OSPF route does not show in center router

Answered Question
Jan 1st, 2010

I practice for my CCNA on three 2500 seriers routers.  When I setup OSPF it seems the "O" route shows on my two far end routers but only "C" for interfaces on the center router.  I have setup three routers from a book, and making up my own networks.


Router A E0 to Router B E0

Router B S0 to Router C S0


I know the cost is lowest for connected but shouldn't I see the route in show ip route for all routers?  Show ip protocol shows OSPF stats.


Also no auto-summary is not an avail. command.


Just wondering if I am to expect to only see "C" connected on one router.




Thanks,

Jake

Correct Answer by Giuseppe Larosa about 7 years 1 month ago

Hello Jake,

the central router RB can see OSPF routes only if RA and RC are advertising links that are not directly connected to RB.


on RA:


int loop1

ip address 192.168.254.1 255.255.255.255


router ospf xx

network 192.168.254.1 0.0.0.0 area 0


I suppose you are using area0 otherwise use the area-id in use.


no auto-summary applies to protocols with distance vector characteristics. OSPF and ISIS are link state and they haven't this command because they don't need it they never perform summarization automatically at a major network (class A, B,C) boundary.


to be noted you can see from the fact that RA sees OSPF route for RB-RC link that OSPF is working.

As suggested by Anser there are several show commands that can help

sh ip ospf neighbors

sh ip ospf interface


sh ip ospf database


sh ip ospf database adv-router


note: connected routes have a lower administrative distance not cost that is like metric. In IP routing table in the [x/y] x is AD and y is metric.



Hope to help

Giuseppe

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Muhammad Anser Khan Fri, 01/01/2010 - 22:39

Dear Jake,


Can you post the output of the following commands from all the routers:


#show ip ospf interface brief


#show run | begin router ospf


Regards,

Anser

Correct Answer
Giuseppe Larosa Sat, 01/02/2010 - 02:34

Hello Jake,

the central router RB can see OSPF routes only if RA and RC are advertising links that are not directly connected to RB.


on RA:


int loop1

ip address 192.168.254.1 255.255.255.255


router ospf xx

network 192.168.254.1 0.0.0.0 area 0


I suppose you are using area0 otherwise use the area-id in use.


no auto-summary applies to protocols with distance vector characteristics. OSPF and ISIS are link state and they haven't this command because they don't need it they never perform summarization automatically at a major network (class A, B,C) boundary.


to be noted you can see from the fact that RA sees OSPF route for RB-RC link that OSPF is working.

As suggested by Anser there are several show commands that can help

sh ip ospf neighbors

sh ip ospf interface


sh ip ospf database


sh ip ospf database adv-router


note: connected routes have a lower administrative distance not cost that is like metric. In IP routing table in the [x/y] x is AD and y is metric.



Hope to help

Giuseppe

brent.sodders Sat, 01/02/2010 - 08:18

That would explain it because I do see 2 neighbors.  Two different labs, and even removing loopbacks seemd to produce the same routes.  So I assumed this was normal.


What some labs fail to show are the route tables on all routers. (Which is handy since you are trying to achive routing between all three.)   Many are very good about showing the running config. for each router.   When I see different routes on one of three, I start top question if I have set it up correctly.


Thanks for clarifying the results.

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