Basic question on BGP no auto-summary & advertisement

Answered Question
Mar 1st, 2010

Hello,

Since I have 'no auto-summary' option on under the bgp process, I was expecting to networks such as 10.2.80.0/24 included under the advertisement 'network 10.2.0.0 mask 255.255.0.0'. However, that did not happen. In order to see the network 10.2.80.0/24 advertised on R4 (BGP 200), I had to create a 'network 10.2.80.0 mask 255.255.255.0' under the bgp 100 on R2.


Am missing something here? If 10.2.0.0/16 and no auto-summary is in place, why 10.2.80/24 is not included?

R2

#
router bgp 100
no synchronization
bgp log-neighbor-changes
network 10.2.0.0 mask 255.255.0.0 
network 10.177.206.0 mask 255.255.255.0
neighbor 10.2.47.193 remote-as 200
neighbor 10.2.255.47 remote-as 100
no auto-summary
!
ip forward-protocol nd
!
!
ip http server
no ip http secure-server
!
!
!
!
!
control-plane
!
#
R2(config)#router bgp 100
R2(config-router)#network 10.2.80.0 mask 255.255.255.0
R2(config-router)#
R2#
*Mar  1 00:43:09.315: %SYS-5-CONFIG_I: Configured from console by console
R2#


R2#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     4.0.0.0/32 is subnetted, 1 subnets
B       4.4.4.4 [20/0] via 10.2.47.193, 00:18:03
     10.0.0.0/8 is variably subnetted, 4 subnets, 3 masks
O       10.2.80.0/24 [110/2] via 10.177.206.1, 00:23:21, FastEthernet0/0
C       10.2.60.104/30 is directly connected, FastEthernet1/0
C       10.177.206.0/24 is directly connected, FastEthernet0/0
C       10.2.47.192/28 is directly connected, FastEthernet0/1
     192.168.1.0/32 is subnetted, 1 subnets
C       192.168.1.2 is directly connected, Loopback0
R2#

R4#ping 10.2.80.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.2.80.1, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
R4#ping 10.2.80.1 <============= After entering the  net 10.2.80.0 mask /24 on R2 and I can see the route advertised on the border router on AS 200:

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.2.80.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 372/668/1352 ms
R4#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     4.0.0.0/32 is subnetted, 1 subnets
C       4.4.4.4 is directly connected, Loopback1
     10.0.0.0/24 is subnetted, 4 subnets
C       10.2.47.0 is directly connected, FastEthernet0/1
C       10.2.55.0 is directly connected, FastEthernet0/0
B       10.2.80.0 [20/2] via 10.2.47.204, 00:00:59
B       10.177.206.0 [20/0] via 10.2.47.204, 00:07:26
     192.168.1.0/32 is subnetted, 1 subnets
C       192.168.1.4 is directly connected, Loopback0
R4#

I have this problem too.
0 votes
Correct Answer by Jon Marshall about 6 years 9 months ago

Marlon

The reason why I had to include 'network 10.2.80.0 mask 255.255.255.0' is because I must advertise at least one network which is part of the aggregate-address entry, correct?

Exactly. Glad you got it working.

Jon

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KARUPPUCHAMY MA... Mon, 03/01/2010 - 21:56

Hi,

BGP auto-summary rule says that ,Take a example that you are having network that 10.2.80.0/24 and you want to advertise into BGP with auto-summary,

So you need to advertise this network via network 10.0.0.0 in the bgp process with auto-summary.you cannot adverise 10.2.0.0 mask 255.255.0.0.it will not work.

so your configuration would be (with auto-summary)

R2

#

router bgp 100

no synchronization

bgp log-neighbor-changes

network 10.0.0.0

network 10.177.206.0 mask 255.255.255.0

neighbor 10.2.47.193 remote-as 200

neighbor 10.2.255.47 remote-as 100

Now try to ping , it should work.

for more information, have a look into this below URL

https://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a00800949e8.shtml#five

regards

Karuppu

Jon Marshall Mon, 03/01/2010 - 23:21

Marlon

It's not included because with the network statement under BGP there must be an exact match in the IGP routing table for it to be advertised. So when you enter 10.2.0.0/16 as a network statement that is exactly what the router looks for in the IGP table. And when it sees 10.2.80.0/24 it does not consider this a match because the subnet mask is different as is the 3rd octet.

Having "no auto-summary" on does not change the above rule.

Jon

news2010a Tue, 03/02/2010 - 07:13

Oh, that is true!

So imagine I have a number of subnets:10.2.30.0/24, 10.2.41.0/24, 10.2.42.0/24, etc.

Then I want to minimize the number of 'network' statements under the 'bgp router' statement.


I have the option to use the 'aggregate-address' to summarize this under the BGP process, correct?

Jon Marshall Tue, 03/02/2010 - 07:16

news2010a wrote:

Oh, that is true!

So imagine I have a number of subnets:10.2.30.0/24, 10.2.41.0/24, 10.2.42.0/24, etc.

Then I want to minimize the number of 'network' statements under the 'bgp router' statement.


I have the option to use the 'aggregate-address' to summarize this under the BGP process, correct?

