CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

Maximum Hop Count Hop count metric increases each time a route passes through a router.
It is able to solve counting to infinity problem by defining a
maximum hop count. This is not a good solution since packets will
still loop in the network until all routers removed or flushed the bad
route from their routing tables.
Holddown When a router (RT1) did not receive the periodic routing update for a
particular subnet from another router (RT2) after the invalid timer is
expired (due to the passive interface configuration or RT2 is down),
the route will be placed into holddown state (x is possibly down).
During this period, the router will ignore or suppress any incoming
updates about alternative routes to that subnet (from routers other
than RT2) until either a route with better metric is received (from
RT2) or the holddown timer expires.
It is able to prevent counting to infinity problem that split horizon
does not solve, eg: networks with redundant links (Figure 11-4).
Triggered Updates Without waiting for the regular scheduled routing update timer to
expire, a router will immediately send out an update as soon as a
directly connected subnet has changed state (up or down), which
intends to speed up convergence time. Also known as flash updates.
- Note: Some loop-avoidance features, eg: holddown, slow down convergence. Loop-avoidance
features on distance-vector routing protocols are enabled and activated by default.
172.16.3.0 1
RT2 RT3
172.16.3.0 2 172.16.3.0 16
2
1
RT1
172.16.3.0
172.16.3.0 16
Figure 11-4: Counting to Infinity on Redundant Network
- Figure 11-4 shows another routing loop scenario – counting to infinity on redundant network
(network with redundant links).
- Imagine that RT3 send out an update to RT1 and RT2 right after the link to 172.16.3.0 has failed.
However, RT1’s update timer expires at the same time and it sends out an update contains route
to 172.16.3.0 to RT2 (RT1 does not send the update to RT3 due to split horizon). RT2 chooses
the metric 2 route through RT1 between the RT3 infinite metric route and RT1 metric 1 route.
Note: Routers normally (and should) advertise the metric to a destination network based on the
metric in the routing table. However, RIP advertises metric + 1 in the routing updates.
- On RT2’s next update, it does not advertise the infinite metric route to 172.16.3.0 to RT3 (due to
split horizon), but it does advertise the route with metric 2 to RT3. After a while, RT3 also
advertise the route with metric 3 to RT1 after its update timer expires. Counting to infinity occurs
in this scenario even with split horizon enabled. Holddown is used to prevent this problem.
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Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com
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