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Need Help with BGP

Hello Experts @Georg Pauwen  @balaji.bandi  @paul driver  @Richard Burts  @MHM Cisco World 

 

I have two sites where underlying ospf is configured and then BGP is configured.  Site A is advertising 189.75.62.x/24 to BGP and Site B is advertising 189.75.61.0/24 to BGP and its working fine. 

 

Site A is going to decommission so i want to use subnet 189.75.62.0/24 at site B . I tried following things

1. On site A, removed command network 189.75.62.x/24 and added same command at site B and refresh the BGP topology but it stopped working and subnet 189.75.62.x was totally unreachable.

 

Please let me know how should i approach this issue?

 

I also need to help with commands to get following results:

1. BGP command to show neighborship 

2. BGP command to show advertise routes

3. BGP command  to show learning routes 

 

Thanks 

 

Thanks

4 Accepted Solutions

Accepted Solutions

Hello @LovejitSingh1313 ,

for site B router to be able to advertise in eBGP the new subnet it must know of it via OSPF.

So on site B border router you should verify

show ip route 189.75.62.0

you should learn the /24 prefix via OSPF via the Site A - Site B link.

 

if site B border router has no knowledge of the subnet it will not advertise it.

On the other end, if on site B you have created a new SVI referring to the subnet 189.75.62.0/24 your issue is a duplicated address.

 

Another possible option is that the subnet is actually subnetted and from OSPF you learn subnets of it.

If this is the case either use the aggregate-address command in BGP or create a static route to null 0 with high AD in order to have the prefix advertised as a /24.

 

Hope to help

Giuseppe

 

View solution in original post

I believe that @Giuseppe Larosa is on the right track, one of the essential requirements for BGP to advertise a prefix using the network command is that there must be an entry in the IP routing table for the prefix (and matching network mask). If there is not a matching entry in the IP routing table then BGP will not advertise the prefix, even though there is a network statement in BGP. 

 

I do not necessarily agree that the solution is for OSPF to advertise the prefix to B. To give good advice about how to get B to advertise 189.75.62.0/24 we need a better understanding of the topology of A and of B, and in particular where 189.75.62.0/24 is connected.

HTH

Rick

View solution in original post

why Null0?
because  it use as Aggregate route i.e. if there are many route/prefix share same NET then you can config one static NET and advertise it through BGP as single route/prefix.

why BGP not work before?
it the tech use by BGP to be sure that the route in found in RIB before advertise it.
if it not found then BGP will never advertise.

View solution in original post

Hello @LovejitSingh1313 ,

modern BGP implementations default to

no auto-summary

 

this means that an exact match for the prefix needs to be found in the IP routing table in order for BGP to be able to advertise it.

 

>> If this is the case either use the aggregate-address command in BGP or create a static route to null 0 with high AD in order to have the prefix advertised as a /24.

You have found the following:

 

>> ip route vrf INTERNET 189.75.62.0 255.255.255.0 Null0 254 permanent

 

This means the prefix is actually subnetted in more specific subnets and the static route above is one of the two methods to build an aggregate route that can be advertised using network 189.75.62.0 255.255.255.0.

The other one if you are interested is to use the BGP aggregate-address command

 

router bgp <YourASN>

address-family ipv4 vrf INTERNET

aggregate-address 189.75.62.0 255.255.255.0

 

However, the aggregate-address would disappear if all the component subnets are removed from BGP table.

Here the static to Null0 with an high AD is used to create a permanent stable BGP advertisement.

The use of the keyword permanent in this case being a route to null0 that always exists can be considered not necessary.

 

Hope to help

Giuseppe

 

View solution in original post

15 Replies 15

balaji.bandi
Hall of Fame
Hall of Fame

After the advertisement have you done the reset the BGP soft - both Site A and Site B

 

clear ip bgp x.x.x.x  soft

then check it should be learning only the place you made a network advertised,

 

still an issue post the config of BGP both the side

 

here is BGP troubleshooting commands :

 

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/19345-bgp-noad.html

 

BB

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I already tried this after removing the command from site A but it does not help. 

 

I will post config shortly.

 

Thanks 

After remove the network, did you anounce other BGP process ?

