BGP and auto-summary enabled or disabled with "network" command

[zillah2004]

http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a00800949e8.shtml#five

I just modified the below quoted by adding numbers

[[[

1- When auto-summary is enabled, it summarizes the locally originated BGP networks to their classfull boundaries. (Auto-summary is enabled by default in BGP).

2- When auto-summary is disabled, the routes introduced locally into the BGP table are not summarized to their classfull boundaries.

3- When a subnet exists in the routing table and the following three conditions are satisfied, then any subnet of that classfull network in the local routing table will prompt BGP to install the classfull network into the BGP table.

* Classfull network statement for a network in the routing table

* Classfull mask on that network statement

* Auto-summary enabled

For example, if the subnet in the routing table is 75.75.75.0 mask 255.255.255.0, and you configure network 75.0.0.0 under the router bgp command, and auto-summary is enabled, BGP introduces the classfull network 75.0.0.0 mask 255.0.0.0 in the BGP table.

4- If these three conditions are not all met, then BGP does not install any entry in the BGP table unless there is an exact match in the local routing table.

Note: If the AS that performs BGP does not own the complete classfull network, Cisco recommends that you disable auto-summary using the no auto-summary]]].

My understanding is this:

A- I hope my numbering to the quoted above is right ,,,I numbered it to be easy to point to it.

B- What does it mean by locally originated, does it mean within AS ?

1- auto-summary is enabled it summarizes the locally originated BGP networks to their classfull boundaries without caring to check if there are any entry in IGP routing table,,,,Am I right ?

2- auto-summary is disabled, the routes introduced locally into the BGP table are not summarized to their classfull boundaries, without caring to check if there are any entry in IGP routing table,,,,Am I right ?

3- I could not match his example with his explanation

4- It is obvious.

[Harold Ritter]

B- locally originated is from a local router standpoint.

1- auto-summary will always care about entries in the routing table. There is two different cases:

a) If a network statement is configured for a classful network, IOS will look for a perfect match or more specific subnet.

b) If redistribution is configured, IOS will automatically summarize to the classful network boundary.

2- with "no auto-summary" these is two cases.

a) with network statement, IOS will look for a oerfect match with a routing table entry. No summarization occur.

b) with redistribution, the redistributed subnets will been in BGP exactly as they are in the routing table (no summarization).

3- his example is a reflection of answer 1(a) where the routing table has an entry for 75.75.75.0/24 and the network statement configured under the bgp process is "network 75.0.0.0". With "auto-summary" prefix 75.0.0.0/8 will be originated in BGP.

Let me know if I answered your questions,

[zillah2004]

Thanks hritter

Please let us concentrate on "network" statement.

[[[1/a If a network statement is configured for a classful network, IOS will look for a perfect match or more specific subnet]]].

1- What about if a network statement is configured for a classless network?

2- What did you mean by perfect match ? Is it similar to exact or it isn't ?

3- What did you mean by "more specific subnet" ? have we got "not specific subnet" ?

4- If perfect is not similar to exact , that means in the the case of "auto-summary" we just care about the entry in a routing table

[[[2/a with network statement, IOS will look for a perfect match with a routing table entry]]]

1- Does that mean with "no auto-summary" we do not care about the exact match, we do care about perfect match (the reason I am saying that because I can feel there is a difference between these two words perfect and exact,,,,I hope I am right)

[Harold Ritter]

1- Then a perfect match is required in this case and no summarization occurs.

2- Perfect match is the same a exact match. Sorry for the confusion.

3- More specific subnet means any subnet that belongs to the classful network.

Let me know if I answered your questions,

[zillah2004]

Thanks hritter

I think you forgot to answer this :

[[[2/a with network statement, IOS will look for a perfect match with a routing table entry]]]

1- Does that mean with "no auto-summary" we do not care about the exact match

The author says:

"3- When a subnet exists in the routing table and the following three conditions are satisfied, then any subnet of that classfull network in the local routing table will prompt BGP to install the classfull network into the BGP table.

* Classfull network statement for a network in the routing table

* Classfull mask on that network statement

* Auto-summary enabled "

Correct my understanding to the above quoted

The author says: For example, if the subnet in the routing table is 75.75.75.0 mask 255.255.255.0 {match the above quoted : “When a subnet exists in the routing table”}.

The author says : and you configure network 75.0.0.0 under the router bgp command (for my understanding it does not match the above quoted first comdition : “Classfull network statement for a network in the routing table”},,,,,,,he says 75.0.0.0 under bgp table and the above quoted says under routing table (IGP table)

[Harold Ritter]

The reason I didn't answer 2/a is that you were asking for clarification between perfect and exact match, which I had already responded that they are one and the same.

With "no auto-summary" the network statement has to match exactly with an entry in the routing table, otherwise the prefix is not originated in BGP.

Let me know if I answered your question,

[zillah2004]

Could you kindly comment this

[[[Correct my understanding to the above quoted

The author says: For example, if the subnet in the routing table is 75.75.75.0 mask 255.255.255.0 {match the above quoted : “When a subnet exists in the routing table”}.

The author says : and you configure network 75.0.0.0 under the router bgp command (for my understanding it does not match the above quoted first comdition : “Classfull network statement for a network in the routing table”},,,,,,,he says 75.0.0.0 under bgp table and the above quoted says under routing table (IGP table) ]]]

[Harold Ritter · Accepted solution]

Assuming "network 75.0.0.0" and "auto-summary" are configured under the bgp process, the presence of route 75.75.75.0/24 in the routing table would trigger the generation of prefix 75.0.0.0/8 in the bgp table since route 75.75.75.0/24 is a subnet belonging to network 75.0.0.0/8.

Assuming "network 75.0.0.0" and "no auto-summary" are configured under the bgp process, the presence of route 75.75.75.0/24 in the routing table would NOT trigger the generation of prefix 75.0.0.0/8 in the bgp table. Only the presence of 75.0.0.0/8 in the routing table (exact match) could trigger the generation of this prefix in bgp.

Hope this helps. Please rate this posting if it does.