Routing

Hi All !

 

I have a question regarding IGP (OSPF or ISIS) link-protection and ECMP.

 

Topology is triangle where R1 is connected to R2 and R3, and R2 to R3.

 

On R1 when node-link-protection is not configure, I will have only one route to reach the R2's loopback :

 

schalal@VR-DEVICE.re0:R1# run show route table inet.0 10.200.1.2/32                   

inet.0: 365 destinations, 691 routes (365 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

10.200.1.2/32      *[IS-IS/18] 00:00:17, metric 10
                    > to 10.200.112.2 via ge-0/0/1.112

 

but when I configure node-link-protection, I have two routes to reach R2's loopback :

 

schalal@VR-DEVICE.re0:R1# run show route table inet.0 10.200.1.2/32              

inet.0: 365 destinations, 691 routes (365 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

10.200.1.2/32      *[IS-IS/18] 00:19:55, metric 10
                    > to 10.200.112.2 via ge-0/0/1.112
                      to 10.200.113.3 via ge-0/0/1.113

 

So primary path is R1=>R2, backup path is R1=>R3=>R2

 

 

Until there no problem. But what about ECMP ? It seems I can enable ECMP between the primary and backup path. But why the backup path has the same metric as the primary ? the path is longer which make the cost higher !

 

Let's try ECMP, but here is the output before ECMP :

 

schalal@VR-DEVICE.re0:R1# run show route forwarding-table destination 10.200.1.2/32    
Logical system: R1
Routing table: default.inet
Internet:
Destination        Type RtRef Next hop           Type Index NhRef Netif
10.200.1.2/32      user     1 10.200.112.2       ucst 14084    23 ge-0/0/1.112

 

[edit]
schalal@VR-DEVICE.re0:R1# show routing-options forwarding-table 
export [ LB ];

schalal@VR-DEVICE.re0:R1# run show route table inet.0 10.200.1.2/32                

inet.0: 365 destinations, 691 routes (365 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

10.200.1.2/32      *[IS-IS/18] 00:00:07, metric 10
                    > to 10.200.112.2 via ge-0/0/1.112
                      to 10.200.113.3 via ge-0/0/1.113


schalal@VR-DEVICE.re0:R1# run show route forwarding-table destination 10.200.1.2/32    
Logical system: R1
Routing table: default.inet
Internet:
Destination        Type RtRef Next hop           Type Index NhRef Netif
10.200.1.2/32      user     1                    ulst 1048614     8
                              10.200.112.2       ucst 14084    11 ge-0/0/1.112
                              10.200.113.3       ucst 14057    15 ge-0/0/1.113

 

As you can see ECMP is active for the route to R2's loopback although the theoretical metric for  the primary and backup path are different.

 

My question is : Do we have ECMP (when policy is configured on the forwarding-table) active when we enable IGP node-link-protection even if the theoretical cost of the primary and backup path are different ? Why ???

 

Thanks for you help

 

 

 

 

Use the extensive keyword when showing the forwarding-table. You will see a Weight. Lowest Weight always wins, so it's not really ECMP.

Thanks for your reply.

 

 

When I use extensive output I found the same weight :

 

schalal@VR-DEVICE.re0:R1> show route forwarding-table destination 10.200.1.2/32 extensive 
Logical system: R1
Routing table: default.inet [Index 25] 
Internet:
    
Destination:  10.200.1.2/32
  Route type: user                  
  Route reference: 1                   Route interface-index: 0   
  Flags: sent to PFE, rt nh decoupled  
  Next-hop type: unilist               Index: 1048614  Reference: 8    
  Nexthop: 10.200.112.2
  Next-hop type: unicast               Index: 14084    Reference: 23   
  Next-hop interface: ge-0/0/1.112  Weight: 0x1  
  Nexthop: 10.200.113.3
  Next-hop type: unicast               Index: 14057    Reference: 8    
  Next-hop interface: ge-0/0/1.113  Weight: 0xf000
    
Destination:  10.200.1.2/32
  Route type: cloned                
  Route reference: 1                   Route interface-index: 0   
  Flags: none 
  Next-hop type: unilist               Index: 1048614  Reference: 8    
  Nexthop: 10.200.112.2
  Next-hop type: unicast               Index: 14084    Reference: 23   
  Next-hop interface: ge-0/0/1.112  Weight: 0x1  
  Nexthop: 10.200.113.3
  Next-hop type: unicast               Index: 14057    Reference: 8    
  Next-hop interface: ge-0/0/1.113  Weight: 0xf000

 

 

 

 

Two different weights:

Next-hop interface: ge-0/0/1.112 Weight: 0x1
[...]
Next-hop interface: ge-0/0/1.113 Weight: 0xf000

Weight 0x1 wins. You are seeing two routes (user and cloned), that's a forwarding table thing, hence the reason for seeing 0x1 twice. But within the same route, weights are different.

Oups, Sorry I was looking the wrong line : yes two different weight.

 

So here, no ECMP. Correct ?

Correct

Thanks for you help !