Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

BGP

Hi experts,

We have the following senario and I need a sloution for it please.

.

The current senarion is one router installed in HQ for each romote sites and BGP is configured among them.

The needed senarion:

On 3845 router installed in HQ and connected to more than 4 remote sites, each remote site has it is own AS.

How can we configure the HQ router wich connected to more than router with more than AS.

Thanks... 

10 REPLIES
Cisco Employee

Re: BGP

Hello,

In short - you cannot. A router can run only one BGP instance and therefore belong only to a single AS. Within BGP paradigm, the boundary between autonomous systems is on links, not on routers.

Having each of your routers running BGP in a different autonomous system should not be a problem at all. Do you have a problem configuring your router to run BGP peering with other neighbors in different ASes?

Best regards,

Peter

New Member

Re: BGP

Thanks for your reply..

I just want you to know that now we have three routes in HQ connected to three remote sites each router has it is instant . Now we want to bring a big router and plug all the links to it.

Regards..

Ammar Al Hayyani

IP Telephony Specialist

Information Technology

Abu Dhabi Ports Company

Phone +971 2 695 2142   +971 50 6146553

Fax +971 2 695 2111

Email ammar.alhayyani@adpc.ae

PO Box 54477, Abu Dhabi, UAE

Website www.adpc.ae

Hall of Fame Super Silver

Re: BGP

Hello Ammar,

as Peter has noted building 3-4 eBGP sessions is not a problem.

Use a new and different AS number on HQ router.

router bgp 65001

neigh 10.10.10.2 remote-as 65002

neigh 10.10.20.2 remote-as 65003

neigh 10.10.30.2 remote-as 65004

neigh 10.10.40.2 remote-as 65005

some notes :

use private AS numbers in range (64512-65535) it can be helpful for the future.

if you want to build sessions using loopbacks you need to provide:

static routes to tell how to reach the BGP endpoint

you need to use

conf t

ip route 20.20.20.2 255.255.255.255 10.10.10.2

router bgp 65001

neigh 20.20.20.2 remote-as 65002

neigh 20.20.20.2 update-source loop1

neigh 20.202.20.2 ebgp-multihop 2

Hope to help

Giuseppe

Cisco Employee

Re: BGP

Giuseppe,

Thanks a lot for adding to my answer.

In situations where two directly connected routers should be peered in eBGP using their loopbacks, I often like to use the disable-connected-check option instead of ebgp-multihop. The disable-connected-check skips the test whether the peer is on a directly connected network but still sends the TCP packets using the TTL=1.

Best regards,

Peter

VIP Super Bronze

Re: BGP

paluchpeter wrote:

Giuseppe,

Thanks a lot for adding to my answer.

In situations where two directly connected routers should be peered in eBGP using their loopbacks, I often like to use the disable-connected-check option instead of ebgp-multihop. The disable-connected-check skips the test whether the peer is on a directly connected network but still sends the TCP packets using the TTL=1.

Best regards,

Peter

Hello Peter,

Good seeing you on the forum.

Regarding BGP disable-connected-check, this command has to deployed in addition to ebgp-multihop command not instead.

So, I am wondering, what is the use of disable-connected-check command if you have to use it in conjunction with ebgp-multihop.  You may as well just use "ebgp-multihop 2"and not worry about disable-connected-check command at all.

Thanks,

Reza

This command is required only when the neighbor ebgp-multihopcommand is configured with a TTL value of 1. The address of the single-hop eBGP peer must be reachable. The neighbor update-sourcecommand must be configured to allow the BGP routing process to use the loopback interface for the peering session.

Examples

In the following example, a single-hop eBGP peering session is configured between two BGP peers that are reachable on the same network segment through a local loopback interface on each router:

BGP Peer 1

Router(config)# interface loopback 1 

Router(config-if)# ip address 10.0.0.100 255.255.255 

Router(config-if)# exit 

Router(config)# router bgp 64512 

Router(config-router)# neighbor 192.168.0.200 remote-as 65534 

Router(config-router)# neighbor 192.168.0.200 ebgp-multihop 1 

Router(config-router)# neighbor 192.168.0.200 update-source loopback 2 

Router(config-router)# neighbor 192.168.0.200 disable-connected-check

Router(config-router)# end 
Cisco Employee

Re: BGP

Hello Reza,

Thank you for your kind words - I am also glad to have found some time to visit the forum again.

