Solved: OSPF ABR behavior in redistribute static route

shiro.kishibe · ‎09-27-2010

Hi,

Two ABR in same area redistribute same static route , and that static route's AD is 200.

One ABR install static route in routing table, other one install ospf route in routing table.

ABR-1、ABR-2 config

router ospf 10

redistribute static metric 500 metric-type 1 subnets

network 10.1.1.0 0.0.0.255 area 1

network 10.1.2.0 0.0.0.255 area 0

ip route 192.168.1.0 255.255.255.0 FastEthernet0/0 10.1.1.254 200

ABR-1#sh ip route 192.168.1.0

Routing entry for 192.168.1.0/24

Known via "static", distance 200, metric 0

Redistributing via ospf 10

Advertised by ospf 10 metric 500 metric-type 1 subnets

Routing Descriptor Blocks:

* 10.1.1.254, via FastEthernet0/0

Route metric is 0, traffic share count is 1

ABR-2#sh ip route 192.168.1.0

Routing entry for 192.168.1.0/24

Known via "ospf 10", distance 110, metric 510, type extern 1

Last update from 10.1.1.254 on FastEthernet0/0, 00:31:32 ago

Routing Descriptor Blocks:

* 10.1.1.254, from 10.1.2.1, 00:31:32 ago, via FastEthernet0/0

Route metric is 510, traffic share count is 1

Does anyone knows why don't ABR-1 install ospf route that has higher priority than static route?

And, When I change area 1 to stub area, both ABR-1 and ABR-2 install static route in routing table.

But, both ABR install external route from area 0.

Why don't stub ABR intstall external route redistributed myself?

Thanks.

Giuseppe Larosa · ‎09-27-2010

Hello Shiro,

when the field called forwarding address is not 0.0.0.0 (that means the router itself) but points to a real next-hop one of the two ABRs suppresses its own LSA type 5.

This is made because there is no need of two copies of the LSA in backbone area as the load balancing effect can be achieved by the fact of two copies of inter area route to Forwarding address.

>> Does anyone knows why don't ABR-1 install ospf route that has higher priority than static route?

ABR-1 cannot generate the external route if a non
OSPF route is not in the table ( at lesat not a route coming from the SAME ospf process)

so ABR-1 installs the static route and generates the OSPF external route

ABR-2 suppresses its own OSPF generation and as a result of this accepts the route coming from companion node.

>> Why don't stub ABR intstall external route redistributed myself?

again it is not done for the reason above

ABR-X cannot generate the external route if a non
OSPF route is not in the table ( at least not a route coming from the SAME ospf process)

Edit:

to be noted this behaviour is standard per RFC 2328

Hope to help

Giuseppe

View solution in original post

Giuseppe Larosa · ‎09-27-2010

Hello Shiro,

when the field called forwarding address is not 0.0.0.0 (that means the router itself) but points to a real next-hop one of the two ABRs suppresses its own LSA type 5.

This is made because there is no need of two copies of the LSA in backbone area as the load balancing effect can be achieved by the fact of two copies of inter area route to Forwarding address.

>> Does anyone knows why don't ABR-1 install ospf route that has higher priority than static route?

ABR-1 cannot generate the external route if a non
OSPF route is not in the table ( at lesat not a route coming from the SAME ospf process)

so ABR-1 installs the static route and generates the OSPF external route

ABR-2 suppresses its own OSPF generation and as a result of this accepts the route coming from companion node.

>> Why don't stub ABR intstall external route redistributed myself?

again it is not done for the reason above

ABR-X cannot generate the external route if a non
OSPF route is not in the table ( at least not a route coming from the SAME ospf process)

Edit:

to be noted this behaviour is standard per RFC 2328

Hope to help

Giuseppe

shiro.kishibe · ‎10-08-2010

Thanks to replay.

>> ABR-1 cannot generate the external route if a non
>> OSPF route is not in the table ( at lesat not a route coming from the SAME ospf process)

>> so ABR-1 installs the static route and generates the OSPF external route

>> ABR-2 suppresses its own OSPF generation and as a result of this accepts the route coming from companion node.

I understood that this behavior is ospf standard.

Thanks.

Peter Paluch · ‎09-27-2010

Shiro,

A couple of things must be stressed in this scenario.

First, a router in OSPF that performs redistribution is by definition an ASBR, not an ABR.

Second, the redistribution is performed over object in the routing table. Only objects already present in the routing table can be redistributed into other routing protocols. In your example, if a static route is redistributed into the OSPF, it must first be present in the routing table and it must remain there so that it can be redistributed indefinitely. If the route stops being static and becomes a different type, say, OSPF-learned, then the command redistribute static no longer applies to it because the route is not static anymore.

Third, you have a race condition scenario in your network. One of the routers learns about the redistributed route from the other, and because OSPF-learned routes have lower AD than your static route AD of 200, it will replace the static route in that router's routing table. The network will in this case converge on one router redistributing the static route into OSPF and the second router simply learning that route via OSPF. A worse-case scenario is that both routers will redistribute this route into OSPF, advertise it simultaneosly, learn about it from each other simultaneously, replace their static route in their routing tables with the OSPF-learned route (pointing to each other, creating a temporary routing loop), then removing that route from the routing table because none of the routers has the source static route anymore, thus placing back the static route - and doing this all over again in possibly neverending cycle.

Best regards,

Peter