ISIS Routing Control

[Daniel Smith]

We have a meshed backbone using ISIS as the IGP with BGP VPNV4 and Route Reflectors on top of that. Given the mesh nature of the network and using ISIS, we see points in the network that have an equal cost path to the next hop of a route. I know there is the capability to set path metrics on ISIS links, but this has an impact on all connectivity. I am wondering if it is possible to use inbound route-maps, identifying loopback prefixes of different destinations in the network, and setting a metric value via that means. Low metric would indicate that is the preferred path to a destination, while high would make that a link a secondary choice for specific destinations.

[Giuseppe Larosa]

Hello @Daniel Smith ,

Equal Cost Multi Path is not an issue until all links have MPLS enabled on each of them.

Just avoid to have a mix of IP only links and IP + MPLS links in ECMP because it can break connectivity for MPLS services like MPLS L3 VPN.

So my suggestion is just to have MPLS enabled on all the links and to use ECMP with IS-IS.

L3 VPN traffic is hashed on a per flow basis using source IP and destination IPs

L2 VPN traffic is not balanced in the same way it uses a hash of inner label.

Edit:

about your original question IS-IS is link state so I don't think you can tweak the metric on a per prefix per link basis as you would like to to.

And as I have explained above you don't need to do it.

Hope to help

Giuseppe