Advertised /32 in /24 LAN subnet

[jean1]

Hello,

I am trying to do a basic thing but it's not working.

I would like to adverstived only IP address that is included in my LAN subnet. 198.168.10.1

Example :

interface GigabitEthernet0/0/0.10
encapsulation dot1Q 10
ip address 198.168.10.1 255.255.255.0

!

My test configuration :

ip route 198.168.10.1 255.255.255.255 null0 tag20

!

router bgp 64XXX
address-family ipv4

network 198.168.10.1 mask 255.255.255.255

!

OR

route-map STATIC permit 10

match tag 20

!

router bgp 64XXX
address-family ipv4

redistribute static route-map STATIC

!

Nothing is working !

Note : if i shut interface GigabitEthernet0/0/0.10, my /32 is advertised :

(Beacause AD Connected=0 and STATIC=1, I think )

Router#sh ip bgp nei 10.1.1.1 advertised-routes
*> 198.168.10.1/32 0.0.0.0 0 32768 i

Do you know if it's possible to advertised only IP included in connected interface ?

[Georg Pauwen]

Hello,

when you shut the subinterface, 198.168.10.2/32 is advertised ? What does that IP address belong to ?

Post a diagram of your topology, as well as the full configurations (sh run) of all devices involved.

[jean1]

Hello,

There is an error in initial post, IP send is 198.168.10.1.

Topology is basic, i am trying to do this in lab.

Routers R1,R2 WAN are in Nominal/Backup.

I just want advertise only /32.

Because if /24 is advertised to WAN, ALL IP can be reachable throught LAN.

Maybe others solutions are possible, but i just want to know if it's possible ?

[paul driver]

Hello

---

@jean1 wrote:

Hello,

I am trying to do a basic thing but it's not working.

I would like to adverstived only IP address that is included in my LAN subnet. 198.168.10.1

---

The network command should be applicable  -
router bgp xx
network 192.168.10.0  (no mask is required as its a classful subnet)

[jean1]

Hi Paul,

I just try in lab, but it's not working. only 198.168.10.0/24 is advertised.

[Georg Pauwen]

Hello,

your drawing suggests some sort of LAN redundancy (HSRP/VRRP/GLBP) ?

As stated, when we have the full configs we can get a better picture and come up with an answer...

[Georg Pauwen]

Hello,

post the full running configurations (sh run) of all 4 routers so we can lab this up...

[paul driver]

Hello

---

@jean1 wrote:

I just try in lab, but it's not working. only 198.168.10.0/24 is advertised.

---

That is correct you only have 192.168.10/24 to advertise, are you saying you do NOT wish to advertise .250  if so that wont be applicable as rtr1 &2 lan interfaces are directly connected as such 192.168.10./24 subnet will be advertised

[MHM Cisco World]

config your Null0
no shut the sub interface 
do

show ip route 192.168.10.1 255.255.252.255 longer-prefixes

check which one will appear null0 or sub interface ??

[jean1]

Hello MHM,

Configuration :

ip route 192.168.10.1 255.255.255.255 Null0

!

Router# sh ip route 192.168.10.1 255.255.255.255 longer-prefixes

   192.168.10.0/24 is variably subnetted, 2 subnets, 2 masks
L 192.168.10.1/32 is directly connected, GigabitEthernet0/0/0.10

Router# sh ip route static

not present. 

route is not detect as static but as local

Router# sh ip route connected | i 192.168.10
   192.168.10.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.10.0/24 is directly connected, GigabitEthernet0/0/0.10
L 192.168.10.1/32 is directly connected, GigabitEthernet0/0/0.10

[Georg Pauwen]

Hello,

are you doing this on 'real' routers, or are you working in a simulator ? Since you are using dot1q subinterfaces for the link coming from R3, what is R3...a layer 3 switch ? As stated before, seeing the full configs (sh run) of all devices would enable us to lab this up and give you an answer in probably 15 minutes...rather than the many hours you have already spent on this.

[jean1]

hello Georg,

This is not a simulator, I am testing this configuration on reals Cisco devices. Topology is just an example.

Lab is not hard, there is no specific configuration.

Just 2 routeurs with an ebgp peer and one LAN Connected. Try to annonce an IP /32 included in this LAN. For me is not possible.

This is the question. 

[Richard Burts]

There are several things I would comment on.

- In general your approach of a static route for a /32 with null 0 as the next hop should result in an entry in the routing table.

- However, as has been pointed out, the AD of a connected interface is more attractive than the AD of a static route. So in this particular case the static route for /32 is not present in the routing table.

- This explains why the attempt to redistribute static is not working. I would think that redistribute connected might succeed in advertising the address.

-  I would expect that using the network statement in BGP for the /32 should work. Can you confirm that you configured BGP with the network statement but the route was not advertised?

If that is the case then please post the output of these commands

show ip route
show run | section router
show ip bgp sum
show ip bgp neighbor
show ip bgp neighbor 10.1.1.1 advertised-routes

[Jon Marshall]

Rick 

That's the bit I am unsure of. 

There is a directly connected route in the routing table for 192.168.10.1, it is the L route that IOS puts in there but it seems that is not included when BGP checks the IP routing table for a matching route. 

Any ideas ? 

Jon

[Richard Burts]

Jon

I have never tried this approach but would expect that the network statement should work. But OP says it does not. I hoped that maybe it was a simulator thing, but apparently this is real equipment. It might be interesting to know what model router and what version of code is running.

If the OP has a support contract it might be interesting to know what TAC would say about this.

I would like to see the output of the commands that I requested, just to verify some things.

I wonder if debug for bgp might shed some light on this.

I would think that redistribute connected in bgp (perhaps filtered by a prefix list or route map) should advertise the /32.

I am not clear why the OP wants to advertise the router interface address to the DC but not the LAN subnet. But the question asked was if this is possible, and that is what we should focus on.