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Network
Information Library
Limitations
of Triggered RIP and SAP
Triggered
RIP and SAP dynamically updates routing information without
using excessive bandwidth. To save on bandwidth costs, Triggered
RIP and SAP sends update messages sends only when the router
detects a change in its routing or service database.
However,
there are some limitations on the use of Triggered RIP and
SAP. You can use the equation given to below to calculate
if the use of Triggered RIP or SAP is appropriate for your
network. There are several factors to consider and each
factor is given a value.
Triggered
RIP and SAP operation is limited by:
-
Value
H -- The 180-second period after which a route or service
is deemed unreachable if it is not possible to connect
to the next hop router to the destination. This is configured
as the route/hold-down timer.
-
The
number of calls that can be processed in that period.
The number of calls that can actually be processed depends
on a number of factors:
-
Value C -- The time it takes to set up a call and close
a call, about five seconds, but it could be longer for
international calls.
-
Value
R -- The time it takes to transfer a RIP/SAP update, about
five seconds maximum.
-
Value
I -- The time the link will remain open when transferring
data. This is the greater of the following two values:
-
Value M -- The minimum call timer
-
Value
() -- The idle timer
If
there is no user traffic, each B-channel or virtual circuit
can theoretically run Triggered RIP and SAP to the following
number of sites per B-channel or virtual circuit.
MAX_SITES
= 1 + (H/(C + (maximum of M, R + I)))
In a real world environment, other traffic may be transmitted
at the same time as RIP and SAP information is being exchanged.
Take the following factors into account:
-
Duration of user data
-
The
preemption timer to each destination (and differing circuit
priority)
-
The
triggered update management timer. To begin with, this
timer executes every 30 seconds and preempts a circuit
whose preemption timer has expired, if preemption is required
The
above factors require an arbitrary reduction in the number
of remote routers that RIP or SAP can contact within a 180-second
period. To take this into account, Intel Network Systems
recommends running Triggered RIP or SAP on no more than
the total number of sites given using the following equation:
TOTAL_SITES = 1
+ 2/3 * (H/(C + (maximum of M, R + I)))
With
ISDN, if you increase the number of B-channels or virtual
circuits in use, you can increase the number of sites to
which you can run Triggered RIP and SAP. This means that
if you double the number of B-channels, you can double the
number of remote sites. This also applies if you increase
the number of virtual circuits on other media types.
If
you attempt to run Triggered RIP and SAP to more sites than
recommended in the preceding equation, your routes may be
timed out and you may accrue large WAN costs for many continual
retransmissions.
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