An important consideration in selection of LSP protection technique
is not only the amount of time taken to establish and switch over to the
backup, but also how long it takes to detect the initial failure. If the
detection is based on routing protocol hello's then it obviously takes
longer than physical layer failure detection of a directly connected
point to point interface. In the scenarios being discussed, the fast re-route
technique uses failure of a directly connected interface to trigger shift
to the backup path.
dave
At 05:34 PM 7/1/2002 +0100, Dave Humphrey wrote:
>There are various options which increase speed of recovery but require more
>state to be retained.
>
>Secondary LSP's can be configured to back-up a Primary, the secondary LSP's
>can be pre-signalled or not. Pre-signalling cuts down on the time it will
>take traffic to switch to the secondary because it will already be up. the
>downside is that the routers on the secondary path will have to maintain
>"state" for an LSP which is not being used. State ikn the case is the RSVP
>traffic (mainly refreshes) to maintain the LSP.
>
>Fast reroute in Juniper terms should be the quickest way of recovering from
>LSP failure. Each node in the path calculates a detour around it's
>downstream neighbor which will kick in if that neighbot dissappears. When
>the failure is detected the node doing the rerouting will signal back to the
>ingress LSP that fast reroute has been invoked and the ingress LSP can then
>make its own decision about re-routing the LSP.
>
>If you configure a primary LSP reversion will always take place at least
>once. To avoid this configure two secondarys rather than a primary and a
>secondary. With this set-up there is no primary LSP to revert to. Just make
>sure that the path you want to use first is the secondary you configure
>first.
>
>To summarise:
>
>Primary no secondary = slowest. (Primary will recover via IGP if possible)
>Primary and non standby (non pre signalled) secondary = Next slowest.
>Primary and standby secondary )pre signalled) = A bit faster than above.
>Primary and standby secondary with fast reroute = fastest.
>
>If reversion is not wanted then configure no primary and two secondaries.
>
>Dave
>
>-----Original Message-----
>From: Strahler, Carsten [mailto:Carsten.Strahler@lambdanet.net]
>Sent: 01 July 2002 17:14
>To: juniper-nsp@puck.nether.net
>Subject: MPLS redundancy - fast reroute or secondary path
>
>
>Hi,
>
>to minimize the paket loss when a LSP fails Juniper provides two mechansim -
>fast reroute and secondary path.
>What are the pro and cons of both ? Are they revertive ?
>
>Thanks
>
>
>------------------------------------------------------------
>Carsten Strahler
>
>IP Planning
>
>Lambdanet Communications GmbH (AS 13237)
>web: www.lambdanet.net
>-----------------------------------------------------------
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