vector protocols in order to overcome large
routing loops and offer explicit information when a subnet or
network is not accessible. This is usually accomplished by
setting the hop count to one more than the maximum.One way to
avoid inconsistent updates is route poisoning. When Network 5
goes down, Router E initiates route poisoning by making a table
entry for Network 5 as 16, or unreachable. By this poisoning of
the route to Network 5, Router C is not susceptible to
incorrect updates about the route to Network 5. When Router C
receives a router poisoning from Router E, it sends an update,
called a poison reverse, back to Router E. This makes sure all
routes on the segment have received the poisoned route
information. When route poisoning is used with triggered
updates it will speed up convergence time because neighboring
routers do not have to wait 30 seconds before advertising the
poisoned route. Route poisoning causes a routing protocol to
advertise infinite-metric routes for a failed route. Route
poisoning does not break split horizon rules. Split horizon
with poison reverse is essentially route poisoning, but
specifically placed on links that split horizon would not
normally allow routing information to flow across. In either
case, the result is that failed routes are advertised with
infinite metrics. Web Links Route Poisoning
http://www.firewall.cx/distance_vector.php
Content
7.1 Distance Vector Routing 7.1.6
Avoiding routing loops with triggered updates New routing
tables are sent to neighboring routers on a regular basis. For
example, RIP updates occur every 30 seconds. However a
triggered update is sent immediately in response to some change
in the routing table. The router that detects a topology change
immediately sends an update message to adjacent routers that,
in turn, generate triggered updates notifying their adjacent
neighbors of the change. When a route fails, an update is sent
immediately rather than waiting on the update timer to expire.
Triggered updates, used in conjunction with route poisoning,
ensure that all routers know of failed routes before any
holddown timers can expire.Triggered updates go ahead and send
updates because routing information has changed not waiting for
the timer to expire. The router sends another routing update on
its other interfaces rather than waiting on the routing update
timer to expire. This causes the information about the status
of the route that has changed, to be forwarded and starts the
holddown timers more rapidly on the neighboring routers. The
wave of updates propagates throughout the network. Issuing a
triggered update Router C announces that network 10.4.0.0 is
unreachable. Upon receipt of this information, Router B
announces through interface S0/1 that network 10.4.0.0 is down.
In turn, Router A sends an update out interface Fa0/0. Web
Links Sending Triggered Updates
http://www.cisco.com/en/US/tech/ tk365/tk554/ technologies_
configuration_ example09186a00800948a1.shtml
http://www.cisco.com/univercd/cc/ td/doc/product/ software
/ios120/120newft/ 120t/120t1/ trigrip.htm
Content
7.1 Distance Vector Routing 7.1.7
Preventing routing loops with holddown timers A count to
infinity problem can be avoided by using holddown timers:
- When a router receives an update from a neighbor indicating
that a previously accessible network is now inaccessible, the
router marks the route as inaccessible and starts a holddown
timer. If at any time before the holddown timer expires an
update is received from the same neighbor indicating that the
network is again accessible, the router marks the network as
accessible and removes the holddown timer.
- If an
update arrives from a different neighboring router with a
better metric than originally recorded for the network, the
router marks the network as accessible and removes the holddown
timer.
- If at any time before the holddown timer
expires an update is received from a different neighboring
router with a poorer metric, the update is ignored. Ignoring an
update with a poorer metric when a holddown timer is in effect
allows more time for the knowledge of a disruptive change to
propagate through the entire network.
Web
Links Holddown http://www.cs.berkeley.edu/~kfall/
EE122/lec16/ sld035.htm
Content 7.2 RIP
7.2.1 RIP routing process The modern open
standard version of RIP, sometimes referred to as IP RIP, is
formally detailed in two separate documents. The first is known
as Request for Comments (RFC) 1058 and the other as Internet
Standard (STD) 56. RIP has evolved over the years from a
Classful Routing Protocol, RIP Version 1 (RIP v1), to a
Classless Routing Protocol, RIP Version 2 (RIP v2). RIP v2
enhancements include: - Ability to carry additional
packet routing information.
- Authentication mechanism
to secure table updates.
- Supports variable length
subnet masking (VLSM).
RIP prevents routing loops
from continuing indefinitely by implementing a limit on the
number of hops allowed in a path from the source to a
destination. The maximum number of hops in a path is 15. When a
router receives a routing update that contains a new or changed
entry, the metric value is increased by 1 to account for itself
as a hop in the path. If this causes the metric to be
incremented beyond 15, it is considered to be infinity and the
network destination is considered unreachable. RIP includes a
number of features that are common in other routing protocols.
For example, RIP implements split horizon and holddown
mechanisms to prevent incorrect routing information from being
propagated. Web Links Configuring RIP
http://www.cisco.com/en/US/products/sw/
iosswrel/ps1831/products_configuration_guide_
chapter09186a00800d97f7.html#xtocid1
Content 7.2
RIP 7.2.2 Configuring RIP The
router rip command enables RIP as the routing protocol. The
network command is then used to tell the router on
which interfaces to run RIP. The routing process then
associates specific interfaces with the network addresses and
begins sending and receiving RIP updates on these
interfaces.RIP sends routing-update messages at regular
intervals. When a router receives a routing update that
includes changes to an entry, it updates its routing table to
reflect the new route. The received metric value for the path
is increased by 1, and the source interface of the update is
indicated as the next hop in the routing table. RIP routers
maintain only the best route to a destination but can maintain
multiple equal-cost paths to the destination.
A router
running RIP can be configured to send a triggered update when
the network topology changes using the ip rip
triggered command. This command is issued only on serial
interfaces at the router(config-if)# prompt. After updating its
routing table due to a configuration change, the router
immediately begins transmitting routing updates in order to
inform other network routers of the change. These updates,
called triggered updates, are sent independently of the
regularly scheduled updates that RIP routers forward. For
example, the descriptions for the commands used to configure
router BHM shown in the figure are as follows:
- BHM(config)#router rip – Selects RIP as the routing
protocol
- BHM(config-router)#network 10.0.0.0 –
Specifies a directly connected network
- BHM(config-router)#network 192.168.13.0 –
Specifies a directly connected network
The Cisco
router interfaces that are connected to networks 10.0.0.0 and
192.168.13.0 send and receive RIP updates. These routing
updates allow the router to learn the network topology from
neighboring router also running RIP. RIP must be enabled and
the networks specified. The remaining tasks are optional. Among
these optional tasks are: - Applying offsets to routing
metrics
- Adjusting timers
- Specifying a RIP
version
- Enabling RIP authentication