Category Archives: 3. ISIS

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.




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
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

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.


3. Configuring ISIS



######## R6 | AREA 01 ########

interface Loopback0
ip router isis 1
interface FastEthernet0/0.15
ip router isis 1
isis circuit-type level-2-only
interface FastEthernet0/0.45
ip router isis 1
interface FastEthernet0/0.56
ip router isis 1
isis circuit-type level-1

router isis 1
net 01.0000.0000.0006.00

######## R5 | AREA 01 ########
router isis 1
net 01.0000.0000.0006.00

R7(A01) Database:

R7#sh isis database
IS-IS Level-1 Link State Database:
LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL
R5.00-00 0x00000005 0x9F2D 1081 1/0/0
R6.00-00 0x00000005 0x3501 1089 1/0/0
R6.03-00 0x00000001 0xEC56 1082 0/0/0
R7.00-00 * 0x00000005 0x9C34 1083 0/0/0
R7.01-00 * 0x00000001 0x182C 1084 0/0/0
R7.02-00 * 0x00000001 0xF74C 346 0/0/0

Those to boarder routers have the attached bits set on the links state packets. Attachment bits means we have connection to other area or another level and you can use me as a default destination.

It’s automatically set the attached bits and router R7 now has the default route to reach any destination to outside.

R7#sh ip route isis
i*L1 [115/10] via, FastEthernet0/0.56
[115/10] via, FastEthernet0/0.46 is subnetted, 2 subnets
i L1 [115/20] via, FastEthernet0/0.56 is variably subnetted, 5 subnets, 2 masks
i L1 [115/20] via, FastEthernet0/0.56
[115/20] via, FastEthernet0/0.46

We still have full reachability the difference is just we have and using a shorter view to match or find the path to destination.