6 address, it sends out a NS to ensure no other host is configured with that address. Autoconfiguration of Hosts Hosts can be assigned IPv6 addresses one of two ways: manually, or using autoconfiguration. Hosts learn how to autoconfigure themselves from Router Advertisements (RA’s). Two types of autoconfiguration exist, stateless and stateful. When using Stateless Autoconfiguration, a host first assigns itself a linklocal IPv6 address. It accomplishes this by combining the link-local prefix (FE8) with its interface ID (MAC address in EUI-64 format). The host then sends a Router Solicitation multicast to the all-routers multicast address, which provides one or more network prefixes. The host combines these prefixes with its interface ID to create its site-local (or aggregate global) IPv6 addresses. Stateful Autoconfiguration is used in conjunction with stateless autoconfiguration. Stateful Autoconfiguration utilizes DHCPv6 to provide additional information to the host, such as DNS servers. DHCPv6 can also be used in the event that there is no router on the link, to provide stateless autoconfiguration. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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Cisco CCNA Study Guide - Router Alley

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