Bgp notes

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  • Publicado : 22 de noviembre de 2011
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* BGP is an external Routing protocol ( egp – external gateway protocol)
* BGP runs on top of tcp port 179
* Udates are only sent when is needed and only sends the new info
* Slow for converdgence
* Normaly in companys that have dual isp connections so they have redundancy
* Both isps get the bgp routes
* Bgp finds the best routes for each connection ( maultihomming)
* Styles of bgp
* There is two type of bgp ibgp and ebgp
* You can advertise your network to two or more isps
* All outgoing access uses default routes
* Can set one isp to better for one link and another isp to another link
* Uses a bgp table to store routes
* Bgp Algortihium
* Distant vector
* Without tunning bgp acts like RIP ( ieit uses hop count of autonimous systems to get to a route)
* Bgp packets and types
* Open packet: starts a seson
* Keep alive: keep alive
* Update message: update message
* Notification: never a good nodification
* Tables
* Neighbor table: connected neighbors ( you have to maunaly type in the neighborrs)
* Bgp table: all the bgp routes
*Routing table: best routes
* IBGP = Internal
* Ebgp = External
* BGP uses Autonimous Systems
* Each organisesation useally gets one AS number
* 2 types of AS public and private
* IGBP is were two routers in the same AS connect up
* Egbp is were two routers from different AS join up
* Ibgp neighbor routers don’t haveto be dirrectly connected ie:
* the 2 IBGP routers can be nighbors
* BGP neighbors have to be maunally set
* Bgp does not send any info to form connected
* The main reason why you would run ibgp is if the two ibgp routers that are connected to egpb routes ( ie connected to two different ISP’s) run ibgp then they can share routes with each otherallowing them to use easch othrs info to find best paths
* Two isps can go through you to connect to each other. Ie a transit network, only issue is that if the isp 1 sends info for isp 2 the ospf router dosnt have the bgp table so it has no idea about the external networks. This will course a black hole
* To fix this you have two options:
1. give the ospf router thewhole bgp table ( not good idea)
2. connect the two igbp routers dirrectly


neighbors manually configured
router BGP 6500

each router can only be in one authonmous system

bgp router (6500)
neighbor ( ) remote-as ( 5500)

( show ip bpg summery)

neighbors can be formed with out any neighbor info being sent

IGBP neighbors (R4)Router BGP (5500)
Neighbor ( remote-as ( 5500)
Neighbor ( update-source loopback ( loopback interface of ie loopback 1)

( when setting up neighbor’s in IGBP it is best to use a loop back ip address so that there is redundancy ie:

this way by using a loopback if one link goes down the router can use the other one.
( make sure you advertise the loop back outvia ospf. )

the reason why you need the update source cmd is so that when R2 recives the info from R4 it knows who to send it back to. This is becouse BGP dosnt have a neighbor relashonship with any other network then the network ( as the neighbors have to be manually configured).

So this means that when R2 gets the info from R4 it sees the source as ether or10.1.24.0 ( depening on the network ospf selects) and as BGP dosnt have these networks it will drop the packet. So the neighbors don’t form.

Egpb router neighbors must be directly connected

Unless you use egbp multihop ( you use this if you want to use loopback interface) so that the routers will allow egpb neighbors to be more then 1 hop away.

No networks were being advertised into BGP...
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