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

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[Vinit Jain]

I believe there is a typo in the question. the sub-command would be ebgp-multihop. ebgp-multihop command is used when the EBGP peers are not directly connected or are connected via loopback. As a best practice, the hop-count of the BGP peer should be specified. If the hop-count is not specified, then the maximum value would be set to 255. 

In an ideal condition, the EBGP peers will not be so many hops away.

[Monica Lluis]

Another question:

Can your router-id and update source interfaces be different?

[arvindsingh1989]

Yes it can. 

[Vinit Jain]

Yes, both can be different. 

[Monica Lluis]

Another question:

Can you configure more than one Route Distinguisher attribute for a network?

[Vinit Jain]

A route distinguisher (RD) is unique to VRF and router. So there can be multiple customers who might have same internal network subnets but will be configured under different VRFs on the PE and thus be under different RD values.

[Monica Lluis]

Another question:

ISP limits to send prefixes and if prefixes reaches its maximum. BGP goes down... How
can we check how many prefixed ISP have set?

[Vinit Jain]

when the maximum-prefix command option is specified, the default behavior is to reset the BGP neighborship. The behavior can be modifed as well from the command line. 

show ip bgp neighbor <neighbor-ip> routes command can be used to identify the routes received from the peer.

The show ip bgp summary command will also show the number of prefixes received from a peer.

[Monica Lluis]

Another question:

When is next-hop self needed in BGP?

[Vinit Jain]

next-hop-self command is generally used on the ASBR routers. When a router is having an IBGP connection on one side and EBGP on the router side and the Internet AS network does not know how to reach the external link, next-hop-self command can be used. This tells the peers (IBGP peers) that in order to reach an external subnet advertised by me, I am the next-hop. I have the information on how to reach to that subnet/prefix.

R1-----IBGP------R2--------EBGP-------R3.

Hope this clarifies.

[Monica Lluis]

Another question from the live audience

What is use of "send-community extended " command in BGP?

[arvindsingh1989]

This command is used to advertise the extended communities like RT to the neighbor. By-default  BGP extended community is not advertise to neighbor. 

[Vinit Jain]

This command is used in Cisco IOS to advertise extended BGP communities to the peers. This command is generally configured for peers under the vpnv4 address-family. If both option is configured, then BGP advertises both standard as well as extended BGP communities.

[Monica Lluis]

Another question from the live event:

When is  MP-BGP  used?

[arvindsingh1989]

It is used to carry LDP labels.