RIP: Loop Avoidance Mechanisms
A routing loop is a serious network problem which happens when a data packet is continually routed through the same routers over and over.
A packet continuously routed within the network in endless circle called routing Loop.
A routing loop can create critical situation in a network, and in some cases it completely disabling the network.
Several loop avoidance mechanisms are available to prevent routing Loop:
1. Split-Horizon
2. Route-Poisoning
3. Hold-Down Timers
Split-Horizon: - The split horizon is the one of the feature of the distance vector routing protocols which prevents routing loop. if a neighbor router sends a route to a router, the receiving router will not advertise this route back to the advertising router on the same Link.
Route-Poisoning: - The route poisoning is the methods to prevent the router from sending packets via route which has become the invalid within the network.
When a router finds that one of its directly connected network has failed, it advertisement that route with an infinite metric called route poisoning. A router that receives the update knows that the network has failed doesn't use this route.
Hold-Down Timers: - Hold-down is a loop-prevention mechanism in RIP. This feature prevents routing loop when a router start learning new information about a failed route. When a router receives an information about an unreachable route, a hold-down timer is start, in a hold-down state, routers will neither advertise the route nor accept about it for a specific interval called the hold-down period.
RIP Passive Interfaces:
Passive interface command is use to disable sending updates out from a particular interface. This means that the router will able to receive updates on that passive interface but can't be send out.
Take a Look of Passive Interfaces Configuration:
Configure ip address as per this diagram: -
R4(config)# router rip
R4(config-router)# network 30.1.1.0
R4(config-router)# network 4.4.4.4
R4(config-router)# passive-interface f0/0 (for Single interface)
R4(config-router)# network 30.1.1.0
R4(config-router)# network 4.4.4.4
R4(config-router)# passive-interface f0/0 (for Single interface)
Or
R4(config-router) # passive-interface default (for all interface)
Or
R4(config-router) # no passive-interface default (Disable passive interface)
Verify passive interface configuration:
R3# show ip rip database
4.0.0.0/8 is possibly down
4.0.0.0/8 is possibly down
10.0.0.0/8 auto-summary
10.0.0.0/8
[1] via 20.1.1.1, 00:00:26, FastEthernet0/1
20.0.0.0/8 auto-summary
20.1.1.0/24 directly connected, FastEthernet0/1
30.0.0.0/8 auto-summary
30.1.1.0/24 directly connected, FastEthernet0/0
R4#sh ip protocols | begin rip
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 19 seconds
Invalid after 180 seconds, hold down 180, flushed after 240
Redistributing: rip
Default version control: send version 1, receive any version
Interface Send Recv Triggered RIP Key-chain
Loopback0 1 1 2
Automatic network summarization is in effect
Maximum path: 4
Routing for Networks:
4.0.0.0
30.0.0.0
Passive Interface(s): FastEthernet0/0
Routing Information Sources:
Gateway Distance Last Update
30.1.1.1 120 00:00:23
Distance: (default is 120)
