This article discusses one of the scenarios where RSVP LSP fail to re-optimize to IGP shortest path when there is a network change and the following message is seen:
CSPF: computation result ignored, new path less avail bw
Topology:
The above topology has two LSPs created. One LSP To-R5 is from R1 to R5 using strict path as R2->R3->R4->R5 and reserved bandwidth of 900mbps. Another LSP To-R4 is from R2 to R4, which does not contain any path explicitly configured. It has two paths to R4 (i.e one directly connected to R4 and another via R3) . The To-R4 LSP chooses the directly connected path to R4 since IGP chooses the best path as the cost is 1.
lab@R2# run show route 4.4.4.4
inet.0: 14 destinations, 15 routes (14 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
4.4.4.4/32 *[OSPF/10] 00:00:10, metric 1
> to 10.1.24.4 via ge-0/0/1.0
If for some reason, a change is needed in OSPF metric so that the LSP from R2 to R4 re-routed via R3->R4. If we change the OSPF metric on directly connected interface to R4 ge-0/0/1 to 5, our OSPF will choose ge-0/0/2 as the best path whereas RSVP would still choose the old path and the error message is seen from
'show mpls lsp extensive'
output:
lab@R2#set protocols ospf area 0 interface ge-0/0/1.0 metric 5
lab@R2# run show route 4.4.4.4
inet.0: 14 destinations, 15 routes (14 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
4.4.4.4/32 *[OSPF/10] 00:00:06, metric 2
> to 10.1.23.3 via ge-0/0/2.0 <-- IGP chose the lesser metric path
inet.3: 1 destinations, 1 routes (1 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
4.4.4.4/32 *[RSVP/7/1] 00:00:01, metric 2
> to 10.1.24.4 via ge-0/0/1.0, label-switched-path to-R4 <-- RSVP did not choose the same path as IGP.
lab@R2# run show mpls lsp extensive
Ingress LSP: 1 sessions
4.4.4.4
From: 2.2.2.2, State: Up, ActiveRoute: 0, LSPname: to-R4
ActivePath: (primary)
LSPtype: Static Configured, Penultimate hop popping
LoadBalance: Random
Encoding type: Packet, Switching type: Packet, GPID: IPv4
*Primary State: Up
Priorities: 7 0
OptimizeTimer: 2
SmartOptimizeTimer: 180
Reoptimization in 0 second(s).
Computed ERO (S [L] denotes strict [loose] hops): (CSPF metric: 5)
10.1.24.4 S
Received RRO (ProtectionFlag 1=Available 2=InUse 4=B/W 8=Node 10=SoftPreempt 20=Node-ID):
10.1.24.4
8 Jan 26 21:34:43.683 CSPF: computation result ignored, new path less avail bw
7 Jan 26 21:33:50.507 CSPF: computation result ignored, new path no benefit[3 times]
Since the existing LSP from R1 to R5 has a reserved BW of 900mbps, which has a strict path R2->R3->R4->R5. While RSVP tries to re-optimize to choose the path as IGP, it identifies the available BW on the newer path is lesser than the old path
lab@R2# run show rsvp interface
RSVP interface: 3 active
Active Subscr- Static Available Reserved Highwater
Interface State resv iption BW BW BW mark
ge-0/0/0.0 Up 1 100% 1000Mbps 1000Mbps 0bps 0bps
ge-0/0/1.0 Up 1 100% 1000Mbps 1000Mbps 0bps 0bps
ge-0/0/2.0 Up 1 100% 1000Mbps 100Mbps 900Mbps 900Mbps <-- Available BW is 100Mbps
This is the expected behavior as per the topology and the configuration.
By default, traffic engineering support is disabled in OSPF. To enable, below configuration need to be done to populate the traffic engineering database. The traffic engineering database is used exclusively for calculating explicit paths for the placement of LSPs across the physical topology. The Constrained Shortest Path First (CSPF) algorithm uses the traffic engineering database to compute the paths that MPLS LSPs take. RSVP uses this path information to set up LSPs and to reserve bandwidth for them.
#set protocols ospf traffic-engineering
To learn more, refer to the technical documentation on Examples: Configuring OSPF Traffic Engineering
For more on the criteria for re-optimization results to be accepted, refer to
Optimizing Signaled LSPs
After clearing the LSP, it will take the same path as IGP best path to form the LSP.
lab@R2# run clear mpls lsp
lab@R2# run show mpls lsp extensive
Ingress LSP: 1 sessions
4.4.4.4
From: 2.2.2.2, State: Up, ActiveRoute: 0, LSPname: to-R4
ActivePath: (primary)
LSPtype: Static Configured, Penultimate hop popping
LoadBalance: Random
Encoding type: Packet, Switching type: Packet, GPID: IPv4
*Primary State: Up
Priorities: 7 0
OptimizeTimer: 2
SmartOptimizeTimer: 180
Reoptimization in 0 second(s).
Computed ERO (S [L] denotes strict [loose] hops): (CSPF metric: 2)
10.1.23.3 S 10.1.34.4 S
Received RRO (ProtectionFlag 1=Available 2=InUse 4=B/W 8=Node 10=SoftPreempt 20=Node-ID):
10.1.23.3 10.1.34.4
16 Jan 26 21:37:17.640 CSPF: computation result ignored, new path no benefit
15 Jan 26 21:37:15.737 Selected as active path
14 Jan 26 21:37:15.724 Record Route: 10.1.23.3 10.1.34.4
13 Jan 26 21:37:15.724 Up
12 Jan 26 21:37:15.660 Originate Call
11 Jan 26 21:37:15.660 CSPF: computation result accepted 10.1.23.3 10.1.34.4
10 Jan 26 21:37:15.660 Clear Call
9 Jan 26 21:37:15.659 Deselected as active
8 Jan 26 21:34:43.683 CSPF: computation result ignored, new path less avail bw
7 Jan 26 21:33:50.507 CSPF: computation result ignored, new path no benefit[3 times]