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Summarization of Routes on ASBR

Hello Guys,

I am studying in ASBR route summarization, I have a situation where I sumarizo networks with the end command summary-address / mask and still zero then the area contained the summarized route and the subordinate routes. For the ospf theory should remove the subordinate routes and consider only summarized or am I wrong?

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Accepted Solutions

Re: Summarization of Routes on ASBR

Hi,

then the area contained the summarized route and the subordinate routes

external routes do not belong to a particular Area, they are flooded within the whole OSPF domain. Special, non-reguar areas types (stub, NSSA, totally stub/nssa) filter them out, stubs then replace them automatically by a default summary route, whereas the injection of a default-route in a NSSA is optional.

Just for the sake of knowledge...

But now to your actual question.

Have a look at this simple example:

R1(config-router)#do show ip ospf database | b Type-5

Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag

192.168.0.0     1.1.1.1         11          0x80000004 0x008E22 0

192.168.1.0     1.1.1.1         16          0x80000001 0x008929 0

192.168.2.0     1.1.1.1         16          0x80000001 0x007E33 0

192.168.3.0     1.1.1.1         16          0x80000001 0x00733D 0

I have 4 static routes, all /24, redistributed into OSPF.

Now I summarize them on the ASBR to a /22:

R1(config-router)#summary-address 192.168.0.0 255.255.252.0

Result:

R1(config-router)#do show ip ospf database | b Type-5

Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag

192.168.0.0     1.1.1.1         17          0x80000005 0x007D35 0

R1(config-router)#do show ip ospf database external

OSPF Router with ID (1.1.1.1) (Process ID 1)

Type-5 AS External Link States

LS age: 62

Options: (No TOS-capability, DC)

LS Type: AS External Link

Link State ID: 192.168.0.0 (External Network Number )

Advertising Router: 1.1.1.1

LS Seq Number: 80000005

Checksum: 0x7D35

Length: 36

Network Mask: /22

Metric Type: 1 (Comparable directly to link state metric)

TOS: 0

Metric: 20

Forward Address: 0.0.0.0

External Route Tag: 0

However, the /24 static routes are still in the routing table, but they are not populated as OSPF external LSAs. There are several possible reasons why you still would see them as external OSPF routes, like another ASBR for those routes which doesn't summarize, or a misconfiguration.

HTH

Rolf

1 REPLY

Re: Summarization of Routes on ASBR

Hi,

then the area contained the summarized route and the subordinate routes

external routes do not belong to a particular Area, they are flooded within the whole OSPF domain. Special, non-reguar areas types (stub, NSSA, totally stub/nssa) filter them out, stubs then replace them automatically by a default summary route, whereas the injection of a default-route in a NSSA is optional.

Just for the sake of knowledge...

But now to your actual question.

Have a look at this simple example:

R1(config-router)#do show ip ospf database | b Type-5

Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag

192.168.0.0     1.1.1.1         11          0x80000004 0x008E22 0

192.168.1.0     1.1.1.1         16          0x80000001 0x008929 0

192.168.2.0     1.1.1.1         16          0x80000001 0x007E33 0

192.168.3.0     1.1.1.1         16          0x80000001 0x00733D 0

I have 4 static routes, all /24, redistributed into OSPF.

Now I summarize them on the ASBR to a /22:

R1(config-router)#summary-address 192.168.0.0 255.255.252.0

Result:

R1(config-router)#do show ip ospf database | b Type-5

Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag

192.168.0.0     1.1.1.1         17          0x80000005 0x007D35 0

R1(config-router)#do show ip ospf database external

OSPF Router with ID (1.1.1.1) (Process ID 1)

Type-5 AS External Link States

LS age: 62

Options: (No TOS-capability, DC)

LS Type: AS External Link

Link State ID: 192.168.0.0 (External Network Number )

Advertising Router: 1.1.1.1

LS Seq Number: 80000005

Checksum: 0x7D35

Length: 36

Network Mask: /22

Metric Type: 1 (Comparable directly to link state metric)

TOS: 0

Metric: 20

Forward Address: 0.0.0.0

External Route Tag: 0

However, the /24 static routes are still in the routing table, but they are not populated as OSPF external LSAs. There are several possible reasons why you still would see them as external OSPF routes, like another ASBR for those routes which doesn't summarize, or a misconfiguration.

HTH

Rolf

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