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Border Gateway Protocol (BGP) Fundamentals and Troubleshooting - FAQ from Live Webcast

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Introduction

 Cisco Ask the Expert      

              Vinit Jain is a technical lead with the High-Touch Technical Support (HTTS) team supporting customers in areas of routing, MPLS, TE, IPv6, and multicast. He also supports a wide variety of platform issues such as high CPU; memory leaks; Cisco IOS, IOS XE, and IOS XR Software; and NxOS code base. He has delivered training within Cisco on various technologies as well as platform troubleshooting topics. He has also written a workbook about Cisco IOS XR Software fundamentals on the Cisco Support Community.
Vinit has expertise in troubleshooting Service Provider, Enterprise and Data Center environments and holds holds CCIE certification (no. 22854) in R&S, Service Provider, and Security, as well as multiple certifications on programming and databases.

 

You can download the slides of the presentation in PDF format here. The related Ask The Expert sessions is available here. The complete recording of this live Webcast can be accessed here.

 

Border Gateway Protocol (BGP) Fundamentals and Troubleshooting

 

Q. What values can we set for SPD, Input Queues and small buffers in heavy bgp environments?

A. http://www.cisco.com/web/about/security/intelligence/spd.html see this link it has very useful info, let me know for any follow up question/put it on the community.

 

 

Q.  Why we need IBGP however IGP can also serve the same?

A: BGP is open standard, scalable, and available attributes helps tweak incoming and outgoing traffic. In IGP you can influence egress routes but no knob to influence neighbor's decision.

 

Q.  Can you repeat the alternative to GNS?

A: You can use VIRL.

 

Q.  Are the Open packets the equivalent to the TCP 3-way handshake?

A: Yes, BGP uses tcp port 179 to communicate with neighbor and 3 way handshake is underline mechanism.

 

Q.  What is use of IPV4 UNICAST cmd?

A: BGP supports multiprotocol.. it means you can redistribute eigrp/rip etc into bgp, so the command ipv4 unicast prepares the router for this capability
Refer this link for more info, -http://www.cisco.com/c/en/us/td/docs/ios/iproute_bgp/command/reference/irg_book/irg_bgp1.html#wp1110597

 

Q.  Do you have any quick troubleshooting commands ?

A: Troubleshooting BGP
http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/22166-bgp-trouble-main.html

Q.  In the BGP route-map example, what is the local-preference attribute used for?

A: Local preference helps determine which path to choose for exiting should you have multi path available.. higher the better and default value is 100.

Q.  I have a question, at CE end ASR9k & i'm not aware of PE end, my bgp session is being established but gets disconnected after 90 sec. automatically, what could be the issue here?

A: 1) http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/116377-troubleshoot-bgp-mtu.html 2) debug ip bgp 3) ensure route to neighbor is not flapping 4) layer 1 issue/ match config with provider 5) high cpu verification are good starting

 

Q.  Can we use bfd over bgp to improve convergence time?

A: Yes, and here is good doc for the same http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bfd/configuration/15-mt/irb-15-mt-book/irb-bi-fwd-det.html#GUID-228FE545-B864-4819-ABDA-BC915874384E

Q.  What is use of send-community cmd in BGP?

A: ISP can distinguish routes from various customers and apply policies accordingly, its easy way to recognize bunch of routes collectively.. so send-community is used to send that info out for provider to recognize it and take action if any.

Q.  We have ospf internal and bgp external - could we have some troubleshooting commands to see each customer receive routes please?

A: For bgp : show ip bgp neigh x.x.x.x receive routes and show ip bgp neigh x.x.x.x adv routes. for ospf you have to ensure you are advertising desired networks and they are in ospf database "show ip ospf database"

Q.  What is the best way to influence inbound traffic? In our organization outbound Netflix traffic leaves the higher bandwidth interface and is inbound on our lower bandwidth interface. We can influence outbound traffic.

A: You can use route-map to set as path prepend or modify MED value (lower the better) to influence routing. AS path prepend can be used to add extra AS path on less preferred path as shorter is preferred.

Q.  What is the preference level in all filtering method

A: http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html - this is how path selection works.

Q.  What is the use of MED?

A: Use of MED, its kind of metrics like we have in IGP, its again used as an attribute and used to influence traffic e between different AS.

Q.  Can we advertise subnets less than /24 ?

A: Yes, we can.

Q.  What is the role of router id in BGP?

A: Router id really helps kind of making uniquely identify particular information while at the same time it has some information that is exchanged between the routers. It’s a good practice to configure loop back addresses as router id and that has been recommended also so that you know how to reach particular router within the network. Apart from that it’s not really required for forming any kind of BGP neighboring.

Q.  What does mean of "Prefix" in BGP?

A: Prefix is something that is advertised to a peer. It becomes the route when it is selected as the best and installed in the routing table.

 

Q.  Why we are using OSPF and iBGP both are same IGP?

 

A: If you notice the IBGP configuration we did the peering. When you are using the peering via loopback address, you need to have the loopback reachability; these IBGP provides the loopback to loopback reachability. You can also have static routes but if you look at within a autonomous system environment having 100s of routers, it is not really a scalable way to run a static route everywhere. Better option is to configure IBGP within those 100 routers and have reachability to any loopbacks across the network. You can form IBGP connection between any Router A / Router C/ Router G which are sitting 20 hops away and not direction is required.

 

Q.  How to see the BGP routes coming from a BGP peer?

A: "show ip bgp neighbors [ip-address] routes" to look at the bgp routes coming from BGP peer.

Q.  Why is it recommended to use a router id when configuring bgp?

A: Why it is recommended is that say if you are not using and say you are using a different address family using vrs, it becomes difficult if you are using vrs to identify which ip is router id unless you are describing in the description. Better option is using a router id you can identify the node properly and it makes really simpler to manage your network.

 

 

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