BGP Notes




Port 179 TCP ports

eBGP AD = 20

iBGP AD = 200



Path Vector Protocol.



When an update it sent it sends an Ack, updates are incremental… only sends updates when something changes.


Incremental and triggered.


Slowest convergence time.




BGP styles


Default Route Only – > Edge router sends a static route from the edge router to BGP through


Possible to advertise your networks into Bgp from IGP



Partial Updates – >


Business–à BGP -> 2nd Business.

-à 2nd BGP ->>>> 3rd business.


Having routes only for certain sites..


So routes for only 1st bgp to first Edge router etc.



Full updates


Edge Router gets the entire Routing Table of the internet..


BGP table, BEST paths to routes on the internet.


Without Tuning BGp acts similar to RIP! – > so if not configured it will hop from autonomous system to autonomous system.


Single Honed BGP – > Having one connection from a Router out to BGP

Dual Hones BGP – > two options

a.)    one router connected to 2 links out to a BGP

b.)    2 routers connected to two different links out to the same BGP


This is setup for load balancing and for Redundancy



One router or multiple routers going out to different BGPs per router.


BGP Speaker – > Any router that Runs BGP






BGP table – > a list of all BGP routes.

Routing table – > list of best Routes.


Public and Private AS#’s


iBGP – > 2 routers in the Same AS

eBGP – > Neighbor relationship linking 2 different AS


Peering – > When two routers form a Neighbor relationship


iBGP relationships – > Do not have to be physically connected together…  Since it runs over other routers in that AS, the Data/packets for new routes flow through the routes through non IBGP’s.



Blackhole – > since the traffic goes over a IGP router out to BGP… if it cant find the destination since it doesn’t have it.. it take a look at the destination does not know what exactly to do with it and it possibly drops the packet.


BGP Configuration..


BGP Neighbors are manually configured.


Config t

Router bgp 6500(AS)



How to configure a Neighbor? EBGP


Neighbor (interface that connects) remote-as 5500(as system externally in)


Neighbors can peer with other neighbors without sending any data first.


BGP offers passive and active similar to EIGRP, Active is searching for a route



IBGP – > when configuring IBGP networks internally the best thing to do to neighbor then is to use loopbacks.. just incase the router that is attached to it goes down.



How to Configure a Neighbor?  IBGP?

Router bgp

neighbor remote-as 5500


doing this the relationship will never form it will stay at active… this is only for internal


reason why is since they are attached via loop back address.  Since the router that see’s as its neighbor will not have a interface that has a address of being that it is a loop back.  So when it gets the neighbor packet from a lets say source it is not going to form that neighbor relationship…. so on this router we have to do the following… this all has to do with the source address..


so we have to do the following..


neighbor update-source lo1(the loopback interface on that neighbor)


this will move the neighbor relationship from active to passive/how many prefixes.


EBGP Multihop


neighbor multi-hop 2(hops)


This is a bit confusing but, intstead of 1 hop to a loopback its 2… since you use a interface connected to a router + the loopback.


This command is really used for interfaces like a loopback that are not directly connected.




How to advertise networks


2 ways, Network command and Redistribution..


Using Network Command


Network – > Will auto summarize this network into like RIP

Every other BGP router will have in its network..

Network mask – > this has to match the subnet mask for what I have set the interface for.


By default auto-summarazation is turned on.


So any redistributed route is Classful.


BGP Syncronization


Do not advertise a route learned via IBGP Until you learn from that route from the IGP……. So a route has to be learned via ospf eigrp etc etc until BGP will syncronize and send that route.


or…. you can turn off router syncronization..


Simply typing in

router bgp 6500

no sycnchronization


BGP next Hop process…


When configuring routes, keep in mind.


When IBGP forms routes it will keep the same Next hop this is bad.

When EBGP forms route it will change appropriately


So if I have routes to outside of my Own AS, my igrp will have that address instead of the router that is correctly adjacent to it which should not get the right IP.


Neighbor next-hop-self – > tells that other router( For all its routing that the next hop will be itself….. so I telling my neighbor I am the next hop.


BGP tuning and Attributes..


Manditory Attributes – > example is a next hop address..


Well known Attributes..

AS Path – > mandatory

Next Hop – > mandatory

Origin – > mandatory



Local Preference – > discretionary

Atomic Aggregate – > discretionary


Optional Attributes..


Multi-exit Discriminator(Med/Metric)


Goes from top down… Bold are the most important.


How BGP finds the best path / Metric.

Ignore routes with Inaccessible next hop address

Highest Weight

Highest Local_Pref

Prefer the path that was locally originated with Network commands

Shortest AS_Path

Path with lowest origin type

Lowest MED

eBGP over iBGP

Lowest IGP metric to the BGP next hop

Determine If multiple paths require installation for BGP Multipath

If both paths are external, prefer the path received first

Prefer the route that comes from BGP router lowest RID

If the originater Rid if the same for multiple path with the minimum cluster list length

Prefer the path tha comes from the lowest Neighbor address.


Weight / Local_pref, AS_Path etc etc are all set the same.. so they are all tied by default.. so they have to bet set.


Weight = Cisco Propriatary, Stays Local to a BGP router,

You can specificy a router to have higher weight from R1 over R2 , if you get a router that has multiple routes… it will say the weight of R1 is higher so take R1 instead of R2.


Weight is automatically set to 0

More Weight = the better

This is Local per Router.


Setting the weight..

R1(config –router)BGP 5500

Neighbor weight 500(sets the weight for all the routes to that neighbor to 500)

So for neighbor any routes I know that that neighbor has it will send it there opposed to the other weights if they were 0







If weight is tied we go to LOCAL_PREF


Local Pref – > Something that is advertised to other routers…


AS_path – > Highly unlike but it is Hop count from the original BGP router.


More attribute Information…




Next Hop Address


Local Preference



Things that would not allow a BGP route to be in the IP Route table..

1st.) Syncronization

2.) Remember to look at the next hop address.. remember that EBGP will change the Hop to the next hop AS.  Next Hop Self would work.



Clear ip BGP *



Neighbor shutdown  – > shuts down that neighbor but, it will keep all the attributes……….. so to add that neighbor again it will come back up with all the attribute settings.



Origin Attribute – > Where the route came from…. Typically its from a network command if it says i… if it is denoted at ? it is normally redistributed into bgp.



Local_pref – > Path with Highest local preference. The difference between the Local preference and weight is that Local Preference will advertise to the entire AS, does not get advertised outside of the AS.


Default = 100


Using local to make a BGP Router in a AS..


Bgp default local-preference 600

So every single route it advertises to other routes it will mark as 600, keep in mind if the weight is different that will break the ties.



Metric attribute…


0 is default

Tries to influence other AS

Works between AS

Different than a IGP Lower = Better.


<Config-router>Default-metric 200


Or you can do a route map.


Config-route-map#set metric 200




Neighbor command more than likely just tells the packet where to go to what destination…












Post a comment or leave a trackback: Trackback URL.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: