RIPv2

Why it exists

Before link-state IGPs dominated, networks exchanged distance vectors — “I can reach this prefix at this cost.” RIP is the simplest surviving form of that idea, and it still teaches the failure modes every distance-vector protocol must contain.

RIPv2 (RFC 2453) keeps hop count as the metric, raises the ceiling to a usable fifteen hops, and adds subnet masks, next hop, tags, authentication, and multicast advertisement. The infinity problem does not go away; the mitigations become visible. RFC 2453 §1JustificationRFC 2453 · INTERNET STANDARD · November 1998

A Response carries mask and metric

Two routers exchange RIPv2 Responses on UDP/520. Each route entry names a prefix, a subnet mask, an optional next hop, and a hop-count metric — not the classful advertisement RIPv1 sent.

How it works

A Response message on UDP/520 carries one or more twenty-octet route entries. Version 2 entries include subnet mask and next hop. Metric 1–15 is usable; 16 is infinity. RFC 2453 §3.6Message FormatRFC 2453 · INTERNET STANDARD · November 1998

Split horizon does not advertise a route back out the interface it was learned on. Poison reverse advertises it as 16 on that interface. Triggered updates announce changes without waiting for the periodic timer. RFC 2453 §3.4.3Split horizonRFC 2453 · INTERNET STANDARD · November 1998

On multi-access networks RIPv2 multicasts Responses to 224.0.0.9. That is an operational difference from RIPv1’s broadcast, and a reason version mismatches look like “one side never sees updates.” RFC 2453 §4.5MulticastingRFC 2453 · INTERNET STANDARD · November 1998

On the wire

Constructed examples, encoded from the field table below them — not captured traffic.

Configure it

When it breaks

Symptom first, because that is what you have when it happens.

1. A prefix’s metric climbs and then sticks at 16 after a link failure.

   Narrow it down

   1. Check whether split horizon or poison reverse is active on the interface that faces the neighbour.
   2. Compare the two routers’ RIP databases for the prefix during the climb.
   3. Look for a missing triggered update after the failure.

   Cause

   Without split horizon, neighbours re-advertise a lost prefix to each other and increment the metric until infinity.

   Fix

   Ensure split horizon / poison reverse on the shared interface; after the event, confirm the prefix is gone rather than held at 16 as a stale black hole.

   RFC 2453 §3.4.2Preventing instabilityRFC 2453 · INTERNET STANDARD · November 1998

2. One router shows RIP neighbours or routes; the other never installs the prefix.

   Narrow it down

   1. Confirm both ends send and accept version 2.
   2. Check whether one side still expects broadcast RIPv1 while the other multicasts to 224.0.0.9.
   3. Verify UDP/520 is not filtered.

   Cause

   RIPv1 and RIPv2 are not interchangeable on the wire. Mask and multicast behaviour differ.

   Fix

   Force version 2 on both ends and confirm multicast reachability to 224.0.0.9.

   RFC 2453 §4.5MulticastingRFC 2453 · INTERNET STANDARD · November 1998

3. A subnet is advertised as a classful network and traffic blackholes for more-specific destinations.

   Narrow it down

   1. Inspect the RTE mask in the Response — or whether auto-summary collapsed it.
   2. Compare the installed prefix length on each hop.

   Cause

   RIPv1 behaviour or classful auto-summary discarded the subnet mask RIPv2 was meant to carry.

   Fix

   Run version 2 end to end and disable auto-summary where the platform still offers it.

   RFC 2453 §4.3Subnet maskRFC 2453 · INTERNET STANDARD · November 1998

4. After a failure, the alternate path appears only after a long delay.

   Narrow it down

   1. Check whether triggered updates are suppressed or filtered.
   2. Compare invalid and flush timers against the expected failover.

   Cause

   Distance-vector recovery waits on timers when triggered updates do not propagate the loss.

   Fix

   Confirm triggered updates; accept that RIP will not match link-state failover times.

   RFC 2453 §3.4.4Triggered updatesRFC 2453 · INTERNET STANDARD · November 1998

5. Updates ignored on a passive interface.

   Narrow it down

   1. List passive interfaces
   2. Confirm the related configuration and counters.

   Cause

   Interface configured passive so it will not send updates.

   Fix

   Remove passive where adjacency updates are required.

Design notes

A diameter greater than fifteen hops cannot be expressed. Metric 16 is unreachable, not “a long way.” Networks that outgrow fifteen hops need a different IGP, not a creative metric scheme. RFC 2453 §3.2Limitations of the ProtocolRFC 2453 · INTERNET STANDARD · November 1998

RIPv2 is still a teaching and edge protocol. Redistributing it into OSPF or EIGRP without filtering is how a lab accident becomes a production routing table.

Misconceptions

“Metric 16 means a distant but usable route.”

Sixteen is infinity. The prefix is unreachable. Usable metrics stop at 15. RFC 2453 §3.4.2Preventing instabilityRFC 2453 · INTERNET STANDARD · November 1998

“Split horizon is an optional tuning knob with no correctness role.”

It is the primary defence against two-router count-to-infinity. Turning it off to “see more updates” recreates the textbook loop. RFC 2453 §3.4.3Split horizonRFC 2453 · INTERNET STANDARD · November 1998

“RIPv2 is just RIPv1 with a new version number.”

Version 2 adds subnet masks, next hop, tags, authentication, and multicast. Running “RIP” without pinning version 2 often leaves you in classful behaviour. RFC 2453 §4Protocol ExtensionsRFC 2453 · INTERNET STANDARD · November 1998

More walkthroughs

Terms

Split horizon

A distance-vector rule: do not advertise a route out the interface it was learned on. Poison reverse advertises it as unreachable instead.

Check yourself

• In RFC 2453, what does metric 16 mean?

  • Fifteen hops plus one optional backup
  • Infinity — the prefix is unreachable
  • The administrative distance of RIP
  • A poisoned route that is still forwarded
• What does simple split horizon do?

  • Encrypts the Response
  • Omits a route from advertisements out the interface it was learned on
  • Raises every metric by two
  • Converts RIP to OSPF
• How does poison reverse differ from simple split horizon?

  • It disables triggered updates
  • It advertises the route back with metric 16 instead of omitting it
  • It uses broadcast instead of multicast
  • It removes the subnet mask
• Which RIPv2 RTE field lets classless prefixes be advertised correctly?

  • The command byte alone
  • The subnet mask
  • The must-be-zero field
  • The UDP checksum
• On multi-access media, where do RIPv2 Responses go?

  • 255.255.255.255 only
  • 224.0.0.9 (and UDP port 520)
  • 224.0.0.5
  • The router’s unicast RIP address only
• Without split horizon, what happens after a prefix’s true path fails?

  • Both routers delete it immediately
  • Neighbours can re-advertise it to each other while the metric climbs to 16
  • OSPF takes over automatically
  • The metric freezes at 1
• What are triggered updates for?

  • Replacing split horizon
  • Announcing route changes without waiting for the periodic timer
  • Authenticating every packet
  • Electing a DR
• What is the largest usable hop-count metric in RIP?

  • 8
  • 15
  • 16
  • 255

Related

• EIGRP

  A distance-vector protocol that keeps a proven loop-free backup ready, so most failures are a switch rather than a recomputation — and asks the network only when it has none.

• OSPFv2

  Each router floods a description of its links, then independently computes the same shortest-path tree — so the forwarding tables agree without exchanging routes.