BGP looping

Dear all,
I have some problem, and would like your advice as below.
I tried to connect a few 1711 router connect together and use EBGP for the routing.
Problem is on the command
- router bgp 100
no synchronization
bgp log-neighbor-changes
network 10.61.177.0 mask 255.255.255.0
network 192.1.1.0
neighbor 10.61.177.69 remote-as 555
neighbor 192.1.1.6 remote-as 65127
I find that after clear the ip bgp * process and sh the ip route, it show all network 0.0.0.0 will be via 192.1.1.6.
Show ip route
Gateway of last resort is 192.1.1.6 to network 0.0.0.0
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
B 10.0.1.0/24 [20/0] via 10.61.177.69, 00:06:15
C 10.61.177.68/30 is directly connected, Vlan1
C 192.1.1.0/24 is directly connected, FastEthernet0
B* 0.0.0.0/0 [20/0] via 192.1.1.6, 00:06:15
sh ip bgp nei 192.1.1.6 advertised-routes
Network Next Hop Metric LocPrf Weight Path
*> 0.0.0.0 192.1.1.6 0 65127 65535 i
*> 10.0.1.0/24 10.61.177.69 0 0 555 i
*> 192.1.1.0 0.0.0.0 0 32768 i
I am not sure whether looping occurred on this router.
Can anyone have ideal on this case.
Thanks

At the first time, maybe bgp looping occur by this default route, checked the neigbor router but no abnormally observed.
After I checked and found that the default route is propagated from the third EBGP router outisde, due to the command
router bgp xx
neighbor 192.xx.xx.xx default-originate
Therefore all the router observed the default route.
Now this question is solved, thanks for your replied.
Best Regards

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We have 50+ sites on our MPLS network. All are connected quite successfully and all have a completely individual private AS number eg router bgp 65001
We are about to bring a new site online and our telco provider has given us the details but this time with an AS number that we have on another site.
When I spotted this and pointed it out, our provider said that it was ok for our sites to have the same AS number and they didn't have to be unique to each site.
Is this correct?

Hi Louis,
It might work, it all depends on the kind of solution you've got in your network.
We all know that BGP loop prevention mechanism works checking the AS-path of the packet and it compares it with your AS. If any of the AS written in the AS-path match with your AS the update will be discarded and your router will not learn that update.
This might represent some problems with LantoLan communications.
According to that, you can use some features that BGP have:
BGP allowas-in: (http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/112236-allowas-in-bgp-config-example.html)
BGP as-override: (https://supportforums.cisco.com/document/87671/understanding-bgp-override-feature)
You can use any of them depending on your network needs.
I Hope this information could be useful to you.
Sorry about my english.

Loop in MPLS

Hi,
Here is the standard MPLS network with vrf INTERNET:
PE2--P2--P1--PE1--INTERNET
|
CPE
PE1-routes
ip route vrf INTERNET 0.0.0.0 0.0.0.0 --> PE1
IP route vrf INTERNET 0.0.0.0 0.0.0.0 -- > INTERNET
vrf INTERNET 10.10.0.0/16 connected
192.168.0.5 is loopback for MP-BGP
PE2-routes
ip route vrf INTERNET 0.0.0.0 0.0.0.0 --> PE1
ip route vrf INTERNET 111.111.111.111 255.255.255.255 10.10.0.10
Route on PE2:
ip route vrf INTERNET 111.111.111.111 255.255.255.255 10.10.0.10 is misconfigured because nexthop is from network 10.10.0.0/16 which is directly is connected on PE1, and address 111.111.111.111 is not active
When I start the ping from CPE to 111.111.111.111 route on PE1 sends packet via P1, and P2 to PE2. PE2 has route that recursively points back to PE1. Ofcourse there is LOOP in routing network.
All devices except CPE are Cisco7600
Here are the questions:
1) With only one continuous PING packet-size 1500Bytes from CPE toward address 111.111.111.111 link utilisation P1-P2 goes up to 150Mbps. And with pings from more CPEs link utilisation rises. Why TTL doesn't prevent increasing of link utilisation?
2) CPU on PE2 goes up to 70-80%. Shouln't Cisco 7600 forward packets in hardware? Or maybe recursive lookup is done with CPU?
output from PE2:
Sh ip route vrf INTERNET 111.111.111.111
Routing entry for 111.111.111.111/32
Known via "static", distance 1, metric 0
Redistributing via bgp 65001
Advertised by bgp 65001
Routing Descriptor Blocks:
* 10.10.0.10
Route metric is 0, traffic share count is 1
PE2#sh ip route vrf INTERNET 10.10.0.10
Routing entry for 10.10.0.0/16
Known via "bgp 65001", distance 200, metric 0, type internal
Last update from 192.168.0.5 6d17h ago
Routing Descriptor Blocks:
* 192.168.0.5 (Default-IP-Routing-Table), from 192.168.0.5, 6d17h ago
Route metric is 0, traffic share count is 1
AS Hops 0
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A

sorry, I made a mistake.
CPE is connected to PE1 like this:
PE2--P2--P1--PE1--INTERNET
|
CPE

