Network command in BGP ?

auos

Hi,

I was so confuse about (network command) in BGP,

1. I do (network 172.16.1.0) without mask and auto-summary are enable and I have entry for that network in my routing table, why BGP NOT advertise that network to BGP peers with default class. No any entry in BGP table. (I mean if I did not mention mask they should be take /16 as default?)

2. I do (network 172.16.1.0 mask 255.255.255.0) and auto-summary are enable and I have entry for that network in my routing table, why BGP advertise that network with /24 and did not care about auto-summary.

chmorin

I'm not a BGP expert... heck I'm not even a BGP novice. But I found this:

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

Q. How does BGP behave differently with auto-summary enabled or disabled?

A. Auto-summary behavior has changed across Cisco IOS software releases. Initially, auto-summary was enabled by default. However, with Cisco bug ID CSCdu81680 ( registered customers only) this behavior has changed. In the latest Cisco IOS, auto-summary is disabled by default. When auto-summary is enabled, it summarizes the locally originated BGP networks to their classfull boundaries. Auto-summary is enabled by default in BGP. When auto-summary is disabled, the routes introduced locally into the BGP table are not summarized to their classfull boundaries. When a subnet exists in the routing table and these 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.

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.

The last part there. Someone else also had as similar problem:

https://supportforums.cisco.com/message/3027790

Since I have 'no auto-summary' option on under the bgp process, I was expecting to networks such as 10.2.80.0/24 included under the advertisement 'network 10.2.0.0 mask 255.255.0.0'. However, that did not happen. In order to see the network 10.2.80.0/24 advertised on R4 (BGP 200), I had to create a 'network 10.2.80.0 mask 255.255.255.0' under the bgp 100 on R2.

With the answer:

BGP auto-summary rule says that ,Take a example that you are having network that 10.2.80.0/24 and you want to advertise into BGP with auto-summary. So you need to advertise this network via network 10.0.0.0 in the bgp process with auto-summary.you cannot adverise 10.2.0.0 mask 255.255.0.0.it will not work.

So I guess we would need to see your routing table to tell you why. By default with auto-summary enabled it gives 172.16.1.0 a mask of 255.255.0.0 (class . Since you specified an exact mask though it must have matched something in the routing table, and worked.