[bgpcfgd]: Change prefix-list generation for "Allow prefix" feature

[pavel-shirshov]

- Why I did it
I was asked to change "Allow list" prefix-list generation rule.
Previously we generated the rules using following method:

For each {prefix}/{masklen} we would generate the prefix-rule
permit {prefix}/{masklen} ge {masklen}+1
Example:
Prefix 1.2.3.4/24 would have following prefix-list entry generated
permit 1.2.3.4/24 ge 23

But we discovered the old rule doesn't work for all cases we have.

So we introduced the new rule:

For ipv4 entry,  
For mask  < 32 , we will add ‘le 32’ to cover all  prefix masks to be sent by T0  
For mask =32 , we will not add any ‘le mask’ 
For ipv6 entry, we will add le 128 to cover all the prefix mask to be sent by T0  
For mask < 128 , we will add ‘le 128’ to cover all prefix masks to be sent by T0 
For mask = 128 , we will not add any ‘le mask’ 

- How I did it
I change prefix-list entry generation function. Also I introduced a test for the changed function.

- How to verify it

1. Build an image and put it on your dut.

2. Create a file test_schema.conf with the test configuration

{
    "BGP_ALLOWED_PREFIXES": {
        "DEPLOYMENT_ID|0|1010:1010": {
            "prefixes_v4": [
                "10.20.0.0/16",
                "10.50.1.0/29"
            ],
            "prefixes_v6": [
                "fc01:10::/64",
                "fc02:20::/64"
            ]
        },
        "DEPLOYMENT_ID|0": {
            "prefixes_v4": [
                "10.20.0.0/16",
                "10.50.1.0/29"
            ],
            "prefixes_v6": [
                "fc01:10::/64",
                "fc02:20::/64"
            ]
        }
    }
}

3. Apply the configuration by command

sonic-cfggen -j test_schema.conf --write-to-db

4. Check that your bgp configuration has following prefix-list entries:

admin@str-s6100-acs-1:~$ show runningconfiguration bgp | grep PL_ALLOW
ip prefix-list PL_ALLOW_LIST_DEPLOYMENT_ID_0_COMMUNITY_1010:1010_V4 seq 10 deny 0.0.0.0/0 le 17
ip prefix-list PL_ALLOW_LIST_DEPLOYMENT_ID_0_COMMUNITY_1010:1010_V4 seq 20 permit 127.0.0.1/32
ip prefix-list PL_ALLOW_LIST_DEPLOYMENT_ID_0_COMMUNITY_1010:1010_V4 seq 30 permit 10.20.0.0/16 le 32
ip prefix-list PL_ALLOW_LIST_DEPLOYMENT_ID_0_COMMUNITY_1010:1010_V4 seq 40 permit 10.50.1.0/29 le 32
ip prefix-list PL_ALLOW_LIST_DEPLOYMENT_ID_0_COMMUNITY_empty_V4 seq 10 deny 0.0.0.0/0 le 17
ip prefix-list PL_ALLOW_LIST_DEPLOYMENT_ID_0_COMMUNITY_empty_V4 seq 20 permit 127.0.0.1/32
ip prefix-list PL_ALLOW_LIST_DEPLOYMENT_ID_0_COMMUNITY_empty_V4 seq 30 permit 10.20.0.0/16 le 32
ip prefix-list PL_ALLOW_LIST_DEPLOYMENT_ID_0_COMMUNITY_empty_V4 seq 40 permit 10.50.1.0/29 le 32
ipv6 prefix-list PL_ALLOW_LIST_DEPLOYMENT_ID_0_COMMUNITY_1010:1010_V6 seq 10 deny ::/0 le 59
ipv6 prefix-list PL_ALLOW_LIST_DEPLOYMENT_ID_0_COMMUNITY_1010:1010_V6 seq 20 deny ::/0 ge 65
ipv6 prefix-list PL_ALLOW_LIST_DEPLOYMENT_ID_0_COMMUNITY_1010:1010_V6 seq 30 permit fc01:10::/64 le 128
ipv6 prefix-list PL_ALLOW_LIST_DEPLOYMENT_ID_0_COMMUNITY_1010:1010_V6 seq 40 permit fc02:20::/64 le 128
ipv6 prefix-list PL_ALLOW_LIST_DEPLOYMENT_ID_0_COMMUNITY_empty_V6 seq 10 deny ::/0 le 59
ipv6 prefix-list PL_ALLOW_LIST_DEPLOYMENT_ID_0_COMMUNITY_empty_V6 seq 20 deny ::/0 ge 65
ipv6 prefix-list PL_ALLOW_LIST_DEPLOYMENT_ID_0_COMMUNITY_empty_V6 seq 30 permit fc01:10::/64 le 128
ipv6 prefix-list PL_ALLOW_LIST_DEPLOYMENT_ID_0_COMMUNITY_empty_V6 seq 40 permit fc02:20::/64 le 128

- Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[pavel-shirshov]

retest this please

[pavel-shirshov]

retest vsimage please