IS-IS

IS-IS

IS-IS was originally developed by DEC for CLNS, but it was adapted so that it could also route IP. It is now called integrated IS-IS.
Link state protocol. Rarely runs the SPF algorithm, prefers partial route calculation (PRC).
IS-IS is a highly scalable routing protocol, which is why it is often used on large service provider network backbones.
It’s not encapsulated in an IP packet, requires an OSI address, connectionless network service address (CLNS)
Ethernet Header
IS-IS Header
IS-IS Data
FCS

Area and Router roles

Entire router is in an area not interfaces. IOS default is Level 1-2.
  • Level 1 system - intra-area router, only knows what the local area looks like.
  • Level 2 system - backbone router
  • Level 1-2 system - router performs both roles, creates separate level 1 and 2 link-state databases and two SPF trees.
Sends hello packets to form neighbour adjacencies, will only form on same level
L1 routers try to find the closest L1-L2 exit
IS-IS Design

LSP (Link State Packet)

  • one or more prefixes
  • Adjacent neigbours
  • Metric (0-63 old devices, 10 is default)
 
✉️
NSAP Addresses

IS-IS Configuration

IS-IS Network Diagram
#PE1
router isis
	net 49.0051.1111.1111.1111.00
	is-type level-1
	
int s0
	ip router isis
	isis metric 35
	
#P1
router isis
	net 49.0051.1111.5151.1111.00
	is-type level-1-2
	
int s0
	ip router isis
	isis circuit-type level-1
	isis metric 35
	
int fa0/1
	ip router isis
	isis circuit-type level-2-only
	
#C1 IOS-XR
router isis 1
	net 49.0010.1111.1111.1111.00
	is-type level-2-only
	int gi 0/1/0/1
		address-family ipv4
show clns neighbors

show clns interface