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IS-IS route selection over wide-metric

Ratheesh mv
Beginner
Beginner

ISIS-ROUTESELECTION.PNG

 

R5 has been learning 200.200.200.200/32 from R710 via EIGRP and being redistributed to IS-IS.(R5 redistribute configuration "redistribute EIGRP 100 metric 100 metric-type external level-1")


R6 has one of the loopbacks interface with IP address 200.200.200.200/32.
R6 loopback configuration:-
----------------------------
interface Loopback200
ip address 200.200.200.200 255.255.255.255
ip router isis
isis metric 110


When R7 receive these 2 routes always prefers that which has been received from R6.(Confirmed via traceroute).

I would like to know the reason of behind that always choosing the R6 route.(If I shutdown R6 loopback traffic moves to EIGRP domain)

 

Could someone explain the route-selection mechanism of IS-IS wide metric ?

1 ACCEPTED SOLUTION

Accepted Solutions

Harold Ritter
Cisco Employee
Cisco Employee

Hi @Ratheesh mv ,

 

On R7, the route 200.200.200.200/32 learned from R5 and the one learned from R6 have the same metric.

 

From R5 = (20 (isis metric to R5) + 100 (metric used on redistribute statement)) = 120

From R6 = (10 (isis metric to R6) + 110 (metric used on the loopback interface) = 120

 

So isis will pick one or the other since the next hop is the same anyway.

 

The way to confirm which route is preferred on R7, is to do a show ip route 200.200.200.200 and look at the "from" field. The traceroute from R7 to 200.200.200.200 is misleading as it will always stop on R6 since R6 has that address configured locally, even if the metric from R7 to R5 was lower. This test is therefore not valid.

 

Regards,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

View solution in original post

1 REPLY 1

Harold Ritter
Cisco Employee
Cisco Employee

Hi @Ratheesh mv ,

 

On R7, the route 200.200.200.200/32 learned from R5 and the one learned from R6 have the same metric.

 

From R5 = (20 (isis metric to R5) + 100 (metric used on redistribute statement)) = 120

From R6 = (10 (isis metric to R6) + 110 (metric used on the loopback interface) = 120

 

So isis will pick one or the other since the next hop is the same anyway.

 

The way to confirm which route is preferred on R7, is to do a show ip route 200.200.200.200 and look at the "from" field. The traceroute from R7 to 200.200.200.200 is misleading as it will always stop on R6 since R6 has that address configured locally, even if the metric from R7 to R5 was lower. This test is therefore not valid.

 

Regards,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
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