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We are running eBGP between two routers. We had a 250 M PVC between the two routers on OSM STM4 ATM interface. We need to increase the B/W between the two routers to 350M, but because there's a limitation on the PVC size for the OSM STM4 ATM card (Max. size of the PVC is 300M), so we created new PVC between the two routers on the same circuits (and of course the same interfaces). Now I need to enable the load sharing between the two routers taking the PVC B/W in consideration.. So I prepared this config, is it right and full? Or Am I missing anything here..
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I implmented these changes but did not work, becuase the command neighbor 10.10.1.1 dmzlink-bw is not supported with multihop (Neighbor 10.10.1.1 ebgp-multihop).
I removed the loopback BGP neighbor and created 2 BGP neighbors with the directly connected interfaces IP's and applied same configuration and now it works fine.. Here's show Ip route (Ip's and AS numbers have been chnaged)
R1#sh ip ro 10.10.0.1
Routing entry for 10.10.0.0/20
Known via "bgp xxxxx", distance 20, metric 10
Tag xxxx, type external
Last update from x.x.x.x 00:00:24 ago
Routing Descriptor Blocks:
* x.x.x.x, from x.x.x.x, 00:00:24 ago
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AS Hops 1
Route tag xxxxx
y.y.y.y, from y.y.y.y, 00:00:24 ago
Route metric is 10, traffic share count is 83
AS Hops 1
Route tag xxxx
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Hello all,
We have an MPLS setup with all remote sites connected to site X and site X is directly connected to site Y(datacenter). Note: site Y is not connected to MPLS cloud till now. All the traffic is passing from site X to site Y for accessing servers at datacenter. site X and site Y are connected to each other through Metro E. Now we have new circuit thats being provisioned for site Y(datacenter) that is going to be connected to MPLS cloud directly. We do bgp peering with ISP for MPLS connectivity and Internal routing protocol is Eigrp. We are doing mutual redistribution at each location for propogating routes at each site. Now the concern is when site Y is connected to MPLS and when I will do the route redistribution on site Y with MetroE connected to site X will there be a routing loop occurance or will there be a best path selection based on the metrics calculated and will choose the optimal path.
Need suggestions. Thanks in advance.

Okay no problem, it can be a little daunting when you haven't done it before :-)
1) No this isn't right.
When I refer to remote sites I mean sites other than X or Y.
So you must not allow either X or Y to redistribute any remote site networks into BGP.
There is no need because the remote site networks should never be advertised from X or Y and if you do allow it it will cause you problems.
So assuming you want X and Y to back each other up -
X redistributes it's own subnets from EIGRP into BGP and also Y's but no remote sites.
Y redistributes it's own subnets from EIGRP into BGP and also X's but again no remote sites.
If X and Y were not connected via a dedicated link this would not be an issue but because they are and they are exchanging EIGRP routes via that link they each receive EIGRP routes for the remote sites networks from each other as well as from their own MPLS router which is why you need to filter.
2) You can do the configuration on your CE routers.
MED or AS prepending are the tools you use.
So at X you advertise it's own subnets without any modification. You advertise Y's subnets but change one of the above BGP attributes.
At Y you advertise it's own subnets without any modification and X's subnets you need to modify the BGP attribute again.
This basically means that traffic will go direct to X or Y as long as their MPLS links are up.
Either should work although it may be worth having a quick chat with your provider and tell them what you are doing to make sure they are not doing anything that would override your settings.
Apart from that internally the L3 switches in X and Y should see their own MPLS routers as the best path to all remote sites because from your description the L3 switch is only one hop away from the MPLS router in X and Y whereas to go via the dedicated link would mean more hops.
I'm assuming it means more hops.
Quick way to check is do a traceroute from X L3 switch to it's MPLS router and then a traceroute from X L3 switch to Y MPLS router and hopefully there should be more hops going to Y.
Do the same from the Y L3 switch.
If it is the same number of hops then you may have to modify the EIGRP metrics.
All of the above about redistribution only applies to EIGRP to BGP.
There is nothing to do about the BGP to EIGRP redistribution.
Concentrate on the redistribution issue because that could seriously affect your network in terms of it working properly.
Regarding the second issue have a chat with your provider about the MED and AS prepending just to clear it with them.
It's not a given that traffic would go in the wrong site, it might, but it wouldn't be the end of the world if you got it wrong temporarily because traffic is doing exactly that to and from the DC at the moment ie. it goes into X and across to Y.
By all means come back if you need more help but like I say focus on the redistribution issue because that really does need addressing.
Jon

