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

[ciscomoderator]

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:

www.cisco.com/en/US/tech/tk365/tk80/tsd_technology_support_sub-protocol_home.html.

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.

[josh94950]

Thanks Sandeep. Although the south bound devices are dual homed but for some reasons they are acitve/standby. We will test this out with Active/Active scenario.

Regards,

Josh.

[lee.ajacs]

Hi,

Can you proide any advice on the best method or tools to monitor route changes. We operate an MPLS network and have various routers configured as VPNs for backup. we run BGP and EIGRP and I'm intersted to know how to track the specific routes and any route changes.

Regards,

Lee

[Sandeep Sharma]

Hi Lee

To track route changes you can try the below EEM script that can update you if any new route got added or removed.

event manager applet route-table-monitor

event routing network 0.0.0.0/0 ge 1

action 0.5 set msg "Route changed: Type: $_routing_type, Network: $_routing_network, Mask/Prefix: $_routing_mask, Protocol: $_routing_protocol, GW: $_routing_lastgateway, Intf: $_routing_lastinterface"

action 1.0 syslog msg "$msg"

Hope it answers you query. In case you have any further query please feel free to post.

Thanks & Regards

Sandeep

[sathvik k v]

Hi Sandeep,

The information provide is very helpful thank you.

I have one more query regarding usage of bgp always-compare-med and  bgp deterministic-med.  My understanding is bgp deteministic-med is used for comparison of metric in same AS and bgp always-compare-med is used for comparing metric from different AS. We had a scenario were we observed stale routes in the routing table.  Was informed by TAC  both bgp always-compare-med and  bgp deterministic-med are not required and was asked to disable bgp deterministic med.

Would be helpful  if you can suggest.

Regards,

Sathvik K V

[Sandeep Sharma]

Hi Sathvik,

Your understanding is correct,bgp deterministic-med command ensures the comparison of the MED variable when choosing routes advertised by different peers in the same autonomous system and bgp always-compare-med command ensures the comparison of the MED for paths from neighbors in different autonomous systems.

But there is a way and sequence of comparison depending if both are enabled or either one is enabled. Please follow the link below that will help you understanding the MED comparison.

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094925.shtml

However regarding disabling bgp deterministic med in your condition i am not sure the reason behind it. As it might depends on your situations and you might required both in certain conditions. May be the above link will answer your question. If you wanted me to check that please share the topology and other information related to the problem.

Thanks & Regards

Sandeep

[snarayan62]

Hallo

Sandeep

I was also looking for the solution for similar problem.  Its a helpful post I tried and its working fine in our networks

Thanks for your valuable guidence

Satnarain gautam

[surajit_c]

Hi Sir,

  We have a Cisco ASR1004 router ESP20/RP2/16GB DRAM acting as our ISP gateway router. It is really an excellent machine. With three peering ISPs with full routing table from each the CPU utilization is <1%. First of all I would like to thank Cisco to manifacture such a wonderful product. Anyway let me come to the point.

1) Recently we tried to peer with a new upstream ISP. But they were not being able to inject full BGP table as their next hop router has some limitations. But they were able to advertise our subnets/ASN to the internet cloud. No issue.

But it turned worse when we requested full BGP feed. They requested us to establish a new peering relation with a multihop router. So we removed the old neighbor statement (which was the next hop router) and added a multihop neighbor (with a static route to reach this multihop neighbor). Though full BGP table was received and our subnets/ASN were visible globally. But any request from our subnets were not reachable even to their next hop router.

e.g. if we ping with providers WAN IP address we can reach everywhere in the internet, but if we ping with our subnet IP address we can not even reach providers next hop router. Earlier with the next hop BGP neighbor this was possible.

So they requested us to add an additional bgp neighbor statement with the next hop IP address that we removed earlier.

This let everything worked. I am confused as their remark was - the multihop neighbor was for receiving full BGP feed and the next hop bgp neighbor statement was to allow traffic from our subnets.

2) Earlier we faced similar situation with our previous ISP also. But they only requested us to add just the multihop bgp neighbor statement (also a static route to this multihop neighbor).  No additional next hop neighbor syntax was required to advertise our subnets/ASN.

Please help me to understand the logic behind both the scenery discussed above.

Thanks and regards

Surajit

Assam, India

[Sandeep Sharma]

Hi Surajit,

Thanks for appreciating the cisco products and good to hear that you are satisfied with the ASR routers.

EBGP multihop explanation:

====================

- As you are aware that BGP works on TCP and no direct connectivity is required to build the bgp peering and just need the IP reachability between the BGP speakers/routers.

In case of Ebgp peering we use ebgp-multihop command so that we can customize/change the TTL value.

