4. IS-IS Path Selection

ISIS Path Selection
– All links default to cost of 10. (Can be manually modified).

– Neighbor must agree on metric-style.

– Level 1 path preferred level 2 path. (Like OSPF Intra-Area over Inter-Area)

TLV(Type of length)
– Use to encode not only metric but use to encode IPV6 information & Extention of MPLS TE.

Note: When you form ISIS adjacencies, 99% of cases you want to set the metric style “Wide”.

Metric-style “Wide” gives a larger bit length, which can encode IPV6 and MPLS TE mean by default ISIS will not support TE & IPv6 routing.

IOS CONFIGURATION:
#ROUTER ISIS 1
#METRIC-STYLE WIDE

XRV CONFIGURATION:
#ROUTER ISIS 1
#ADDRESS-FAMILY IPV4
#METRIC-STYLE WIDE
#ADDRESS-FAMILY IPV6
#METRIC-STYLE WIDE

Verification:
IOS – SHOW CLNS PROTOCOL
XR – SHOW ISIS PROTOCOL

This configuration will exchange routes now with other devices that is using wide metric style bec. the device using narrow won’t understand the coding of the attributes.

If you have the device that doesn’t support metric-style “Wide” you can use metric-style “Transition”.

Screenshot from 2018-08-06 11-58-37

** – Means it wasn’t compute the shortest to the destination.

Multi-topology IS-IS
– ISIS supports for both IPv4 & IPv6.
– IPv6 routing can be either

Single Topology
– Share path calculation with IPv4.
– Requires 1:1 correlation of IPv4 and IPv6 interfaces.

Multi Topology
– Independent path calculation from IPv4.
– IPv4 & IPv6 configuration completely independent.

Manually changing the metric:
interface FastEthernet0/0.15
description TO_R6
encapsulation dot1Q 15
ip address 10.1.5.1 255.255.255.0
ip router isis 1
ipv6 address 2001:1:5::1/64
ipv6 router isis 1
isis metric 100 level-2
isis ipv6 metric 200 level-2

Q&A:
1. For ipv4 and ipv6 are we using different transport?
A: neither, we are using CLNS. Protocol directly encapsulated at layer 2.

2. Which would you recommend ISIS or OSPF?
A: Depends if you’re running both ipv4 and ipv6. If youre in a large network ISIS would be preferred because you can run in single topology. There’s are feature difference but in-terms of the core spf calculation they’re barely close to each other in-terms of convergence and scaling.

Reference:
https://ipcisco.com/isis-for-ipv6-configuration-example-on-cisco-ios/
http://wiki.kemot-net.com/is-is-metric
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_isis/configuration/15-sy/irs-15-sy-book/ip6-route-isis.html

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