Routing
Routing

IS-IS Database Questions

‎03-11-2019 03:12 PM

Is there i way i can check the IS-IS database to ensure that there is an alternative path to a specific prefix?

e.g. i want to know that if the primay path goes down, that there is an alternative path already known in the DB

 

 

8 REPLIES 8
Routing

Re: IS-IS Database Questions

‎03-11-2019 05:19 PM

The extensive command will include the prefixes in the database.  If it is large you can pipe it to match with a prefix to just pull all those entries out for review.

 

show isis database extensive | match 1.1.1.0

 

 

Steve Puluka BSEET - Juniper Ambassador
IP Architect - DQE Communications Pittsburgh, PA (Metro Ethernet & ISP)
http://puluka.com/home
Routing

Re: IS-IS Database Questions

‎03-11-2019 06:34 PM

Not really because ISIS doesn't pre-calculate an alternative path. It runs the SPF algorithm and finds the shortest path (lowest total cost) to each End System (router) and then the prefixes advertised by each of them.  The database contains no routes, it describes how things are connected in ISIS (the neighbors, the prefixes, the metrics, system ids, hostnames, and so on). You might be thinking about EIGRP where there are precalculated feasible successors in the topology table.  ISIS does not work that way.  Nor does OSPF. 

What you can do is look at the contents of the database (show isis database extensive) and basically kind of manually verify that you have all the LSPs from all the routers, and that they are advertising the ISIS neighbors and prefixes. Not an easy task for a large network.  But all the show isis commands (show is route, show isis spf results, and so on) will show you only the best path. 

 

Now, if you have equal cost path Junos places the two next hops in the routing table eventhough load balancing does not happen by default. If the selected next hop fails, the other one gets installed in the forwarding table. 

 

VR3.inet.0: 10 destinations, 10 routes (10 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

192.168.10.1/32   *[IS-IS/18] 00:00:15, metric 20
                                 > to 10.10.50.1 via lt-0/0/0.3  <= only this next-hop is used. 
                                    to 10.10.60.1 via lt-0/0/0.4

 

I hope this helps, though probably not the response you were hoping for. 

Yasmin Lara - Juniper Ambassador #QuadE - JNCIE-SP, JNCIE-ENT, JNCIE-DC, JNCIE-SEC
JNCIS-CLOUD, JNCDS-DC, JNCIA-DevOps
Routing

Re: IS-IS Database Questions

‎03-11-2019 06:54 PM

By default Junos will not show you if alternative path available, You should inspect isis db. 

But you can enable LFA, and verify if alternative path available with command "show isis backup coverage"

https://www.juniper.net/documentation/en_US/junos/topics/concept/isis-node-link-protection-understan...

Routing

Re: IS-IS Database Questions

‎03-11-2019 11:43 PM

The below link has some pointers on “Understanding Backup Selection Policy for IS-IS Protocol”,

https://www.juniper.net/documentation/en_US/junos/topics/concept/backup-selection-overview.html

 

 

 

-Python JNCIE 3X [SP|DC|ENT] JNCIP-SEC JNCDS 3X [ WAN | DC|SEC] JNCIS-Cloud JNCIS-DevOps CCIP ITIL
#Please mark my solution as accepted if it helped, Kudos are appreciated as well.
Routing

Re: IS-IS Database Questions

‎03-14-2019 05:31 AM

Just for clarification: the IS-IS database does not contain any paths at all. It is just a source of available information to the protocol which are used by the SPF algorithms as input.

 

The actual result of the SPF calculation can be seen via the "show isis spf results" command. By default SPF only calculates equal cost paths to each destination (no backup paths). If you want to have backup paths, you need LFA (which requires MPLS dataplane and does not cover all topologies unless you use remote LFA).

 

Cheers,

Carsten

 

 

Routing

Re: IS-IS Database Questions

‎03-14-2019 06:02 AM

This is great! I was scratching my head going "humm, I thought LFA was for MPLS only". THANKS!!  

Yasmin Lara - Juniper Ambassador #QuadE - JNCIE-SP, JNCIE-ENT, JNCIE-DC, JNCIE-SEC
JNCIS-CLOUD, JNCDS-DC, JNCIA-DevOps
Routing

Re: IS-IS Database Questions

‎03-14-2019 06:09 AM

In theory you can do without MPLS once Segment Routing v6 is available in JUNOS. :-)

Routing

Re: IS-IS Database Questions

‎03-14-2019 06:14 AM

OH! IPv6 to the rescue?  :-) 
I have to read about that! 

Yasmin Lara - Juniper Ambassador #QuadE - JNCIE-SP, JNCIE-ENT, JNCIE-DC, JNCIE-SEC
JNCIS-CLOUD, JNCDS-DC, JNCIA-DevOps