QID:NF58 - BGP configuration and the no synchronization command

[joshuad31]

Hello,

For the following question below you stated the correct answer is:
1. Subnets of 172.32.0.0/22 do not exist in the BGP table.

However they would exist in the BGP table because the aggregate-address command works similarly to the network command in BGP.  It injects these routes into the BGP table but not the routing table.  the problem is that there would not be a ">" mark next to those routes indicating that they are the best routes because BGP cannot find these routes in the routing table propagated by an IGP. 

Thus the correct answer is:
5. The BGP command no synchronization is missing.

Please correct thanks!

Question:
59. (QID:NF58) During BGP configuration on a router that has peered with other BGP speakers, the BGP command "aggregate-address 172.32.0.0 255.255.252.0" is issued. However, the peers do not receive this aggregate network in BGP advertisements. Also, the router does not have this aggregate network in its BGP table. Which option indicates a possible reason this command did not cause the router to advertise the aggregate network to its peers?

1. Subnets of 172.32.0.0/22 do not exist in the BGP table.
2. The BGP command no auto-summary is missing.
3. Interface NULL 0 is likely shutdown.
4. The IGP running on this router does not have network 172.32.0.0/22 installed.
5. The BGP command no synchronization is missing.
6. The next hop IP address must be a loopback address.

[routermatrix]

joshuad31,
    My understanding of the network and aggregate-address is sort of the opposite from what you desciribed: a BGP speaking NEVER advertises a route it originates with the aggregate-address command unless a longer match prefix already exists in the BGP table.  From what I have read and experienced first hand, this is unconditionally the case (if anyone can show me configuration output demonstrating this is not true, I’d be interested to see it).  For this reason I think the stated solution is the ‘best’ answer since it is always true (synchronization would not be relevant, for instance, if all connections were eBGP). Along the same lines, routes originated with the network command are ONLY inserted into the BGP table (and therefore advertised) if the destination (exact prefix match) is resolvable to a next hop that already resides in the IP routing table: not in the IP routing table then not inserted/advertised BGP wise. 

    I think the following lab output demonstrates this:



------- initial blank config:
RA(config)#do show ip proto

RA(config)#do show run | include Loop
RA(config)#do show ip route | begin Gate
Gateway of last resort is not set

    10.0.0.0/30 is subnetted, 1 subnets
C      10.0.12.0 is directly connected, FastEthernet0/0
RA(config)#
RA(config)#do show ver | in ROM: 3700
ROM: 3700 Software (C3745-ADVIPSERVICESK9-M), Version 12.4(15)T5, RELEASE SOFTWARE (fc4)
RA(config)#

------ apply aggregate-address command w/o subnet in IP routing table:
RA(config)#
RA(config)#router bgp 1
RA(config-router)#aggregate-address 100.0.0.0 255.254.0.0
RA(config-router)#do show ip bgp

RA(config-router)#do show ip route | in 100.0.0.0
RA(config-router)#

------ create two loopback interfaces that are subsets of the aggregate:
RA(config-router)#do show ip int brief | in 100
RA(config-router)#int lo100
RA(config-if)#ip add 100.0.0.1 255.255.255.255
RA(config-if)#int lo101
RA(config-if)#ip add 100.1.0.1 255.255.255.255
RA(config-if)#do show ip bgp

RA(config-if)#

------ add the network command that matches lo100 and the conditions
------ for advertising/inserting both types of originated routes are satisfied:
RA(config-if)#router bgp 1
RA(config-router)#do show ip bgp

RA(config-router)#network 100.0.0.1 mask 255.255.255.255
RA(config-router)#do show ip bgp | begin Network
  Network          Next Hop            Metric LocPrf Weight Path
*> 100.0.0.0/15    0.0.0.0                            32768 i
*> 100.0.0.1/32    0.0.0.0                  0        32768 i
RA(config-router)#

     
------ add a peer and the route will show as advertised:
RA(config-router)#logging con
RA(config)#router bgp 1
RA(config-router)#neighbor 10.0.12.2 remote-as 1
RA(config-router)#
*Mar  1 01:35:39.899: %BGP-5-ADJCHANGE: neighbor 10.0.12.2 Up
RA(config-router)#do show ip bgp 100.0.0.1 | section Advertised
  Advertised to update-groups:
        1
RA(config-router)#do show ip bgp 100.0.0.0 | section Advertised
  Advertised to update-groups:
        1
RA(config-router)#


Best regards :)

[joshuad31]

wow that was really helpful.  I did not know that.  I will have to look carefully at that again.  Thanks!

[routermatrix]

sure thing - that was good for me to look at as well.