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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[Masoud Pourshabanian]

It is used to carry some BGP extended communities such as route-target as well.

MP-BGP also carries route-distingisher(RD) along with each route.

Masoud

[Vinit Jain]

Multi-Protocol BGP (MP-BGP) is an extension to BGP that allows various address-family routes to be carried over BGP. For example, in MPLS VPN deployment, MP-BGP is used to carry vpnv4 routes between the PE routers.

[Monica Lluis]

Another question:

What is the advantage of using as-path over local preference when configuring BGP to
prefer one path over another?

[Vinit Jain]

In BGP best path selection, local preference is number 2 and as-path is number 4.

See this link:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml

local-preference influences route we receive from our neighbor and therefore influences how we will send traffic out. AS-path prepend influences what we advertise to our neighbors and therefore influences how the Internet sends to us. So local-preference controls how we send and as-path prepend controls how we receive.

Recall the session slide where it is mentioned the attributes applied in inbound direction to influence outbound traffic and applied in outbound to influence inbound traffic.

Hope this clarifies.

[Monica Lluis]

Another question:

What is BGP scanner feature?

[Vinit Jain]

BGP Scanner is a Cisco IOS process that performs the following tasks:

• Validate nexthop reachability
• Validate bestpath selection
• Route redistribution and network statements
• Conditional advertisement
• Route dampening
• BGP Database cleanup

The Full BGP scan takes place every 60 seconds.

[rana85.das1]

Sir,

    I have a query --   There is two office network running, both are using ospf and they are update their routing table accordingly . If I put a L3 device between them which is support only static routing

then is there any possibility of  communication  between these two office? .If yes then what to do? 

[Raja_D]

Hi Vinit,

I have noticed an issue in our network setup where the traffic of the source branch location was actually passing through the static path although we have the BGP up in the network.

I have the default static path configured as ip route 0.0.0.0 0.0.0.0 <wan exit path ip> 50 , while we have the bgp session established over the mpls link but still the traffic takes through the static path.

when the ebgp session is up in the network which generally has to consider itselves as the best path to reach to a destination as its AD value is 20 ,what could be possibilities as of why the network is still taking the static path ?

what could be scenarios that could lead to this sort of issues and can be done to isolate this issue and make the traffic to pass through the BGP and not through static path. Kindly advise..

[Vinit Jain]

Hello

Thanks for posting your question.

Could you please check the routing and cef entry for the destination prefix.

- show ip route <dst>

- show ip cef <dst> detail

If you know the source and destination of the packet, Also, please capture 

- show ip cef exact-route <src> <dst>

If you have set AD value of 50 for the default route, then it should be least preffered but I wonder if the destination IP is learnt via BGP or default (does not exist in the network). Did you verify that the destination prefix is not flapping ?

Will wait for the logs

Regards

Vinit

[johnnylingo]

Are there plans to support BGP Confederations and/or Route Reflectors on the ASA?  

[Vinit Jain]

Though ASA supports BGP but they have not been designed to perform routing tasks and heavy replication tasks as required by router running as route-reflector or Confederations.

I dont think there are any plans of supporting any of these features on ASA unless there is really a need for it.

Regards

Vinit

[jaidabdulkarim]

Hello Vinit,

Could you please answer below question.

I am confuse with BGP scanner and NHT.

My question is as both are enable by default and both Validate nexthop reachability.
So why we need to run both process for same purpose and what is different between them.

Thanks In Advance

[Vinit Jain]

Hello

Yes, both are enabled by default in all the recent software releases. 

BGP Scanner is created from BGP Router process. When BGP starts, it starts with BGP Router process. BGP Scanner is a low priority process that performs periodic tasks.
After being invoked, it enters an infinite loop, in which it scans the routing tables for all AFs.
In this loop, for each AF;
1. Scan of the BGP RIB to determine if prefixes and attributes should be deleted and if route map or filter caches should be flushed
2. Scan of IP RIB to ensure that all the BGP next-hops are still valid
2.1 If the next-hop is unreachable, all BGP entries using that next-hop are removed from the BGP RIB. The criteria of validity check are simple. BGP entries are considered as valid as long as there are any routes in IP RIB which matches to is next-hop of BGP entries regardless of prefix-length or route source.
2.2 If the IGP metric to next-hop is changed since the previous scan, it updates the BGP RIB and performs the bestpath calc
3. BGP dampening information updates
4. Conditional advertisement checks

The scanner then sleeps for Scanner Interval (60sec by default). Upon waking up, it starts from the beginning of the while loop.

The challenge now with BGP Scanner process is, it is CPU intensive and also can cause convergence issues in case of the Next-Hop node down situation. 

The periodic nature could lead to maximum delay of interval (60sec) for detecting the BGP Next-Hop path events. The recent software implementations have the new architecture “Address Tracking Filter (ATF)” which enables event driven quick detection of BGP Next-Hop reachability changes.  To solve the BGP convergence issues due to BGP Scanner, “BGP Next-Hop Tracking” feature was developed, which works on the ATF architecture.

Note: though reducing the BGP scan time is one way to overcome convergence challenges but its not an advised solution.

BGP Scanner process still performs its periodic checks on Cisco IOS along with the other tasks that it performs but you cannot disable the BGP scanner process. It is possible to disable the BGP NHT mechanism.

Hope this helps.

Regards

Vinit

[johnnylingo]

In the case where BGP is running on multiple VRFs, is it possible to have some VRFs be part of a BGP confederation and others not be?

[Vinit Jain]

I dont think vrf aware confederation is supported. Also, bgp confederation is a global command option, not per address-family.

Hope this clarifies.