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.
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.
BGP will always advertise a network if that network exist in the routing table:
1. - Do you have the default class (summarized) in your routing table? I suspect not
2. - You have the network statement with /24 in BGP and also in your routing table, so BGP will advertise that network.
For example, if the subnet in the routing table is 220.127.116.11 mask 255.255.255.0, and you configure network 18.104.22.168 under the router bgp command, and auto-summary is enabled, BGP introduces the classfull network 22.214.171.124 mask 255.0.0.0 in the BGP table.
When you do not specify a mask in network command and auto-summary is enabled, then any subnet of the classful network is enough to cause an origination of the classful network. Now you are saying that's not the case in your scenario 1. Can you post the related output from your config, routing table and bgp table?
If you do specify a longer mask in the network statement, then auto-summary doesn't matter and you need an exact match for the prefix (same network, same mask) in the routing table for the network to be originated by BGP with that mask. So, scenario 2 agrees with that.
One of them is a network command is configured with a classful mask subnet for example network 126.96.36.199 or network 188.8.131.52 or network 192.168.1.0. In this case, the classful aggregate is installed in to BGP table if there is a prefix in the IGP table that is a subnet to the classful network. for example if you advertise network 184.108.40.206 than it would work if any of the prefixes 220.127.116.11/24 or 18.104.22.168/24 is the IGP table.
Classful, mask, auto-summary, etc. All this sounds very confusing to me. The exact match in the routing table requirement is generally the case and scenario 2 is clear.
In scenario 1 the network command doesn't have 172.16.0.0 with the mask misssing. It has 172.16.1.0 with the mask missing. If it had 172.16.0.0 then any subnet of major network would suffice. As I see it, this is not an intuitive behavior.
The second statement (mask) match a route in the IGP and thus won't import any prefix if there is not a match in the routing table.
So inoder for a prefix to be imported, it must exactly match the specification, - i.e. if should have the same subnet number and mask. for example, if you have a loopback0 with the ip address 10.1.1.1/24, then th command would be "network 10.1.1.0 mask 255.255.255.0 not "network 22.214.171.124 mask 255.255.255.0. The second statement will not match any route in the IGP and thus wont import any prefix.
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