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

[Monica Lluis]

Welcome to this Cisco Support Community Ask the Expert conversation. This is an opportunity to learn and any ask questions about how to configure and troubleshoot and design netowrk using  Border Gateway Protocol with Cisco expert Vinit Jain.

Ask questions from Tuesday Janury 12 to Friday January 22, 2016

Border Gateway Protocol (BGP) is one of the most widely deployed protocols in networks today and is the definitive routing protocol in the Internet. BGP is a flexible protocol, in that a variety of options are available to network engineers. Furthermore, extensions and implementation enhancements make BGP a powerful and yet complex tool. Because of BGP's pervasive deployment across multiple networks and the large number of use-cases that BGP can be applied to, it is becoming increasingly important to understand how to troubleshoot BGP issues quickly and easily. Viinit will be helping you answer your queries on how to troubleshoot BGP

Vinit will be helping you with all your queries on all of the above.

Vinit Jain presented at  Cisco Live in June 2015 on Troubleshooting BGP 
Click here for More Information

Vinit Jain, 3X CCIE #22854 is a Technical Lead in HTTS (High Touch Technical Support) team supporting customers in areas of routing, MPLS, TE, IPv6, multicast and a wide variety of platform issues like High CPU, Memory leak, etc IOS, IOS XE, IOS XR and NxOS code base. Has been delivering trainings within Cisco on various technology as well as platform troubleshooting topics. He has also written workbook on IOS XR fundamentals on Cisco Support Community. Vinit has CCIE in R&S, SP and Sec and holds multiple certifications on programming and databases.

Vinit  might not be able to answer each question due to the volume expected during this event. Remember that you can continue the conversation on the WAN, Routing and Swiching sub-community

Find other  https://supportforums.cisco.com/expert-corner/events.

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[Monica Lluis]

Another question from the audience:

Can we configure S2S VPN tunnel over BGP?

[arvindsingh1989]

yes, it is possible because BGP not use multicast like IGP protocols EIGRP or OSPF.  

[Monica Lluis]

Another question from the live audience:

Can we have multiple BGP processes running on a router or a L3  catalyst switches?

[Vinit Jain]

No, there can only be one BGP process that we can run per router / switch.

[Monica Lluis]

another question from the live audience:

when should we use the Community attribute? Please explain any scenario.

[Vinit Jain]

BGP Community attributes are useful in controlling BGP Routing Policies. There are some pre-defined BGP communities which perform certain tasks. 

Please refer to the below CCO document.

http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/28784-bgp-community.html

http://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_6-2/bgp_communities.html

[Monica Lluis]

Another question from the live event:

How best should I influence my iBGP to select the default route learned from a
multi-homed eBGP with my ISP. Where my iBGP router 1 would prefer eBGP gateway 1 to my
ISP and my iBGP router 2 would prefer eBPG gateway 2 to my ISP.?

[Monica Lluis]

Another question from the live audience:

What is the point of TTL value while configuring eBGP? s]ShouldIi use "ebgp multihop 2" 

command or disable directly command check command while configuring eBGP over loopbacks?

[Vinit Jain]

As a best practice, disable-connected-check command should be used than using ebgp-multihop 2 when peering over loopbacks for a directly connected eBGP peer.

[Monica Lluis]

another question:

Can you give us some real life  examples of when to use AS confederations?

[Monica Lluis]

Another question from live webcast:

When configuring BGP to import/ export VRFs, can I configure multiple BGP AS systems
to separate out different types of services / networks?

[Vinit Jain]

Globally, you can only configure one AS per BGP process. But if you want to hide your AS number, you can use neighbor local-as local-asn command. But there are certain limitations that you need to keep in mind when you configure this command:

Local-AS cannot be customized for individual peers in a peer group.

Local-AS cannot have the local BGP protocol AS number or the AS number of the remote peer.

The local-as command is valid only if the peer is a true eBGP peer. It does not work for two peers in different sub-ASs in a confederation.

Hope this clarifies.

[Monica Lluis]

Another question:

Is there a way to view advertised routes before they hit our OUTBOUND fillers and
also after they hit OUTBOUND filters ?

[Vinit Jain]

You can view your BGP table using the show ip bgp command, which is most likely the prefixes that are supposed to be advertised to the BGP peers (unless you are advertising IBGP learnt prefixes to another IBGP peer without RR deployment). after applying filters such as permitting or denying prefixes using route-map, you can view what you are advertising to the peer using show ip bgp neighbor <> advertised-routes command. Though, one important thing to remember is that if you are setting any attribute value, those wont be seen locally on the router using this command. It can be verified from the remote peer only.

Hope this helps.

[Monica Lluis]

Another question from the live event:

When do you use the iBGp multihop subcommand of 255?