Advertising a NAT pool to upstream router

[xuoguoto]

Hello all,

I have a small PPPoE/NAT Linux device, where some users connect to browse internet. This machine is connected to an upstream router via a small /30 network. The NAT IP address are routed to this device via a static route set in upstream router.

To automate this process, I am thinking of advertising the NAT network via BGP so that when a NAT pool is added, a route is automatically added to the upstream router.

I have tried to add static route in my machine, and do a redistribute static which will get distributed to the router and a route will be added.

My problem is that this is a NAT network, which is not added to any interface and does not have any route as such in my device. Route I added in my machine is a dummy route, pointing to a non existent gateway. My first check point was that whether the route will get redistributed and get correctly added in upstream router, which works fine.

I am seeking the wisdom of this group to figure out how to advertise this route correctly so that when I add a new nat network, a return route is automatically created in my upstream router.

Thanks!

X.

[ton31337]

Please show the outputs first of the networks, interfaces, and what specifically you want to do. Because now this is more like a hypothetic question :)

[xuoguoto]

Adding more details

               ExaBGP                   Frr2                        Frr3
            +-----------+            +----------+               +---------+
            |           |            |          |               |         |
            |           |            |          |               |         |
            +-----+-----+            +------+---+               +------+--+
                  |                         |                          |
      172.16.200.22/30                  .26/30                   .30/30|
                  |                         |                          |
                  |                         |  .25                     |
                  |              +----------+--------+                 |
                  |              |                   |                 |
                  +--------------+      Frr1         +-----------------+
                                 |                   |
                         .21     +-------------------+ .29

As shown in the above diagram, I have an Frr router (Frr1) with 3 interfaces and on each /30 interface another router is connected. This part is working fine and in Frr1 I can see all 3 routers connected.

# sh bgp summary 

IPv4 Unicast Summary (VRF default):
BGP router identifier 192.168.200.114, local AS number 65534 vrf-id 0
BGP table version 6
RIB entries 11, using 2112 bytes of memory
Peers 3, using 2174 KiB of memory

Neighbor        V         AS   MsgRcvd   MsgSent   TblVer  InQ OutQ  Up/Down State/PfxRcd   PfxSnt Desc
172.16.200.22   4      65533        79        81        0    0    0 01:14:02            2        6 N/A
172.16.200.26   4      65532        83        84        0    0    0 01:14:03            2        6 N/A
172.16.200.30   4      65531        85        88        0    0    0 01:14:02            2        6 N/A

Total number of neighbors 3
2vBNG3-114# sh ip route bgp
Codes: K - kernel route, C - connected, S - static, R - RIP,
       O - OSPF, I - IS-IS, B - BGP, E - EIGRP, N - NHRP,
       T - Table, v - VNC, V - VNC-Direct, A - Babel, F - PBR,
       f - OpenFabric,
       > - selected route, * - FIB route, q - queued, r - rejected, b - backup
       t - trapped, o - offload failure

B>* 10.21.0.0/24 [20/0] via 172.16.200.22, znmw, weight 1, 01:14:33
B>* 10.22.0.0/24 [20/0] via 172.16.200.22, znmw, weight 1, 01:14:33

Configuration of Frr1 is basic with minimum configuration needed for BGP to work:

frr version 8.5.3
frr defaults traditional
hostname Ubuntu-114
log file /tmp/frr.log
log syslog informational
log timestamp precision 3
service integrated-vtysh-config
!
debug zebra nht
debug bgp neighbor-events
debug bgp nht
debug bgp updates in
debug bgp updates out
debug bfd peer
!
router bgp 65534
 bgp log-neighbor-changes
 no bgp ebgp-requires-policy
 bgp bestpath as-path multipath-relax
 neighbor 172.16.200.22 remote-as 65533
 neighbor 172.16.200.22 bfd
 neighbor 172.16.200.26 remote-as 65532
 neighbor 172.16.200.26 bfd
 neighbor 172.16.200.30 remote-as 65531
 neighbor 172.16.200.30 bfd
 !
 address-family ipv4 unicast
  redistribute connected
 exit-address-family
exit
!
end

I am advertising two routes from ExaBGP which is seen in the Frr1:

exabgpcli neighbor 172.16.200.21 announce route 10.21.0.0/24 next-hop 172.16.200.22
exabgpcli neighbor 172.16.200.21 announce route 10.22.0.0/24 next-hop self

Now I would like to advertise couple of routes from Frr1 to other three routers. The route I want to advertise is not assigned to any interface in Frr1 I just need to route those networks to Frr1.

The use case is that Frr1 acts as a NAT server and the network I want to advertise are the nat pool. Those IPs are not assigned to any interface on Frr1 but when a packet with that destination ip hits Frr1, it will get translated and sent across the clients.

I could not find any means to advertise an arbitrary network from Frr. From all I could understand Frr expects the ip its advertising to be assigned in any of the devices, eg redistribute connected.

Given this configuration, how can I advertise a network say 172.17.100.0/22 which is not assigned to any if Frr1's interfaces but those packets needs to be routed to it's interfaces from upstream routers.

I toyed with the idea of ExaBGP peered with Frr to advertise this network and it kind of works. In the Frr2 and Frr3 I can see the routes advertised by ExaBGP

Ex in Frr2:

# sh ip route bgp
Codes: K - kernel route, C - connected, S - static, R - RIP,
       O - OSPF, I - IS-IS, B - BGP, E - EIGRP, N - NHRP,
       T - Table, v - VNC, V - VNC-Direct, A - Babel, F - PBR,
       f - OpenFabric,
       > - selected route, * - FIB route, q - queued, r - rejected, b - backup
       t - trapped, o - offload failure

B>* 10.21.0.0/24 [20/0] via 172.16.200.25, ens4, weight 1, 01:55:56
B>* 10.22.0.0/24 [20/0] via 172.16.200.25, ens4, weight 1, 01:55:56
B>* 172.16.200.20/30 [20/0] via 172.16.200.25, ens4, weight 1, 01:55:56
B>* 172.16.200.28/30 [20/0] via 172.16.200.25, ens4, weight 1, 01:55:56

But that has two problems:

1. I need a separate ExaBGP running and that needs to be peered correctly with Frr1 (iBGP??)
2. Frr will not peer with localhost in the version I am running. bgpd: Allow peering via 127.0.0.0/8 #13060 fixes this but its not available in my running version.

I am willing to recompile and run bleeding edge if that's the only way out, but really looking for a more elegant way to configure !

Thanks for reading!

[ton31337]

Now I would like to advertise couple of routes from Frr1 to other three routers. The route I want to advertise is not assigned to any interface in Frr1 I just need to route those networks to Frr1.

Use network X.X.X.X command to advertise the custom network. Also, you most likely need no bgp network import-check command in some cases.