BGP route advertisement and received routes issue

[KevA]

BGP advertised and received routes issue

Can someone explain to me what I am missing my configuration given the issues below? I have attached devices mentioned below as well as others in the AS-PATH as well as the section of topology relevant to my issue.

Objective:
• R7 and R12 are peering (eBGP) with R200 and must only receive a BGP default route from it.
• R7 and R8 must advertise only a BGP default route to their eBGP neighbors. This default route must only be advertised if a default route is already installed on their routing tables.
• R9 should see the default route from it eBGP neighbors R8 and R23 but route is only being received from R8.

ISSUE #1:

On R7, when I run the command

show ip bgp neighbors 10.0.100.1

advertised-routes I get the following output with Total number of prefixes 0. Shouldn't the total number of prefixes be 1 given when I run

show ip bgp

command I can see the prefix in the bgp table?:

R7#sh ip bgp neighbors 10.0.100.1 advertised-routes
BGP table version is 3, local router ID is 10.1.7.7
<Output omitted for brevity>
Originating default network 0.0.0.0

Network Next Hop Metric LocPrf Weight Path

Total number of prefixes 0

However, I can see the prefix exist in the BGP table:

R7#sh ip bgp
BGP table version is 3, local router ID is 10.1.7.7
<Output omitted for brevity>
RPKI validation codes: V valid, I invalid, N Not found

       Network          Next Hop       Metric    LocPrf     Weight     Path
*>   0.0.0.0            10.1.100.1     0                                         65001 i

ISSUE #2:

R9 should be learning this prefix from both its eBGP neighbors but the prefix is being ignored from R23 because it
is already being received from R8. How do I resolve this issue?

R9#sh ip bgp summary
BGP router identifier 10.2.9.9, local AS number 65002
<Output omitted for brevity>
Neighbor          V     AS          MsgRcvd     MsgSent     TblVer    InQ    OutQ   Up/Down      State/PfxRcd
10.1.100.1      4    65001         278            278            4            0       0         04:03:23         1
10.2.10.10      4    65002         44              46              4            0       0         00:37:00         0
172.16.100.9  4     65000          111              117             4               0        0          01:31:57           0

R9#clear ip bgp * soft
*Dec 8 01:23:24.637 EET: BGP: nbr_topo global 10.1.100.1 IPv4 Unicast:base (0xF5AB8BD8:1) refresh_epoch is 3
*Dec 8 01:23:24.640 EET: BGP(0): 10.1.100.1 rcvd UPDATE w/ attr: nexthop 10.1.100.1, origin i, merged path 65001, AS_PATH
*Dec 8 01:23:24.640 EET: BGP(0): 10.1.100.1 rcvd 0.0.0.0/0...duplicate ignored

[MHM Cisco World]

Show ip bgp neighbors advertise-route 

Show what router must advertise to this neighbor 

Show ip bgp

Show router what receive from other routers

So what you see correct

Note:- to see what you receive use 

Show ip bgp neighbors receive 

This issue 1

[KevA]

MHM Cisco World

My issue is with the Show ip bgp neighbors advertise-route output on
R7 should display Total number of prefixes 1 given R3 show ip bgp displays shows received the default route from R7.

R3#sh ip bgp
BGP table version is 3, local router ID is 10.0.3.3
Network      Next Hop    Metric   LocPrf   Weight      Path
*> 0.0.0.0   10.0.100.2   0                                          65001 i

May be a bug with the software version and/or this is simulation
environment?

[MHM Cisco World]

Sorry I confuse now 

Show ip bgp neighbors advertise-route 

Show ip bgp in original post is both for R7' that why I answer you need to use receive.

Any way

Share both show ip bgp of both R7 and R3

Let me take look 

MHM

[KevA]

You are right, I posted the show ip bgp output for R7 but should have been for R3 but now you see in my last post that R3 did receive the default prefix. Sorry for the confusion.

[Harold Ritter]

Hi @KevA ,

This behavior is due to the fact that you use the

neighbour default-originate

command on R7 to advertise the originate the default route to R3. This is also why you see the following in the output:

Originating default network 0.0.0.0

If you used the "network 0.0.0.0" and configured a static route to let's say null0 locally, you would see the "Total number of prefixes" set to 1.

