Learning Routes from BGP for which I'm the source

jeremyarcher · ‎11-03-2011

I'm getting an issue that is causing some route flapping.

My topology is:

192.168.1.0/24 Network --- ASA (EIGRP) --- 3925 Router (EIGRP & BGP) ----- MPLS Provider Router (BGP Peer)

The main issue I'm facing is that the 3935 router redistributes the 192.168.1.0 network into BGP fine (using eigrp redistribution into BGP). It shows as an advertised route while still showing the ASA as the next hop.

However, every few minutes the 3925's routing table flips and it starts sending traffic destined for the 192.168.1.0 network out to the MPLS BGP peer (and the routing table shows that it has learned that route from it's BGP peer) instead of using the ASA (AD 20 vs. 90).

Konstantin Dunaev · ‎11-03-2011

could it be that you announce 192.168.1.0/24 somewhere else in network? is it real IP address or you just take a private as an example?

jeremyarcher · ‎11-03-2011

Thought the same and as a test unplugged the ASA and watched for that route to be advertised but it never did.

So, I don't believe it is being advertised anywhere else.

The address is an example but the real address space in use is also a private address.

Sent from Cisco Technical Support iPhone App

Konstantin Dunaev · ‎11-03-2011

Does MPLS BGP any redistribution also? do you have an access to the BGP router to see what is the "source" of routing information?

jeremyarcher · ‎11-03-2011

That is a provider router so I have no access to it at all.

Konstantin Dunaev · ‎11-03-2011

it can happen only if the provider router get the information from other source with a better metric. may be your provider does some redistribution? may be set a static route and redistibute it back to BGP? I'd ask him to check your VRF

jeremyarcher · ‎11-03-2011

Here is some troubleshooting output.

I think my primary question is 'is it normal for a BGP peer to advertise to it's neighbor routes for which the router is the originating router and therefore, the responsibility of the router to ignore those advertisements or should the peer not advertise those routes to the originating neighbor'?

For example...

If Router A advertises route 1 to Router B should Router B Advertise that route to Router A? Or since, it knows that it is the originating router should it not advertise it to its peer?

Here is a 'sh ip eigrp top 192.168.1.0' output when the problem is happening:

central_wan_rtr#sh ip eigrp topology 192.168.1.0

EIGRP-IPv4 Topology Entry for AS(1)/ID(192.168.51.2) for 192.168.1.0/24

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 28160

Descriptor Blocks:

x.x.x.237, from Redistributed, Send flag is 0x0

Composite metric is (28160/0), route is External

Vector metric:

Minimum bandwidth is 100000 Kbit

Total delay is 100 microseconds

Reliability is 255/255

Load is 1/255

Minimum MTU is 1500

Hop count is 0

Originating router is 192.168.51.2

External data:

AS number of route is 1

External protocol is BGP, external metric is 0

Administrator tag is 65000 (0x0000FDE8)

192.168.51.1 (GigabitEthernet0/0), from 192.168.51.1, Send flag is 0x0

Composite metric is (3072/2816), route is Internal

Vector metric:

Minimum bandwidth is 1000000 Kbit

Total delay is 20 microseconds

Reliability is 255/255

Load is 1/255

Minimum MTU is 1500

Hop count is 1

It appears almost as if the router is learning the route from it's peer because of the redistribution of eigrp to bgp.

Traceroute shows the following:

wan_rtr#trace 192.168.1.1 source 192.168.51.2

Type escape sequence to abort.

Tracing the route to 192.168.1.1

VRF info: (vrf in name/id, vrf out name/id)

1 x.x.x.237 [AS 65000] 38 msec 40 msec 42 msec

2 x.x.x.238 [AS 65000] 42 msec 42 msec 42 msec

3 x.x.x.237 [AS 65000] 82 msec 86 msec 82 msec

4 x.x.x.238 [AS 65000] 78 msec *

192.168.1.1 [AS 65000] 28 msec

Konstantin Dunaev · ‎11-03-2011

jeremyarcher wrote:

Here is some troubleshooting output.

I think my primary question is 'is it normal for a BGP peer to advertise to it's neighbor routes for which the router is the originating router and therefore, the responsibility of the router to ignore those advertisements or should the peer not advertise those routes to the originating neighbor'?

For example...

If Router A advertises route 1 to Router B should Router B Advertise that route to Router A? Or since, it knows that it is the originating router should it not advertise it to its peer?

Here is a 'sh ip eigrp top 192.168.1.0' output when the problem is happening:

actually BGP follows the same "split-horiton" rule as pretty all routing protocols, but in case of externalBGP it uses the AS-PATH information and the router which originally advertised a cirtain prefix will drop the routing update from its neighbour if its local AS appears in prefix's AS-PATH. That is why I said that is possible to get the same prefix from you ISP only in the case if your ISP router gets that prefix from other source (may be locally redistributed) with a better metric.

please show the BGP information of the prefix in both cases: right and wrong routing. EIGRP information doesn't play any role here.

jeremyarcher · ‎11-03-2011

I called the ISP today and after quite some time they found that my router stops advertising the routes completely.

I checked my advertised routes and sure enough, it does stop. The strange thing is though that it stops advertising routes completely and totally - not just redistributed routes or networks defined under the BGP config but all networks, even connected networks.

Kishore Chennupati · ‎11-03-2011

Hi jeremy,

it's probably a BUG or something. I would check the bug toolkit and maybe go one train lower. There are heaps of BUGs with this new universal 15.0 IOS images.

The only thing that surprises me though is that redistributed route in your topology table. Are you using network command in BGP to advertise that subnet and then redistribute BGP into EIGRP?

HTH

Kishore

jeremyarcher · ‎11-03-2011

Thanks for the help all.

After a call with TAC we thought it may be related to our service provider. When my router stops advertising it also starts learning those routes from the BGP peer.

I called the carrier back again and showed them this and it turned out that this circuit is in the same vrf as another circuit we have to a different remote location. As a result, they were advertising those routes to the other circuit and in turn, back to me.

Why this just started out of the blue I wouldn't know. However, they seperated the two circuits to two seperate vrfs and that fixed the problem.