BGP LOAD BALANCING

[s4sandyad]

BGP chooses only a single best path to reach a specific destination. BGP is not designed to perform load balancing..

what i learned from every doc... but suppose this two ebgp neighbor have multiple path, creating neighborship by using loopback ip, to reach loopback router have 2 equal cost igp routes...in that case after recursive lookup router will load balance the traffic ...

Plzzz let me know am i right... or what is the meaning of this statement :

At one side it is said bgp is not made for load balancing specially in ccna, ccnp other exam books, but after doing google found following link, saying load balancing/load sharing can be done.... Why this confusion...

http://ccieblog.co.uk/bgp/bgp-unequal-load-cost-sharing

Please tell me what is truth...

[Reza Sharifi · Accepted solution]

For BGP routes only one of the interfaces/IP will be used as the best next hop.  Once that interface is down it will pick the other interface but does not load balance between the interfaces.

HTH
 

[Vasilii Mikhailovskii · Accepted solution]

Hello, Reza.

Could you please provide a proof-link, that only one IP-address will be in use?!

If ebgp-neighbor's loopback is known via ECMP (2 statics, or IGP), there would be a load-sharing by ip cef, even though you have a single [ebgp] entry in RIB for the particular prefix.

[s4sandyad]

Milan,

link will help me lot...

but...

The maximum-paths command for BGP works if your router has multiple parallel paths to different routers in the same remote AS; this command affects only the number of routes kept in the IP routing table, not the number of paths selected as best by BGP. For BGP, the paths parameter defaults to one.

            Suppose R1 have 2 paths to reach any destination; Without the maximum-paths command under the router bgp, there is only one path in R1’s routing table. After the maximum-paths 2 command is added to the R1 BGP configuration, both paths appear in the IP routing table, However, only one path is still selected as the best in the BGP table.

Vasilii & Reza,

means what is right statement for bgp load balancing ....or is it changed case to case..?? please clear me...

Guies ...can you also tell me bgp support for "load balancing" & "load sharing"???

[milan.kulik]

Hi, yes, only one path is still chosen as the best one and advertised to the BGP neighbors. BUT as you (and Mikhail) said: Multiple paths can be used by the router itself to deliver the traffic to the neighbor to which either CEF or BGP maximum-paths create parallel paths of equal costs. See also here: http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html#bgpmpath for a better understanding. Best regards, Milan

[Vasilii Mikhailovskii]

Hello.

If we started talking about best path selection, then you might find following article interesting:

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-3s/irg-xe-3s-book/irg-additional-paths.html

It's about advertising more than 1 best path.

[s4sandyad]

Hi

Guies .... I found very good link regards same....

At one side it is said bgp is not made for load balancing specially in ccna, ccnp other exam books, but after doing google found following link, saying load balancing/load sharing can be done.... Why this confusion...

http://ccieblog.co.uk/bgp/bgp-unequal-load-cost-sharing

what's your opinion about it....

[sathvik k v]

Hello,

Load balancing and load sharing are often misunderstood.In Load sharing traffic is sent across the parallel paths to the destination(ex :R1 to R2  has 2*2 Mbps links) but the load on the links doesn't need to be equal.

On the other hand in load balancing the load/traffic on the links needs to be distributed equally.

With the use of dmz-link in BGP load sharing can be achieved on unequal bandwidth links. Example between R1 and R2 if we have one 2 Mbps and one 4 Mbps link then traffic is shared in 2:1 ratio. But again the traffic is not distributed equally. When CEF is enabled communication between a source and destination usually flows  the same link so the utilization of the link depends on the amount of data transfer between them,

BGP can only perform load sharing.

-Sathvik

[milan.kulik]

Hi, you are talking about the first scenario described here: http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13762-40.html#conf1 You can also use maximum-paths command as described in the same document to reach some kind of restricted load balancing in BGP. Best regards, Milan

[s4sandyad]

Thanks ...doc is very good...

i found that term "load balancing"  & "load sharing" both r different ...

please can you explain it??

[milan.kulik]

Hi, well, some documents use one term while some document the other. If you want a clear definition, it's here: http://routing-bits.com/2009/06/03/load-sharing-vs-load-balancing/ But it always depends on the document author what kind of terms is he/she using. Best regards, Milan