Which routing loop avoidance / prevention mechanism should be disabled on a multi-point interface?
- spanning tree
- reverse path forwarding
- route poisoning
- split horizon
EXPLANATION
A routing loop is a serious network problem which
happens when a data packet is continually routed through the same
routers over and over. The data packets continue to be routed within
the network in an endless circle. A routing loop can have a
catastrophic impact on a network, and in some cases, completely
disabling the network. Normally Routing Loop is a problem associated
with Distance Vector Protocols.
How routing loops affect network performance?
• A major portion of the precious bandwidth which is
available for normal user traffic of the affected routers will be
consumed by looping IP datagram packets.
• The major portion of the processing power of the affected routers is used to process the looping IP datagram packets.
Routing Loop can happen in large
internetworks when a second topology change emerges before the network
is able to converge on the first change. Convergence is the term used to describe the condition when all routers in an internetwork have agreed on a common topology. Link state protocols tend to converge very quickly, while distance vector protocols tend to converge slowly.
The following methods are used to avoid Routing Loops.
Maximum hop Count
Maximum hop count mechanism can be used to prevent Routing Loops. Distance Vector protocols use the TTL (Time-to-Live) value in the IP datagram header
to avoid Routing Loops. When an IP datagram move from router to router,
a router keeps track of the hops in the TTL field in the IP datagram
header. For each hop a packet goes through, the packet’s TTL field is
decremented by one. If this value reaches 0, the packet is dropped by
the router that decremented the value from 1 to 0.
Split Horizon
A split horizon is a routing configuration that
stops a route from being advertised back in the direction from which it
came.
Split Horizon mechanism states that if a neighbouring router sends a
route to a router, the receiving router will not propagate this route
back to the advertising router on the same interface.
Route Poisoning
Route Poisoning is another method for avoiding
routing loops. When a router detects that one of its connected routes
has failed, the router will poison the route by assigning an infinite metric to it.
Hold-down Timers
Hold-down timer is another
mechanism used to prevent bad routes from being restored and propagated
by mistake. When a route is placed in a hold-down state, routers will
neither advertise the route nor accept advertisements about it for a
specific interval called the hold-down period.
|
