ISIS Notes


ISIS is not an IP protocol
ISIS is part of the CLNS stack it is a layer 2 transport protocol to form neighborship and to send data


ISIS supports routing for both ipv4 and ipv6
  - IPv6 routing can be either on:
• single topology
  shares path calculation with ipv4
  NB requires 1:1 correlation of ipv4 and ipv6 interfaces if not you can break ipv4 and ipv6 routing.
• multiple topology
  independent path calculation from IPv4 and IPv6
  ipv4 & ipv6 configuration completely independent


CLNS router-ID
we need to define the network entity title essentially CLNS router-ID (like ospf router-id to calculate the shortest path); the process id is locally significant uses ISO NSAP addressing format ( max 20 bytes, min 8bytes )
(AA.AAAA.AAAA.AAAA.AAAA.AAAA.AAAA.SSSS.SSSS.SSSS.NN)
A- Area - not like OSPF Area meaning the area doesn't limit the SPF domain
S- System-ID router-ID
N- N-Selecto always zero (00)

ISIS packets
ISIS have only one LSA called LSP and the hello packets

IS-IS can form 2 levels of adjacencies level2 and/or level 1
IS-IS level 1:
• L1 LSP describes connected links/prefixes on the router
• L1 LSP are only forwarded by L1 routers (or L1-2 ) and can only be flooded within an area
• Intra area adjacency is like NSNA and in OSPF (intra area routes, default route out, redistribution allowed)

IS-IS level 2:
• L2 LSP describe all links/prefixes within the area
• L2 LSP are only forwarded by level-2-capable routers and cross area boundaries
• Inter or intra-area adjacency is like area 0 in OSPF

ISIS level 1/ level2 router:
L1 and L2 Routers are used as exit points from L1 to L2 (like ABR in OSPF, ). It injects a default route into level-1, it sets the attached bit which informs other routers that it is an ABR
NB:
ISIS prefer the L1 route over the L2 route
level1 / level2 router converts L1 to L2 but not L2 to L1

IS-IS network type
IS-IS support only 2 network types
broadcast:
• default on multipoint interfaces
• uses DIS instead of DR
• there is no backup no BDR
point-to-point:
• Default on point-to-point interfaces

DIS Election
• designated intermediate system
• Like OSPF DR but no BDR no backup DIS
• Election is dynamic; preemption can occur
• Separate election for L1 and L2
• Occurs by highest priority. You can set it under the interface or then the highest SNPA (MAC)address

 

Forming IS-IS adjacency
  level and adjacency

       • area must match if the L1 adjacency
       • area can be different on L2 adjacency
  network type
       • broadcast
       • point-to-point
  Authentication must match
  MTU must match

ISIS path selection
- all links default to a cost of 10 (can be manually modified)
- neighbours must agree on metric style
• narrow (default)
• wide (needed for MPLS TE and IPv6)
• transition (run both narrow and wide at the same time)
- L1 paths are preferred over L2 paths (like ospf intra-area over inter-area)

 

 

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