RIP Split-Horizon , RIP Timers

Lessons Learned:

RIP Auto-summary

RIPV1 – is default when configuring RIP

RIPv2 is classless but does automatic classful summarization by default.

-Disabled with no auto-summary

VLSM is supported within the same major network.

Advertisements between major network boundaries are summarized to classful boundary

-Can result in traffic black holes.

Split Horizon –

If enabled will stop routes from being sent back out the interfaces they we’re learned on.

RIP is a distance vector protocol.

Updates received in an interface will not be sent back out the same interface.

Basically if you advertise a route to me, I don’t want to send the same route back to you. This could result in a loop in the data-plane.

-Undesirable on partial mesh NBMA networks.

Enabled by default on all interfaces except main interface in Frame Relay. It will be enables on all other interfaces.

Ex: in multipoint it would be enabled on the sub interfaces.        

-no ip split-horizon to disable,

Verifcation

#sh ip interface

This is show if Split horizon is enabled on the interface.

Note: remember once a RIP route has been down for over 240 seconds they will be removed from the routing table.

We can also use the debug IP RIP to see what routes are being advertised. And what is being received.

R1#sh ip interface fa0/0

FastEthernet0/0 is up, line protocol is up

  Internet address is 192.168.12.1/24

  Broadcast address is 255.255.255.255

  Address determined by non-volatile memory

  MTU is 1500 bytes

  Helper address is not set

  Directed broadcast forwarding is disabled

  Multicast reserved groups joined: 224.0.0.9

  Outgoing access list is not set

  Inbound  access list is not set

  Proxy ARP is enabled

  Local Proxy ARP is disabled

  Security level is default

  Split horizon is enabled – Split Horizon is enabled.

  ICMP redirects are always sent

  ICMP unreachables are always sent

  ICMP mask replies are never sent

  IP fast switching is enabled

  IP fast switching on the same interface is disabled

  IP Flow switching is disabled

  IP CEF switching is enabled

  IP CEF Fast switching turbo vector

  IP multicast fast switching is enabled

Debug IP RIP :

*Mar  1 00:04:42.303: RIP: sending v2 update to 224.0.0.9 via FastEthernet0/0 (192.168.12.1)

*Mar  1 00:04:42.307: RIP: build update entries

*Mar  1 00:04:42.307:   10.1.1.0/24 via 0.0.0.0, metric 1, tag 0

*Mar  1 00:04:43.075: RIP: sending v2 update to 224.0.0.9 via Loopback1 (10.1.1.1)

*Mar  1 00:04:43.079: RIP: build update entries

*Mar  1 00:04:43.079:   192.168.12.0/24 via 0.0.0.0, metric 1, tag 0

*Mar  1 00:04:43.091: RIP: ignored v2 packet from 10.1.1.1 (sourced from one of our addresses) – shows split horizon at work – the update is not process and the route is ignored

 

Debug IP routing – will show any and all maintenance on the routing itself.

RIP Convergence Timers:

Global – will affect all interfaces

-Timers basic (update) (invalid) (Holddown) (flush)

 

Interface level – will affect just the interface.

-#ip rip advertisement (interval)

 

Note: All the timers start counting at the same time index. The default is 30 seconds.

Once a router receives an update in , the rest of it’s timers which are the invalid, hold down, and flush timers. These all start counting up.

 

Basically from the last time the router received an update, if the time gets to 180 seconds, the route becomes “invalid” and it goes into “hold down”.  Most of the time the invalid and hold down have the same value. Technically  they have two different purposes.

The hold down says – when I receive a new update in regarding a prefix, only accept it if it has the same or better metric. To verify the routing information is not looping around the network while its aging out.

 

The Invalid says – the route is possibly down in the routing table.

Flush timer is 240 sec by default, if it does past that time –remove the route.

 

Verify the timers:

 

# sh ip protocols

R3#sh ip protocols

Routing Protocol is "rip"

  Outgoing update filter list for all interfaces is not set

  Incoming update filter list for all interfaces is not set

  Sending updates every 30 seconds, next due in 16 seconds – send updates

  Invalid after 180 seconds, hold down 180, flushed after 240 – invalid, hold, and flush

  Redistributing: rip

  Default version control: send version 2, receive version 2

    Interface             Send  Recv  Triggered RIP  Key-chain

    FastEthernet0/0       2     2                                   

    Loopback1             2     2                                   

    Loopback33            2     2                                   

  Automatic network summarization is not in effect

  Maximum path: 4

  Routing for Networks:

    1.0.0.0

    10.0.0.0

    192.168.23.0

  Routing Information Sources:

    Gateway         Distance      Last Update

    192.168.23.2         120      00:00:22

  Distance: (default is 120)

The time timer values apply to either RIP version  1 or 2.

To lower the values.

R3(config-router)#timers basic 5 20 20 30

This says – change the updates to 5 seconds / invalid to 20 seconds / Hold to 20 seconds and flush to 30 seconds.

Note: if you set the hold down to 0, the route will never go into hold down.

Verification:

R3#sh ip protocols

Routing Protocol is "rip"

  Outgoing update filter list for all interfaces is not set

  Incoming update filter list for all interfaces is not set

  Sending updates every 5 seconds, next due in 3 seconds

  Invalid after 20 seconds, hold down 20, flushed after 30

Note: the timers in RIP are not 100% accurate, the RIP process uses something known as RIP Jitter. Which is basically a randomization of the timers to make sure the router is not sending or receiving updates out the interface at the exact same time index. This is why the timers are large by default.                                        

There is a limit to the packet format to RIP. So some updated do not make it in each update.