[j-nsp] conditions [and negation] in bgp import policies

[Michael Hare]

Anyone have experience using conditions in bgp import policies?

Normally in our core we point v4/v6 default to discard like I suspect most here do.  I have default route from a transit provider of last resort that I want to activate "when I need to".  With all of its caveats of the approach in general, I'm looking at use 'policy-options condition' to install said default if I believe I've become isolated.  In my lab I'm testing with v4 [would use this for v4 as well] and am using 8.8.4.0/24 as a proof of concept.

'if-route-exists' does not have a built in negation so I tried to roll my own as follows.  0.0.0.0/0 is still being rejected [by import policy] despite no 8.8.8.0/24 in the RIB.  This occurs even from a BGP clear when I can confirm 8.8.8.0/24 was not in the local RIB during the entire BGP startup process.

I thought about doing a generated route, but I like the safety net idea of requiring the advertisement over BGP from this specific peer to be present.

Thought I'd try asking here before going to JTAC.  I respect this may evolve into a "bad idea in general" discussion.

-Michael

==========/=================


########## my config

cooldude at r-mx2010-lab-re1# show policy-options policy-statement conditional-bgp 
term google-reachable {
    from {
        route-filter 0.0.0.0/0 exact;
        condition route-to-google-dns;
    }
    then reject;
}
term google-unreachable {
    from {
        route-filter 0.0.0.0/0 exact;
    }
    then {
        preference 1;
        accept;
    }
}

cooldude at r-mx2010-lab-re1# show policy-options condition route-to-google-dns 
if-route-exists {
    8.8.8.0/24;
    table inet.0;
}


############## 8.8.8.0/24 not present

cooldude at r-mx2010-lab-re1# run show route 8.8.8.0/24 

{master}[edit]
cooldude at r-mx2010-lab-re1# 


################## hidden default

cooldude at r-mx2010-lab-re1# run show route 0.0.0.0/0 exact hidden extensive table inet.0 

inet.0: 595731 destinations, 1191437 routes (595721 active, 9 holddown, 3 hidden)
0.0.0.0/0 (3 entries, 1 announced)
        State: <FlashAll>
TSI:
KRT in-kernel 0.0.0.0/0 -> {}
OSPF area : 0.0.0.0, LSA ID : 0.0.0.0, LSA type : Extern
Page 0 idx 0, (group iBGP-reflector type Internal) Type 5 val 0x2023c080 (addpath tsi)
 Addpath Path ID: 1
     Source route: Static
   Advertised metrics:
     Nexthop: Self
     Localpref: 100
     AS path: [65010] I
     Communities:
     (AddPath PathId)
    Advertise: 00000007
Path 0.0.0.0 Vector len 4.  Val: 0
         BGP                 /-101
                Next hop type: Router, Next hop index: 775
                Address: 0xaa35900
                Next-hop reference count: 1415662
                Source: X.Y.33.86
                Next hop: X.Y.33.86 via ae1.3106, selected
                Session Id: 0x14b
                State: <Hidden Ext>
                Inactive reason: Unusable path
                Local AS: 65010 Peer AS:  3128
                Age: 5:00 
                Validation State: unverified 
                Task: BGP_3128.X.Y.33.86+52615
                AS path: 3128 I
                AS path: Recorded
                Localpref: 100
                Router ID: X.Y.32.2
                Hidden reason: rejected by import policy
         BGP                 /-101
                Next hop type: Router, Next hop index: 774
                Address: 0x26409e0c
                Next-hop reference count: 1571275
                Source: X.Y.33.76
                Next hop: X.Y.33.76 via ae0.3105, selected                                                                                                                
                Session Id: 0x14c
                State: <Hidden Ext>
                Inactive reason: Unusable path
                Local AS: 65010 Peer AS:  3128
                Age: 5:00 
                Validation State: unverified 
                Task: BGP_3128.X.Y.33.76+62875
                AS path: 3128 I
                AS path: Recorded
                Localpref: 100
                Router ID: X.Y.32.1
                Hidden reason: rejected by import policy