Regarding the snippet from the command reference you have posted - I believe it should be understood in the following way: The disable-connected-check is meaningful only if the ebgp-multihop is set to 1. Having the ebgp-multihop set to higher value than 1 also implies the disable-connected-check though it may not be visible in the configuration.

Note that for eBGP peers, the ebgp-multihop 1 is the default value:

R2(config)#router bgp 1
R2(config-router)#nei 1.2.3.4 remote-as 2
R2(config-router)#nei 1.2.3.4 ebgp-multihop 1
R2(config-router)#exit
R2(config)#do show run | sec router bgp
router bgp 1
no synchronization
bgp log-neighbor-changes
neighbor 1.2.3.4 remote-as 2
no auto-summary
R2(config)#

So - yes, the command reference is correct, the disable-connected-check should be used with ebgp-multihop 1, however, that is already the default value with eBGP peers. It does not have any effect with ebgp-multihop set to 2 or more.

Does this make more sense?

Best regards,

Peter

VIP Super Bronze

Re: BGP

paluchpeter wrote:

Hello Reza,

Thank you for your kind words - I am also glad to have found some time to visit the forum again.

Regarding the snippet from the command reference you have posted - I believe it should be understood in the following way: The disable-connected-check is meaningful only if the ebgp-multihop is set to 1. Having the ebgp-multihop set to higher value than 1 also implies the disable-connected-check though it may not be visible in the configuration.

Note that for eBGP peers, the ebgp-multihop 1 is the default value:

R2(config)#router bgp 1
R2(config-router)#nei 1.2.3.4 remote-as 2
R2(config-router)#nei 1.2.3.4 ebgp-multihop 1
R2(config-router)#exit
R2(config)#do show run | sec router bgp
router bgp 1
 no synchronization
 bgp log-neighbor-changes
 neighbor 1.2.3.4 remote-as 2
 no auto-summary
R2(config)#

So - yes, the command reference is correct, the disable-connected-check should be used with ebgp-multihop 1, however, that is already the default value with eBGP peers. It does not have any effect with ebgp-multihop set to 2 or more.

Does this make more sense?

Best regards,

Peter

Hello Peter,

Yes, it does make sense.  I think what is confusing about ebgp-multihop command is that usually a default command is not a command you manually have to add.  for example when you create a new BGP session as the one you did here, the command "bgp log-neighbor-changes" and/or "no auto-summary" is already included so you don't need to added them manually.  But in the case ebgp-multihop you have to added it with value on 1 and then it does not show it in the config.  The other interesting part of this command is that if you enter ebgp-multihop without any values, it will assign 255 which is the highest.

T-1(config-router)#neighbor 10.10.10.1 remote-as 100  
T-1(config-router)#neighbor 10.10.10.1 ebgp-multihop  
T-1(config-router)#do sh run | sec bgp                
router bgp 65222
neighbor 10.10.10.1 remote-as 100
neighbor 10.10.10.1 ebgp-multihop 255

Thanks,

Reza

Cisco Employee

Re: BGP

Hi Reza,

I would personally put it down this way:

    1. There are default commands which are visible in the configuration, and there are default commands which are invisible. This is a matter of the configuration parser in IOS and there is little to be done about it. It is in my opinion one of the confusing things about IOS CLI - it's not consistent.
    2. For eBGP peers, the ebgp-multihop 1 is the default setting.
    3. Specifying only ebgp-multihop without specifying the TTL value causes the TTL 255 to be used. Obviously, we have to distinguish between a default (i.e. used when not explicitely specified) command, which is the ebgp-multihop 1, and a default parameter of an explicitely specified command ebgp-multihop, which is 255.

Best regards,

Peter

Re: BGP

Hi Peter,

I agree, just one comment:

ad 1. default commands which are visible in the configuration:

IMHO, commands which are default in the latest IOS but were not in some previous IOS releases are visible and inserted automatically to keep backward compatibility.

BGP "no auto-summary" is a good example - command default behavior changed to disabled since 12.2(8)T.

BR,

Milan

Cisco Employee

Re: BGP

Hi Milan,

You're absolutely correct. That makes perfect sense. Thanks!

Best regards,

Peter

782
Views
5
Helpful
10
Replies