Default-route in L1 ISIS database

Objective: To generate a default-route in L1 database by an L1-only IS.

Commands/configurations are from FTOS. Hope, the ISIS behavior will be same across all network vendors.

  • The default behaviour in ISIS: If an L1L2 IS has L2 adjacency with another IS in *different* area, then L1L2 IS will send L1 LSP with attached bit set to all its L1 neighbors. L1 IS on seeing this attached bit will insert a default route in its routing table and sends inter-area traffic to nearby L1L2 IS.
  • default-information originate” command will generate a default-route in L2 database of an L1L2 IS. But it will not generate default-route in L1 database.
  • redistribute <protocol> level1” command can redistribute all <protocol> routes (except default-route) to L1 database.
  • advertise level2-into-level1 <list>” command will advertise L2 database into L1 matching the prefix-list. But this command will not have any effect on default-route even if the prefix-list matches default route.
  • So, the only way it seems to generate a default-route in L1 database is by using “default-information originate” command along with a route-map. In the route-map we need to (match the default-route) set the level as L1.
  • R1(L2 IS. Area:01)— L2 adj—R2(L1L2 IS. Area:01) — L1 adj — R3 (L1IS. Area:01)

< Here, R2 will not set attached bit and hence R3 will not have a default-route installed>

R3#show run isis
!
router isis
 is-type level-1
 log-adjacency-changes
 metric-style wide level-1
 metric-style wide level-2
 net 01.0000.0000.0002.00
R3#show isis neighbors
System Id Interface State Type Priority Uptime Circuit Id
R2 Gi 0/2 Up L1 64 00:05:37 R3.01
R3#
R3#show ip route isis
 Destination Gateway Dist/Metric Last Change
 ----------- ------- ----------- -----------
R3#
R2#
R2#
R2#show run isis
!
router isis
 log-adjacency-changes
 metric-style wide level-1
 metric-style wide level-2
 net 01.0000.0000.0001.00
R2#
R2#
R2#show isis database level-1 detail | grep IP
 IP Address: 10.1.1.2
 Metric: 10 IP 10.1.1.0/24
 IP Address: 10.1.1.1
 Metric: 10 IP 10.1.1.0/24
R2#
R2#

<After configuring route-map>

R2#show run route-map l1-default-route
!
route-map l1-default-route permit 10
 set level level-1
R2#
R2(conf)#router isis
R2(conf-router_isis)#$information originate route-map l1-default-route
R2(conf-router_isis)#end
R2#show isis database level-1 detail | grep IP
 IP Address: 10.1.1.2
 Metric: 10 IP 10.1.1.0/24
 Metric: 0 IP 0.0.0.0/0
 IP Address: 10.1.1.1
 Metric: 10 IP 10.1.1.0/24
R2#
R3#show ip route isis
 Destination Gateway Dist/Metric Last Change
 ----------- ------- ----------- -----------
 *i L1 0.0.0.0/0 via 10.1.1.2, Gi 0/2 115/10 00:01:52
R3#
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