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Ask the Expert: Configuring and Troubleshooting Border Gateway Protocol (BGP)

Community Manager
Community Manager

Configuring and Troubleshooting Border Gateway Protocol (BGP)With Sandeep Sharma

Welcome to the Cisco Support Community Ask the Expert conversation.  This  is an opportunity to learn and ask questions about issues encountered while configuring and troubleshooting Border Gateway Protocol (BGP) across various Cisco platforms with expert Sandeep Sharma.

BGP is the most widely deployed routing protocol across service provider and enterprise networks.

For more information, visit the introduction to Border Gateway Protocol at:

Sandeep Sharma is a customer support engineer in the High-Touch Technical Services Routing Protocols team based in Bangalore. He provides support to major service providers and enterprise customers for routing and MPLS technologies. He has more than seven years of experience working with large enterprise and service provider networks. He also holds a CCIE certification (#39002) in routing and switching.

Remember to use the rating system to let Sandeep know if you have received an adequate response. 

Because of the volume expected during this event, Sandeep might not be able to answer every question. Remember that you can continue the conversation in the Network Infrastructure community, subcommunity WAN, Routing, and Switching shortly after the event. This event lasts through October 4, 2013. Visit this forum often to view responses to your questions and the questions of other Cisco Support Community members.

38 Replies 38


Hi Sandeep,

PFA My NW diagram with proposed link.

We are using three ISPs bandwith with eBGP, we have our own IP address and ASN.

Now we are going to start another site with different location with same ASN.

Router A NW IP : advertised with Three ISP

Router A (ASN ) 23456

Router B NW IP  advertised with another ISP in different location with same ASN.

Router B (ASN) 23456

When another ISP b/w goes down then i need my all the traffic going via iBGP (Router A).

My Requirement when link goes down between Router – B to another ISP (Proposed) then my all the traffic working via iBGP.

So what configuration in my both the Router A & B to fulfill my requirement.

Thanks in ADV,


Below is the response on the basis of my understanding to your query:

For giving preference to exit traffic at site A you can use weight attribute as all the ISP's are connected on the same router.

And for influencing the exit traffic for site B you should use the local preference below is the configuration for router B.

router bgp 23456

neighbor remote-as  23456

neighbor remote-as

neighbor route−map setlocalin in         >>>>>>>>. to apply local preference

ip as−path access−list 7 permit ^$       >>>>>>>>> route learned from the ISP4 will be matched here

route−map setlocalin permit 10

match as−path 7

set local−preference 400

route−map setlocalin permit 20

set local−preference 150

>>>>> you can also use default local preference command in place of using AS-path to simplify and you want to use for whole traffic.

- If you want to infulence the incoming traffic you can use MED attribute.

In case you have any specefic query and not answered here please feel free to ask again.

Thanks & Regards


Hi Sandeep,

Thanks for your great help...

Bellow config i have to config in my Router A right ??

I am bit confused If as per your suggested config in Router A , becasue why ISP-4 configuration in Router A becasue it is not directely conneceted with Router A it is connected directely with Router B.

Great help pl clear my dought.

Hi ,

This is router B configuration not router A. I also mentioned in above reply aswell

Please feel free to contact in case you have any furhter query

Thanks & Regards


Hi Sandeep,

Just for clarification you are mention for router B as given bellow.

router bgp 23456

neighbor remote-as  23456 <----------------

I think " neighbor


You are correct just a typo, it will be the IBGP peer IP address and for RTRB router peer is RTRA. so this is RTRA-IP .

Thanks & Regards


Mohammed Imran Khan
Cisco Employee
Cisco Employee

Hello Sandeep Sir,

Just wanted to know what is a BGP slow-peer and what ar/is the way to mitigate this issue.


Hi Imran

BGP Slow peer cases often are reported as "missing update", "slow update", "stopped update" or "session flap due to Hold timer expiry when local BGP is not able to send the updates to neighbor for the time interval of hold time" type issues, rather than being identified as a slow peer issue by the customer.

You can confirm that a case is due to a slow peer by issuing

    show ip bgp all summary

and watching the routing table versions associated with various neighbors. The problem neighbor's version will typically increase slowly, if at all, and frequently, but not always, have a large outQ of unsent BGP messages.

The command

    show ip bgp all update-group

    show ip bgp update-group

Will show you which neighbors are in which update-group. A slow peer only impacts neighbors in the same update-group. If there is more than one update-group, you can check and make sure that the impacted neighbors are indeed in the same update-group as the slow peer.