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Hi, usually when redistributing from BGP into IGP only EBGP routes are redistributed unless the bgp redistribute-internal command is configured which allows the redistribution of IBGP routes. However when doing the same redistribution on a PE router for a IGP running with the CE it seems that this command is not needed and IBGP routes still get redistributed into the IGP. It works without this command. Does anyone know why this is the case?
I have been trying to understand this for some time now and it seems very trivial however its been bugging me and was hoping someone could clarify. Thx

Hi Vikram,
Well, look at it this way, in the case of MPLS VPN, BGP between the PEs (address-family ipv4 vrf x) is used to transport the customer routes between CEs, and thus it is very logical that it should redistribute all kind of routes (in the case of MPLS VPN the customer routes are always going to be iBGP anyway since they are redistributed from IGP (PE-CE) on another PE - except on the local router when the customer is using BGP as the PE-CE routing protocol) - using another prospective, redistributing iBGP routes into IGPs may cause routing loops to form within an autonomous system, but in the case of PE-CE the redistribution is done into a routing protocol outside the BGP domain (customer side).
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BR,
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192.168.100.0/24 (R6) --ospf-- (R4) --BGP-- (R5) --ospf-- (R7) 192.168.200.0/24
Between R4 and R5 is the core network running ospf (R1 - R3).
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Thanks,
Mike

Hi Mike,
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I have issue Reditributing BGP into ISIS network. I have Router R1 running ISIS Level-2 with a neighborn router R2 ISIS L1-L2. Router R1 has a peering BGP with another router, so I want to redistribute network that receives from peering BGP and anounce to its neighbour ISIS.
Here are go setting, all seems to be ok, but withen I check into R2 network 172.16.40.0 it gets "% Subnet not in table"
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R1#sh isis neighbors
R2 L2   Gi9/3       172.30.100.98   UP    24       R1.09
router isis
net 49.0001.0000.0000.0024.00
is-type level-2-only
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route-map Metricas permit 10
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* i172.16.40.0/24 172.16.40.38             0    100      0 i
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The prefix you're trying to redistribute into IS-IS is coming from an iBGP peering. By default, the BGP process will not redistribute iBGP into an IGP because this scenario can frequently create routing loops. If you really want to do this and you're sure your routing setup is stable, you can override this behaviour by adding the "bgp redistribute-internal" configuration command under your "router bgp" process.
Make sure you issue a "clear bgp ipv4 unicast *" to clear the BGP table once you've made the configuration change.

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An AS owns a /20. He advertises the /20 in BGP to ispA and ispB through policies of his own decision (but he definitely advertises the prefix, and the prefix is propagated). Within the /20 is a /24, which at times he wishes to advertise via ispC. ispC propagates said route upstream and traffic traverses ispC by virtue of longest prefix. No problem.
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What am I missing here?

Hi,
ad
"What I don't get is, given the /20 is still propagated in the internet (routers still have it in their RIB, but install the /24 in the FIB as the preferred route), why should there be downtime.")
I can imagine following scenario:
Let's say ispC is not peering directly to ispA nor ispB but there is ispD (or multiple other ISPs on the path) between them.
So ispC has got the /20 prefix received from ispD in his RIB.
So at the time you sent your /24 withdrawal to ispC, it removes the /24 fro his RIB.
And let's say this withdrawal was not delivered to ispD yet.
So what happeans at that moment when ispD receives a packet with a destination address within the /24 from the Internet?
ispD still has got the /24 prefix in his RIB pointing to ispC. So it forwards the packet to ispC.
But ispC has only /20 in his RIB pointing back to ispD!
So there is a routing loop existing until BGP gets converged by the /24 withdrawal delivery to ispD.
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But showing your /24 might be not reachable from some parts on the Internet until the BGP is converged.
Best regards,
Milan

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Dear Team,
Can u explain how does bgp backdoor link will work ,
Scenario :
Two customer sites connected different PE
CE1>>>PE1(mpls core )PE2>>>CE2 this CE 1 and CE2 have back to back connectivity with Eigrp , how does the loop avoidance happens here

Hello.
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Also, if you have same AS for both CE-PE links, then your CE would prevent the prefix to be learnt due to self AS been found in AS-path.
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RP LES No adjacency                     480          0          0
RP LES TTL expired                        0          0      29428
RP LES Fragmentation failed, DF         346          0       1877
RP LES Features                       18434          0      21821
RP LES Unclassified reason               18          0          0
RP LES Neighbor resolution req         1029         20          0
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running, like while(true)
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public static void main(String[] args)
          ClassA test = new ClassA();
Thread thread = new Thread(test.getInstanceOfClassA());I have never seen this idiom. If ClassA instanceof Runnable, you simply write new Thread(test).
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//I do not know what to put
do not know what to put here
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