 

BB

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Hello @balaji.bandi 

 

what does announcing BGP process means, I added same command on site B BGP config ?

yes network anouncement i mean.

 

you can put the network x.x.x.x/24 on site A ( where the side B removing side)

and rest the BGP soft reset to take effective.

Site B should be removed this network anouncing now - Site A should anounce this network.

 

post the config - also look any default route point to SITE B ?

BB

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Hello @LovejitSingh1313 ,

for site B router to be able to advertise in eBGP the new subnet it must know of it via OSPF.

So on site B border router you should verify

show ip route 189.75.62.0

you should learn the /24 prefix via OSPF via the Site A - Site B link.

 

if site B border router has no knowledge of the subnet it will not advertise it.

On the other end, if on site B you have created a new SVI referring to the subnet 189.75.62.0/24 your issue is a duplicated address.

 

Another possible option is that the subnet is actually subnetted and from OSPF you learn subnets of it.

If this is the case either use the aggregate-address command in BGP or create a static route to null 0 with high AD in order to have the prefix advertised as a /24.

 

Hope to help

Giuseppe

 

I believe that @Giuseppe Larosa is on the right track, one of the essential requirements for BGP to advertise a prefix using the network command is that there must be an entry in the IP routing table for the prefix (and matching network mask). If there is not a matching entry in the IP routing table then BGP will not advertise the prefix, even though there is a network statement in BGP. 

 

I do not necessarily agree that the solution is for OSPF to advertise the prefix to B. To give good advice about how to get B to advertise 189.75.62.0/24 we need a better understanding of the topology of A and of B, and in particular where 189.75.62.0/24 is connected.

HTH

Rick

Hello,

 

--> 

Site A is going to decommission so i want to use subnet 189.75.62.0/24 at site B . I tried following things

1. On site A, removed command network 189.75.62.x/24 and added same command at site B and refresh the BGP topology but it stopped working and subnet 189.75.62.x was totally unreachable.

 

189.75.62.x is unreachable from where ? Is that network locally configured on site B ?

Hello @Georg Pauwen , 

 

 

Site B is connected with ASR router to receive and send traffic.

 

siteA --> siteB --> ASR router at site B to go to internet 

 

These are Public subnets we used for Internet connectivity.

 

Thanks 

Can you draw the topology 

..

Hello


@LovejitSingh1313 wrote:

I have two sites where underlying ospf is configured and then BGP is configured.  Site A is advertising 189.75.62.x/24 to BGP and Site B is advertising 189.75.61.0/24 to BGP and its working fine. 

 


What springs to my mind straight away for me is a bgp feature called BACKDOOR however at this staged I would agree with @Richard Burts not enough information is known about your topology -  okay you have two sites advertising two subnets into bgp - stie A 189.75.61.0/24 & site B 89.75.62.0/24 but what you don't mention is the ospf and bgp relationships?

Is ospf between the two sites
Is the bgp peering between the two sites (ibgp)
Is the bgp peering between the two sites via a isp ASN (ebgp)
Does each site have the same ASN or different ASNs?

As such I believe posting a topology diagram would be beneficial in finding a solution for your issue


Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul

Hello @balaji.bandi @Giuseppe Larosa @Richard Burts @MHM Cisco World @paul driver 

 

 

In our topology ospf is configured as backbone and advertising loopback 0 which is utilized for bgp neighborships and then we using MPLS setup and advertising internet within Internet vrf. 

 

SiteA - site B vrf Internet got ibgp connectivity within MPLS and then site B got ebgp connectivity within INTENRNET vrf 

 

I figured out, the reason why I was not able to move the subnet to site B was because I was not moving the static null route from siteA to site B for that subnet.

ip route vrf INTERNET 189.75.62.0  255.255.255.0 Null0 254 permanent

 

Can anybody explain what this null route do,  why without it Subnet was not working on site B?

 

Thanks 

 

 

 

 

 

why Null0?
because  it use as Aggregate route i.e. if there are many route/prefix share same NET then you can config one static NET and advertise it through BGP as single route/prefix.

why BGP not work before?
it the tech use by BGP to be sure that the route in found in RIB before advertise it.
if it not found then BGP will never advertise.

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