For better understanding refer the below diagram where we wanted to establish the Ebgp peering between A and C and to achieve this we have to perform 2 things.

RTR A------{RTR B------- RTR C}

CE                            PE's

1. ebgp-multihop command on A (CE) and C (PE) router

2. Static route for reachability of IP on which establishing the BGP peering.

Now coming to your situation why your traffic was getting dropped when you moved to EBGP multihop because your traffic was black-holed on router B/Next hop router (means no routes available on ISP next hop router to reach your network subnets)

As a solution to this problem ISP would have recommended to configure EBGP peering with next hop so this router gets the routing information for you subnets.

However previous ISP hasn’t asked you to configure 2 eBGP peerings as they would have running iBGP session between their routers (Next hop router and EBGP multihop router or router B& C as per my diagram) and by this way ISP next hop router was getting your network subnet information.

Hope it clarifies your doubts. Please feel free to contact in case you have any further query.

Thanks & Regards

Sandeep

[sathvik k v]

Hi Sandeep,

I have one cisco 7609 connected to cisco 7206 over multilple links of different bandwidth. Load sharing works well initially. Whenever there is a flap or link goes down the share count value is changed abnormally. This causes few of the links to be higly utilized and leaving the rest under utilized. bgp dmzlink-bw is enabled globally and dmzlink-bw is enabled per neighbor.

Configuration is as follows.

Router bgp XX

neighbor x.x.x.x remote-as --

neighbor x.x.x.x activate

neighbor x.x.x.x send-community both

neighbor x.x.x.x soft-reconfiguration inbound

neighbor x.x.x.x dmzlink-bw

bgp dmzlink-bw

Regards,

Sathvik      

[Sandeep Sharma]

Hi Sathvik

I think this is nothing to do with BGP and depends on mls cef.

If it seen on the 7600 side you might refer the below  link for understanding the loadbalancing, I have taken the below capture  from the same link that may help you  :

http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_example09186a00800ab513.shtml

==========

The mls ip cef load-sharing simple command gives a better load balance and avoids a new adjacency in the forwarding engine. Also, the mls ip cef load-sharing full command is a load balancing algorithm recommended for a single-stage  CEF that includes a load balancing algorithm for L4 ports. In order to  achieve the best CEF load balancing, alternate L3 and L4 hashing on  access, distribution and core routers, and use this type of  configuration:

On access and core routers -mls ip cef load-sharing simple

On distribution routers - mls ip cef load-sharing full

The mls ip cef load-sharing full command can  improve load balancing if there is a good mix of L4 ports in the  network. With the SRB2 image it can used in all adjacencies such as  ip2ip, ip2tag, tag2tag and tag2ip cases. However, with SRA it works  only with ip2ip, ip2tag adjacency.

===========

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

Thanks & Regads

Sandeep

[sathvik k v]

Hi Sandeep,

The information provide is very helpful thank you.

I have one more query regarding usage of bgp always-compare-med and  bgp deterministic-med.  My understanding is bgp deteministic-med is used for comparison of metric in same AS and bgp always-compare-med is used for comparing metric from different AS. We had a scenario were we observed stale routes in the routing table.  Was informed by TAC  both bgp always-compare-med and  bgp deterministic-med are not required and was asked to disable bgp deterministic med.

Would be helpful  if you can suggest.

Regards,

Sathvik K V

[Sandeep Sharma]

Hi Sathvik,

Your understanding is correct,bgp deterministic-med command ensures the comparison of the MED variable when choosing routes advertised by different peers in the same autonomous system and bgp always-compare-med command ensures the comparison of the MED for paths from neighbors in different autonomous systems.

But there is a way and sequence of comparison depending if both are enabled or either one is enabled. Please follow the link below that will help you understanding the MED comparison.

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094925.shtml

However regarding disabling bgp deterministic med in your condition i am not sure the reason behind it. As it might depends on your situations and you might required both in certain conditions. May be the above link will answer your question. If you wanted me to check that please share the topology and other information related to the problem.

Thanks & Regards

Sandeep

[Mahabir Prasad]

Hi Sandeep

Can you throw some light on ospf specific bgp attributes and how they are used in ospf superbackbone?

Thanks

Mahavir

[Sandeep Sharma]

Hi Mahabir

As per my understanding your query is  related to the designing and  ASBR that will run BGP with other ASBRs  external to the AS and OSPF as its IGP. For related to this you can find the compete details in RFC1403. Please refer the link below and feel free to contact in case you need any clarification.

http://tools.ietf.org/html/rfc1403

Thanks & Regards

Sandeep

[Mahabir Prasad]

Hi Sandeep

Thanks for the pointing in to right direction.

Regards

Mahavir