In some situations a router may advertise routes into ISIS from OSPF and overwrite routes which are in OSPF. One option is to increase the IP route preference of ISIS from default value 7 to 130 which is higher than OSPF routes making OSPF more favourable.

ip route preference protocol spbm-level1 130

Alternatively, if the platform allows, use ISIS accept policy to control what is learnt. Increasing the ip route preference is required with ERS 8000 as it does not support ISIS accept policies. It can also be useful to change the route preference during migrations and set back to default when required.

Also, when using IP shortcuts and redistribution of ISIS and OSPF it is recommended to disable ip alternative-routes.

no ip alternative-route

It is sometimes necessary to suppress the IST subnet when redistributing direct into ISIS on the cluster routers.

ip prefix-list “ist” 10.199.1.0/30 id 1 ge 30 le 30

route-map “suppress-ist” 1
no permit
enable
match network “ist”
route-map “suppress-ist” 2
permit
enable
exit

router isis
redistribute direct
redistribute direct route-map “suppress-ist”
redistribute direct enable
exit

isis apply redistribute direct

It is sometimes necessary to control which routes are advertised from OSPF into ISIS. This can be achieved by using a route-map when redistributing OSPF into ISIS.

ip prefix-list “access-nets” 0.0.0.0/0 id 2 ge 0 le 32
ip prefix-list “access-nets” 192.168.10.0/24 id 2 ge 24 le 24
ip prefix-list “access-nets” 192.168.20.0/24 id 2 ge 24 le 24

route-map “announce-access-nets” 1
permit
enable
match network “access-nets”
set injectlist “access-nets”
set metric-type type2
exit

router isis
redistribute ospf
redistribute ospf route-map “announce-access-nets”
redistribute ospf enable
exit

isis apply redistribute ospf

It is possible for the cluster routers to advertise an ISIS route into OSPF and overwrite the ISIS learnt routes on the peer router. To avoid this from occurring use a route map to reject routes from the peer OSPF router-ID.

R1 and R2:

route-map “reject” 1
no permit
enable
exit

R1:

router ospf
accept adv-rtr 10.255.0.2 enable route-map “reject”
exit

ip ospf apply accept adv-rtr 10.255.0.2

R2:

router ospf
accept adv-rtr 10.255.0.1 enable route-map “reject”
exit

ip ospf apply accept adv-rtr 10.255.0.1

Ever wondered why certain routes do not appear in the OSPF LSDB? The GRT shows routes but you do not see them when check the LSDB. In the example topology below R1 and R2 have learnt OSPF_INTRA (preference 20) and OSPF_E2 (preference 125) routes from R3. Checking the OSPF LSDB we see the ASExternal routes but not the OSPF_INTRA routes.

R3 has several loopback / CLIP addresses to simulate external networks. CLIP 2 192.168.10.1/24 and CLIP 4 192.168.30.1/24 have IP OSPF Enabled and CLIP 3 192.168.20.1/24 does not have IP OSPF Enabled. OSPF redistributes direct into OSPF.

This means we have some OSPF_INTRA routes and OSPF_E2 routes advertised by R3.

On R1 or R2 type show ip ospf lsdb adv-rtr 10.255.0.6 detail to see Router Link LSAs and ASExternal LSAs.