If a * is marked in front of the neighbor then that shows that updates are being sent to the neighbor. If the * mark is not removed for a period of a minute then it must be a slow peer.

One way to find the slow peer is issue

    show ip bgp neighbor

    show ip bgp neighbor

Look for "Keepalives are temporarily in throttle due to closed TCP window" or TCP receive Window Size is very low or Zero. Repeat this for all the neighbors in the update group. If a neighbor displays above message then it might be a probable slow peer. Coupls of reasons for slow peer might be

    There is packet loss and/or high traffic on the link to the peer and the throughput of the BGP TCP connection is very low.

    The peer is heavily loaded in terms of its CPU and cannot service the TCP connection at the required frequency.

You can try few wokraround to fix the slo peer issues like :

- If IOS version doesn't support the Slow peer detection & protection feature then identify the slow peer from the steps listed above and move the slow peer to different update-group group by configuring dummy policy or by changing "advertisement-interval" interval different than the rest of the neighbors "neighbor advertisement-interval

How to mitigate the slow peer:


- While fully resolving a slow peer situation requires addressing the issue which is causing it to be slow, such as packet loss between the RR and the peer, or an overloaded CPU on the slow peer, you can mitigate the problem my moving the individual peer into its own update group, so that its slowness does not impact other peers.

- More recent Cisco IOS releases contain automatic slow peer mitigation features which can be turned on.

- for Older releases which do not contain these features. To mitigate a slow peer on these older releases, you need to change the configuration so that the slow peer is forced into its own update-group. you can do this by configuring a dummy route-map and apply it to just the one peer. You may need to remove the peer from a peer-group or other shared configuration in order to do this.

Moving a neighbor into its own update-group causes the router to engage in additional processing, which will increase CPU utilization and memory consumption.

Hope this answers your query, In case you have anny further query please feel free to post.

Thanks & Regards


Thanks Sandeep! That was a clear and lucid explanation. It was helpful!!!



HI Sandeep,

Thanks for open up this discussion on BGP actually I’m looking for a BGP solution, my query as below   

If we have 2 WAN routers and a single MPLS connectivity running BGP AS 200, then how we can use our both WAN routers to get hardware redundancy, as service provider is not ready to give duel BGP peer on single link. 

Attaching diagram for more clarity


Hi Sanjeev,

With reference to you query it is not recommended to use BGP in this setup because as a best practice BGP is a viable solution when used in dual home scenario so here you can configure IGP with your service provider. Or if you wanted to run BGP you have to ask for 2 eBGP peering with provider.

However if you are keen to run BGP with in the specified conditions you can try a workaround of running eBGP peering on HSRP/VRRP virtual IP but it will cause the delay and only the session initiated by provider router will establish the BGP. You can minimize the delay upto some extent by changing the HSRP and BGP timers.

But apart from delays there will be one problem that your eBGP session from the standby router will be in active state and keep on probing and I think would not be acceptable. This is not a recommended solution and just a workaround.

Hope it answers your query.

Thanks & Regards


Hi Sandeep,

Simulated in lab, results are same as you mentioned, thanks for your inputs.




We have a VSS domain, with 2 BGP upstream connections (to the same AS), one on each domain-switch...
In BGP we set maximum-paths 2 I'd like to know if there is a way to load-balance over both links outgoing traffic.
I do see both bgp routes in the routing table but VSS is prefering the link on the active switch (as expected I guess), is there a way to overide this behaviour and send traffic over the vsl-link to the other link?  (don't feel for manipulating bgp attributes for half of the routes).



Hi Josh,

First of all in order to utilize both L3 links, you need to make sure that devices are dual-homed to both VSS chassis with Multi-Chassis EtherChannel (MEC), otherwise traffic will only be sent out from the local chassis which is an expected behavior of VSS.

I have seen the similar issue earlier where customer has the single connectivity between LAN and VSS core and soon as he connected to both VSS switches it started load balancing.

If in your case you already have the dual-homed (between VSS core and LAN).Please share the below captures

- show ip bgp    (from vss) and specify any route

- show ip route   (from vss) for the same route in above capture

- traceroute from your VSS switch and LAN to any IP address in outer segment ( from VSS switch, machine and switch below VSS domain in LAN)

Please feel free to contact in case you have any further query.

Thanks & Regards


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