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Appendix D. Summary of IsRouter Rules

This appendix summarizes the rules for maintaining and using the IsRouter flag in Neighbor Cache entries.

D.1. Overview​

The IsRouter flag in a Neighbor Cache entry indicates whether a neighbor is known to be a router. This information is critical for proper packet forwarding and influences several aspects of Neighbor Discovery behavior.

D.2. Setting the IsRouter Flag​

The IsRouter flag is set to TRUE in the following cases:

  1. Router Advertisement Received: When a Router Advertisement is received from a neighbor, the IsRouter flag for that neighbor MUST be set to TRUE.

  2. Neighbor Advertisement with Router Flag: When a Neighbor Advertisement is received with the Router (R) flag set, the IsRouter flag MUST be set to TRUE.

  3. Router Solicitation Sent: When sending a Router Solicitation, if a Neighbor Cache entry is created or already exists for the destination, the implementation MAY set IsRouter to TRUE (though typically Router Solicitations are sent to the all-routers multicast address).

D.3. Clearing the IsRouter Flag​

The IsRouter flag is set to FALSE in the following cases:

  1. Neighbor Advertisement without Router Flag: When a Neighbor Advertisement is received with the Router (R) flag clear (set to 0), the IsRouter flag MUST be set to FALSE.

  2. Router Lifetime Expiry: When the Router Lifetime for a router in the Default Router List expires, the IsRouter flag remains TRUE, but the router is removed from the Default Router List.

D.4. Using the IsRouter Flag​

The IsRouter flag affects behavior in several ways:

D.4.1. Default Router List Management​

  • Only neighbors with IsRouter = TRUE can be added to the Default Router List
  • When IsRouter changes from TRUE to FALSE, the neighbor MUST be removed from the Default Router List

D.4.2. Redirect Message Processing​

When processing a Redirect message:

  • If the target of the redirect is different from the destination, the target is being indicated as the better first-hop router
  • The IsRouter flag for the redirect target SHOULD be set to TRUE

D.4.3. Neighbor Advertisement Processing​

When a Neighbor Advertisement is received with R=0 from a neighbor with IsRouter=TRUE:

  • Set IsRouter to FALSE
  • Remove the neighbor from the Default Router List
  • This typically indicates the neighbor has ceased to be a router

D.5. Race Conditions and Special Cases​

D.5.1. Router Transitioning to Host​

When a router transitions to being a host:

  1. Router stops sending Router Advertisements
  2. Router sends Neighbor Advertisement with R=0
  3. Hosts receive NA, set IsRouter=FALSE, remove from Default Router List
  4. Router Lifetime on hosts eventually expires (if not already removed)

D.5.2. Host Transitioning to Router​

When a host becomes a router:

  1. New router begins sending Router Advertisements
  2. Hosts receive RA, set IsRouter=TRUE
  3. New router is added to Default Router Lists
  4. Router may send Neighbor Advertisement with R=1 to expedite the process

D.5.3. Simultaneous Messages​

If both a Router Advertisement and a Neighbor Advertisement (R=0) are received close together:

  • Process messages in the order received
  • The last message processed determines the final IsRouter state
  • Router Advertisements always set IsRouter=TRUE
  • Neighbor Advertisements with R=0 always set IsRouter=FALSE

D.6. Implementation Guidelines​

D.6.1. Initialization​

When creating a new Neighbor Cache entry:

  • Default value of IsRouter SHOULD be FALSE
  • Set to TRUE only when confirmed by received messages

D.6.2. Consistency Checks​

Implementations SHOULD ensure consistency:

  • If IsRouter=TRUE and no Default Router List entry exists, this is acceptable (neighbor is a router but not a default router)
  • If a Default Router List entry exists but IsRouter=FALSE, this is an error condition that MUST be corrected by removing the router from the list

D.6.3. Logging and Debugging​

Implementations SHOULD log transitions of the IsRouter flag for debugging:

  • When IsRouter changes from FALSE to TRUE
  • When IsRouter changes from TRUE to FALSE
  • When a router is removed from the Default Router List due to IsRouter becoming FALSE

D.7. Security Considerations​

The IsRouter flag can be manipulated by attackers:

Attack: Malicious Neighbor Advertisement with R=0

  • An attacker sends NA with R=0 for a legitimate router
  • Hosts remove the router from their Default Router Lists
  • Traffic is disrupted

Mitigation:

  • Use SEND (RFC 3971) to authenticate Neighbor Discovery messages
  • Implement reasonable rate limiting on IsRouter flag changes
  • Monitor for suspicious patterns (frequent IsRouter transitions)

D.8. Example State Transitions​

Example 1: Normal Router Discovery​

Initial: No Neighbor Cache entry
Action: Receive Router Advertisement from fe80::1
Result: Create entry, IsRouter=TRUE, add to Default Router List

Example 2: Router Becomes Unavailable​

Initial: IsRouter=TRUE, router in Default Router List
Action: Receive Neighbor Advertisement with R=0
Result: IsRouter=FALSE, remove from Default Router List

Example 3: Redirect to Router​

Initial: No Neighbor Cache entry for fe80::2
Action: Receive Redirect message indicating fe80::2 as better router
Result: Create entry, IsRouter=TRUE (implied by redirect target)

D.9. Summary Table​

EventAction on IsRouterImpact on Default Router List
Receive RASet TRUEAdd to list (if not present)
Receive NA with R=1Set TRUENo direct impact
Receive NA with R=0Set FALSERemove from list
Receive RedirectSet TRUE (for target)Target becomes preferred router
Router Lifetime expiresNo changeRemove from list
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