Rules for IS-IS Adjacencies

Posted: February 20, 2016 in IS-IS

• L1 routers form L1 adjacency with L1 and L1-L2 routers in their area.(R1 only form L1 neighborship with R2 which is L1-L2 router and belongs to same area 49.0000)

• L2 routers form L2 adjacency with L2 and L1-L2 routers in their area or another area.(R4 forms L2 neighborship with R2 and R3 which is L1-L2 router but R2 belongs to area 49.0000 and R3 belongs to 49.0001)

• L1-L2 routers form L1 and L2 adjacency with each other in their area and only L2 adjacency in another area.(R2 and R3 area L1-L2 routers but belongs to different area so they just form L2 adjacency)

• An L1 router does not form an adjacency with an L2 router, regardless of area.(R1 doesn’t form neighborship with R4 even if they are from same area 49.0000)

• The system ID must be unique to each router regardless of area(System ID are different for each router it acts like router-id must be unique)

• Hello intervals and hold times do not have to match.(Hello timer is changed on R2 and R3 still neighborship is up with other routers)

• Network type should match to form neighborship.(if we configure command ‘isis network point-to-point’ on R2  all neighborships will go down)

• TO check IS-IS neighborship commands area
#sh isis nei
#sh clns nei

Below output and configuration of 4 routers connected to same L2 switch with interface Fa0/0 and IP address configured on it is 12.0.0.x/24 (where x is router number)
R1 – L1 router; R2 – L1-L2 router; R3 – L1-L2 router; R4 – L2 router
Only R3 belongs to different area 49.0001 and rest of the routers belongs to area 49.0000

R1#  sh isis nei
System Id      Type Interface   IP Address      State Holdtime Circuit Id
R2             L1   Fa0/0       12.0.0.2        UP    25       R2.01
R1#  sh run | sec is
ip router isis
router isis
net 49.0000.0000.0000.0001.00
is-type level-1
R1#

R2#  sh isis nei
System Id      Type Interface   IP Address      State Holdtime Circuit Id
R1             L1   Fa0/0       12.0.0.1        UP    28       R2.01
R3             L2   Fa0/0       12.0.0.3        UP    115      R4.01
R4             L2   Fa0/0       12.0.0.4        UP    8        R4.01
R2#  sh run | sec is
ip router isis
isis hello-multiplier 4
isis hello-interval 20
router isis
net 49.0000.0000.0000.0002.00
R2#

R3#  sh isis nei
System Id      Type Interface   IP Address      State Holdtime Circuit Id
R2             L2   Fa0/0       12.0.0.2        UP    78       R4.01
R4             L2   Fa0/0       12.0.0.4        UP    8        R4.01
R3#  sh run | sec is
ip router isis
isis hello-multiplier 4
isis hello-interval 30
router isis
net 49.0001.0000.0000.0003.00
R3#

R4#  sh isis nei
System Id      Type Interface   IP Address      State Holdtime Circuit Id
R2             L2   Fa0/0       12.0.0.2        UP    65       R4.01
R3             L2   Fa0/0       12.0.0.3        UP    119      R4.01
R4#  sh run | sec is
ip router isis
router isis
net 49.0000.0000.0000.0004.00
is-type level-2-only
R4#

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Comments
  1. carlrdean says:

    Good work, it’s pleasure to read your interesting articles. Waiting for more

    Like

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