OSPF Confusion

Answered Question
Dec 1st, 2008

Hi Guys,

What are the differences among the following statements:

1)

router ospf 1

summary-address 10.0.0.0 255.0.0.0 tag 11122

redistribute ospf 2 metric 5000 subnets route-map TAG-Question

route-map TAG-Question permit 10

match ip address TAG_ACL

2)

router ospf 1

summary-address 10.0.0.0 255.0.0.0

redistribute ospf 2 metric 5000 subnets route-map TAG-Question

route-map TAG-Question permit 10

match ip address TAG_ACL

set tag 11122

3)

router ospf 1

summary-address 10.0.0.0 255.0.0.0 tag 11122

redistribute ospf 2 metric 5000 subnets route-map TAG-Question

route-map TAG-Question permit 10

match ip address TAG_ACL

set tag 11122

According to my understanding no 1 & 2 does the same thing,please correct me if I am wrong. However I am confused with the no.3.

Regards,

I have this problem too.
0 votes
Correct Answer by Giuseppe Larosa about 8 years 1 day ago

Hello Iqbal,

here you have two statements in each OSPF routing process:

a redistribute OSPF 2 + options

a summary -address + options

the results can be different depending on what routes are present in OSPF 2 DB (and in the IP routing table otherwise they are not redistributed into OSPF1).

If all OSPF 2 subnets are within net 10/8 the first option will:

create and propagate a summary address 10/8 with route-tag 11122 in the route tag field of the LSA external type 5 with seed metric 5000. (subnets are suppressed by the summary-address command they are in the router DB but they are not sent out in OSPF1 domain)

if all OSPF 2 subnets are within net 10/8 second option will:

create and propagate a summary address 10/8 with route-tag 2 in the route tag field of the LSA external type 5 with seed metric 5000.

Can you see the difference ?

if OSPF 2 DB and routing table contains prefixes out of net 10/8 and permitted by the ACL:

all the subnets in 10/8 will be suppressed and the summary 10/8 is created as described above.

Prefixes out of net 10/8 will:

have a route-tag of 2 in case 1)

have a route-tag of 11122 in case 2)

in 3) being the route-tag value the same in both commands:

net 10/8 will have route-tag 11122

if other prefixes out of 10/8 are imported by ospf2 into ospf1 they also have route-tag 11122

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Mon, 12/01/2008 - 10:09

Hello Iqbal,

here you have two statements in each OSPF routing process:

a redistribute OSPF 2 + options

a summary -address + options

the results can be different depending on what routes are present in OSPF 2 DB (and in the IP routing table otherwise they are not redistributed into OSPF1).

If all OSPF 2 subnets are within net 10/8 the first option will:

create and propagate a summary address 10/8 with route-tag 11122 in the route tag field of the LSA external type 5 with seed metric 5000. (subnets are suppressed by the summary-address command they are in the router DB but they are not sent out in OSPF1 domain)

if all OSPF 2 subnets are within net 10/8 second option will:

create and propagate a summary address 10/8 with route-tag 2 in the route tag field of the LSA external type 5 with seed metric 5000.

Can you see the difference ?

if OSPF 2 DB and routing table contains prefixes out of net 10/8 and permitted by the ACL:

all the subnets in 10/8 will be suppressed and the summary 10/8 is created as described above.

Prefixes out of net 10/8 will:

have a route-tag of 2 in case 1)

have a route-tag of 11122 in case 2)

in 3) being the route-tag value the same in both commands:

net 10/8 will have route-tag 11122

if other prefixes out of 10/8 are imported by ospf2 into ospf1 they also have route-tag 11122

Hope to help

Giuseppe

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