Regards,

[KevA]

Hello Harold,

From the instructions given, when R7 receives the default route advertised by R200 it would install it in it routing table making it available to be advertised to its neighbors. So are you saying I would still need to configure a default network on R7 ?

I had attached the config files for devices in question but seem they never went thru. Let me know if you would like to look at them. 

[Harold Ritter · Accepted solution]

Hi @KevA ,

> So are you saying I would still need to configure a default network on R7 ?

If you already receive the default from another neighbor, you do not need the network statement, default static route nor the "default originate" for that matter.

Just remove the "default-originate" towards R3 and you should see the "Total number of prefixes" set to 1 on R7.

Regards,

[KevA]

Harold.

That solved the Issue 1. Would you happen to have any ideas as how to resolve issue 2? I think in this case I missing command(s) or as with issue 1 I configured wrong. 

I attached the configs to the original post. 

[Harold Ritter]

Hi @KevA ,

Glad to see issue #1 is solved.

As far as issue #2, The reason R9 is not receiving the default route from R23 is probably that it advertises the default route to R23 in the the first place and R23 prefers it and is therefore not advertising it back to R9. 

One way to solve this issue is to make sure AS 65002 only advertises its local routes to the other ASs. This could be achieved by making sure R9 only advertises BGP prefixes with an empty AS path. This would prevent AS65002 to become a transit AS, which is normally want for a dually connected data center.

ip as-path access-list 1 permit ^$

route-map local-only permit 10

match as-path 1

route-map local-only deny 20

And then apply the route-map local-only outbound on R9 to neighbor R23 and R8.

Regards,

[KevA]

Harold,

That did not solve the issue. Still getting the same update error on R9 after clearing the bgp session. 

router bgp 65002
bgp router-id 10.2.9.9
bgp log-neighbor-changes
neighbor 10.1.100.1 remote-as 65001
neighbor 10.1.100.1 route-map local-only out
neighbor 10.2.10.10 remote-as 65002
neighbor 10.2.10.10 update-source Loopback0
neighbor 172.16.100.9 remote-as 65000
neighbor 172.16.100.9 route-map local-only out

[Harold Ritter]

Hi @KevA ,

Can you provide a "show bgp ipv4 unicast summary" from R9 and "show bgp ipv4 uni 0.0.0.0" from R23.

Regards,

[KevA]

R9#show bgp ipv4 unicast summary
BGP router identifier 10.2.9.9, local AS number 65002
BGP table version is 3, main routing table version 3
1 network entries using 140 bytes of memory
1 path entries using 80 bytes of memory
1/1 BGP path/bestpath attribute entries using 144 bytes of memory
1 BGP AS-PATH entries using 24 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 388 total bytes of memory
BGP activity 1/0 prefixes, 1/0 paths, scan interval 60 secs

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down
State/PfxRcd
10.1.100.1 4 65001 948 944 3 0 0 13:53:35
1
10.2.10.10 4 65002 927 944 3 0 0 13:53:20
0
172.16.100.9 4 65000 933 945 3 0 0 13:53:35
0

R23#show bgp ipv4 uni 0.0.0.0
BGP routing table entry for 0.0.0.0/0, version 0
Paths: (1 available, no best path)
Not advertised to any peer
Refresh Epoch 2
65001 20000
10.0.100.6 (inaccessible) from 10.0.100.21 (10.0.4.4)
Origin IGP, metric 0, localpref 100, valid, internal
rx pathid: 0, tx pathid: 0

[Harold Ritter · Accepted solution]

Hi @KevA ,

Here's the issue. The default route on R23 has an inaccessible next hop (10.0.100.6). This is why it is not being advertised to R9.

I suppose the route is received from R4 (10.0.4.4), right? The next hop should be 10.0.100.21 not 10.0.100.6. Do you by any chance change the next hop on R4 or R23? Can you please provide the configuration for R4 and R23?

Regards,

[KevA]

Harold,

You are right. Originally R23 AS# was 65111 butt the lab instructed that ACME got purchased by Corp so they wanted ACME to use same AS# 65000. In this case,  the next-hop on R4 for neighborship with R23 will need to be change: 

neighbor 10.0.100.22 next-hop-self

I would imagine then I will still need the route-map local-only configuration that you provided if I was to change the AS# on R23 back to AS65111 and should not need the next-hop-self configured on R4.