Marlon

Yes you can aggregate or summarise them to an EBGP peer but to summarise the above 3 networks you would need to use -

10.2.30.0 255.255.192.0  which would include 10.2.0.0 -> 10.2.63.255 which is obviously a lot more /24 networks than just those 3.

Jon

news2010a Tue, 03/02/2010 - 07:54

Sorry for my ignorance, but if I want to summarize all networks under 10.2.0.0/16:

I considered possible networks to be advertised to Corporate border router as: 10.2.1.0/24 - 10.2.255.254/24

Then I removed the 'network 10.2.80.0 mask 255.255.255.0' and I did like shown below:


R3#
router bgp 100
no synchronization
bgp log-neighbor-changes
network 10.2.0.0 mask 255.255.0.0
network 10.177.206.0 mask 255.255.255.0
aggregate-address 10.2.0.0 255.255.0.0
neighbor 10.2.55.133 remote-as 200
neighbor 10.2.255.15 remote-as 100
no auto-summary
!

However I go to my test border router R4 and I no longer see 10.2.80.0/24 there anymore. So this aggregate range is not working...

Do you mind to clarify how the aggregate-address should look like if I want to include all networks under 10.2.0.0/16?

Jon Marshall Tue, 03/02/2010 - 08:03

news2010a wrote:

Sorry for my ignorance, but if I want to summarize all networks under 10.2.0.0/16:

I considered possible networks to be advertised to Corporate border router as: 10.2.1.0/24 - 10.2.255.254/24

Then I removed the 'network 10.2.80.0 mask 255.255.255.0' and I did like shown below:


R3#
router bgp 100
no synchronization
bgp log-neighbor-changes
network 10.2.0.0 mask 255.255.0.0
network 10.177.206.0 mask 255.255.255.0
aggregate-address 10.2.0.0 255.255.0.0
neighbor 10.2.55.133 remote-as 200
neighbor 10.2.255.15 remote-as 100
no auto-summary
!

However I go to my test border router R4 and I no longer see 10.2.80.0/24 there anymore. So this aggregate range is not working...

Do you mind to clarify how the aggregate-address should look like if I want to include all networks under 10.2.0.0/16?

Marlon

Couple of things

1) can you modify your config -

router bgp 100

no network 10.2.0.0 mask 255.255.0.0

network 10.2.80.0 mask 255.255.255.0

aggregate-address 10.2.0.0 255.255.0.0

2) why do you want the specific routes as well as the aggregate address ?

Jon

news2010a Tue, 03/02/2010 - 08:30

That did it.

The reason why I had to include 'network 10.2.80.0 mask 255.255.255.0' is because I must advertise at least one network which is part of the aggregate-address entry, correct?

Now from R4, I noticed that I can reach for example hosts on 10.2.40.0/24, which does not appear on the border router R4 (AS 200) ip route explicitly, but it works now because is learning how to get there via the 10.2.0.0/16 aggregate.

Thanks much!!

Gateway of last resort is not set

     4.0.0.0/32 is subnetted, 1 subnets
C       4.4.4.4 is directly connected, Loopback1
     10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
B       10.2.0.0/16 [20/0] via 10.2.47.204, 00:00:08
C       10.2.47.0/24 is directly connected, FastEthernet0/1
C       10.2.55.0/24 is directly connected, FastEthernet0/0
B       10.2.80.0/24 [20/2] via 10.2.47.204, 00:00:08
B       10.177.206.0/24 [20/0] via 10.2.47.204, 00:05:13
     192.168.1.0/32 is subnetted, 1 subnets
C       192.168.1.4 is directly connected, Loopback0
R4#ping 10.2.40.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.2.40.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 344/564/684 ms
R4#
R4#show ip route 10.2.40.0
Routing entry for 10.2.0.0/16
  Known via "bgp 200", distance 20, metric 0
  Tag 100, type external
  Redistributing via ospf 1
  Advertised by ospf 1 subnets
  Last update from 10.2.47.204 00:04:18 ago
  Routing Descriptor Blocks:
  * 10.2.47.204, from 10.2.47.204, 00:04:18 ago
      Route metric is 0, traffic share count is 1
      AS Hops 1
      Route tag 100

R4#

Correct Answer
Jon Marshall Tue, 03/02/2010 - 08:46

Marlon

The reason why I had to include 'network 10.2.80.0 mask 255.255.255.0' is because I must advertise at least one network which is part of the aggregate-address entry, correct?

Exactly. Glad you got it working.

Jon

KARUPPUCHAMY MA... Tue, 03/02/2010 - 07:26

Hi,

I have the option to use the 'aggregate-address' to summarize this under the BGP process, correct?---  Yes.

We cannot use aggregate-address  to reduce number "network" command  lines under BGP.

When you are doing aggregate-address, there will be one new network segment(aggregated network)  will install into the BGP routing table and it will advertise to other neighbors and rest all the network's will be suppressed.

Need to be caution that when we are doing aggregation, the aggregated address should not clash with another network which we dont want to do aggregation.

for more info,just have a cisco documentation

http://www.ciscosystems.com/en/US/tech/tk365/technologies_tech_note09186a0080094826.shtml

Regards

Karuppu

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