Routing

Hi there, 

Please help me understand this ISIS concept regarding route preference: 

I have the following topology for demo purposes: 

As illustrated, vMX3 is an L1/L2 router.

In his L1 database will have an entry for 100.10.25.0/24 via TLV 128 and TLV 130 and TLV 135 since this is an external prefix ( injected by export on vMX4). 

In his L2 database will have an entry for 10010.25.0/24 via TLV 128 and TLV130 since this is an internal prefix (passive interface on vMX2). 

[edit protocols isis]
root@vMX3# run show isis database level 2 vMX2 extensive | match 100.10.25....
   IP prefix: 100.10.25.0/24                  Metric:       10 Internal Up
    IP prefix: 100.10.25.0/24, Internal, Metric: default 10, Up
    IP extended prefix: 100.10.25.0/24 metric 10 up

[edit protocols isis]
root@vMX3# run show isis database level 1 vMX4 extensive | match 100.10.25....
   IP prefix: 100.10.25.0/24                  Metric:       10 External Up
    IP external prefix: 100.10.25.0/24, Internal, Metric: default 10, Up
    IP extended prefix: 100.10.25.0/24 metric 10 up

Checking the routing table on vMX3 I can notice that LVL1 external is preferred over LVL2 internal. 

root@vMX3# run show route 100.10.25.0/24 

inet.0: 21 destinations, 21 routes (21 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

100.10.25.0/24     *[IS-IS/160] 00:33:02, metric 20
                    >  to 10.4.34.4 via ge-0/0/1.0

Now the question is, is this how it is supposed to work?  Is Level 1 preferred over Level 2 regardless of route preference ( 160 compared to 18)? 

My initial thought was that L1 routes are preferred over L2 because L1 has a route preference of 15 and L2 has 18. If that's the tie break, shouldn't be LVL2 internal preferred over LVL1 external since the route preference of 18 is less than 160?

If I am understanding your question then Yes that's how it is supposed to work. In-short, OSPF and IS-IS are designed to prefer intra-area routes over inter-area routes. That is why there is a difference in preference. 

Hi there, 

Please help me understand this ISIS concept regarding route preference: 

I have the following topology for demo purposes: 

As illustrated, vMX3 is an L1/L2 router.

In his L1 database will have an entry for 100.10.25.0/24 via TLV 128 and TLV 130 and TLV 135 since this is an external prefix ( injected by export on vMX4). 

In his L2 database will have an entry for 10010.25.0/24 via TLV 128 and TLV130 since this is an internal prefix (passive interface on vMX2). 

[edit protocols isis]
root@vMX3# run show isis database level 2 vMX2 extensive | match 100.10.25....
   IP prefix: 100.10.25.0/24                  Metric:       10 Internal Up
    IP prefix: 100.10.25.0/24, Internal, Metric: default 10, Up
    IP extended prefix: 100.10.25.0/24 metric 10 up

[edit protocols isis]
root@vMX3# run show isis database level 1 vMX4 extensive | match 100.10.25....
   IP prefix: 100.10.25.0/24                  Metric:       10 External Up
    IP external prefix: 100.10.25.0/24, Internal, Metric: default 10, Up
    IP extended prefix: 100.10.25.0/24 metric 10 up

Checking the routing table on vMX3 I can notice that LVL1 external is preferred over LVL2 internal. 

root@vMX3# run show route 100.10.25.0/24 

inet.0: 21 destinations, 21 routes (21 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

100.10.25.0/24     *[IS-IS/160] 00:33:02, metric 20
                    >  to 10.4.34.4 via ge-0/0/1.0

Now the question is, is this how it is supposed to work?  Is Level 1 preferred over Level 2 regardless of route preference ( 160 compared to 18)? 

My initial thought was that L1 routes are preferred over L2 because L1 has a route preference of 15 and L2 has 18. If that's the tie break, shouldn't be LVL2 internal preferred over LVL1 external since the route preference of 18 is less than 160?