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Conference noted::decnis

Title:	DEC Network Integration Server (DECNIS)
Notice:	Please read note 1 to use this conference effectively
Moderator:	MARVIN::WELCH

Created:	Wed Sep 18 1991
Last Modified:	Thu Jun 05 1997
Last Successful Update:	Fri Jun 06 1997
Number of topics:	3660
Total number of notes:	15082

3557.0. "RIP failover/convergence time" by CSC32::J_RYER (MCI Mission Critical Support Team) Mon Feb 24 1997 20:32

    Customer is doing some testing with one DECnis and two Xyplex
    routers on a LAN, using RIP as the routing algorithm for IP.
    There are some subnets which are reachable via either of the
    Xyplex routers.  He's testing failover times (on the DECnis)
    by just disconnecting one or both Xyplex routers from the LAN.
    
    He's found that if he disconnects both Xyplex routers (so that
    there is now no path at all to the subnets he's watching), it
    takes three minutes before the DECnis shows that subnet as
    unreachable.
    
    If he disconnects just the router that the DECnis thinks is the
    path to get to the subnet in question, it takes around 110-120
    seconds before the DECnis will report that the subnet is reachable
    via the remaining Xyplex router.
    
    We think that we understand the three minutes, as the RFC 1058
    does specify a 180-second timeout for routers that are no longer
    being announced.  However, we can't figure out where the 110-120
    second failover is coming from.  I would have thought that it would
    have also taken a full three minutes for the DECnis to realize that
    the route it currently had was invalid and listen for a new one
    to replace it.
    
    Any comments about how RIP is designed to failover to a new path?
    It doesn't appear that there's much that's settable on a DECnis
    to make failover happen any faster - is that just a result of
    using RIP?
    
    Thanks,
    Jane Ryer
    MCI Mission Critical Support Team

T.R Title User Personal
Name Date Lines

3557.1 MARVIN::HART Tony Hart, InterNetworking Prod. Eng. Group Tue Feb 25 1997 08:09 12

T.R	Title	User	Personal Name	Date	Lines
3557.1		MARVIN::HART	Tony Hart, InterNetworking Prod. Eng. Group	`Tue Feb 25 1997 08:09`	12
	RIP will replace a route with an equivalent route through a different gateway if it thinks its current route is going to expire. The criteria for which is that the current age is greater than half the expiration time. As you've seen the expiration time is set to 180 seconds, so half is 90 seconds. Therefore once the route is > 90 seconds old the next update from the remaining router will cause the route to be replaced. That update will appear within the next 30 seconds after the 90 second barrier is reached, so it could be up to 120 seconds before the route is replaced. Tony

RIP will replace a route with an equivalent route through a different gateway
if it thinks its current route is going to expire.  The criteria for which is
that the current age is greater than half the expiration time.  As you've seen
the expiration time is set to 180 seconds, so half is 90 seconds. 

Therefore once the route is > 90 seconds old the next update from the remaining
router will cause the route to be replaced.  That update will appear within the
next 30 seconds after the 90 second barrier is reached, so it could be up to
120 seconds before the route is replaced.

Tony