Router Advertisement
ICMP Type 9 Router Advertisement is sent by routers to announce their presence and IP addresses to hosts on a subnet. Part of ICMP Router Discovery (RFC 1256). Hosts use these messages to discover default gateways without requiring manual configuration or DHCP.
Description
ICMP Router Discovery (RFC 1256) allows hosts to dynamically discover available routers without manual configuration. Routers periodically multicast or broadcast Router Advertisement messages (Type 9) on directly connected subnets. Hosts can also solicit advertisements using Router Solicitation (Type 10).
Code 0 is a normal router advertisement. Code 16 indicates the router does not route common traffic (it only routes mobile traffic per RFC 3344, the Mobile IP standard). This distinction matters for Mobile IPv4 deployments.
Router Discovery has been largely superseded by DHCP for IPv4 default gateway discovery and by NDP (Neighbor Discovery Protocol) Router Advertisements in IPv6. However, Type 9 messages still appear in some embedded and industrial network environments.
Common Causes
- –Router periodically advertising its presence on a subnet
- –Response to ICMP Router Solicitation (Type 10) from a host
- –Mobile IP foreign agent announcing its presence (code 16)
Codes
| Code | Name |
|---|---|
| 0 | Normal Router Advertisement |
| 16 | Does Not Route Common Traffic |