BGP synchronization

FrankGuthrie

Nowadays BGP sync is turned off, but why do you want BGP to sync with andIGP, what will then be the use of BGP, if the IGP also knows the route?

xXErebuS

FrankGuthrie wrote: »

Nowadays BGP sync is turned off, but why do you want BGP to sync with andIGP, what will then be the use of BGP, if the IGP also knows the route?

Frank,

You want a full mesh IBGP topology whether its physical links / route reflectors / confederacys. The use of BGP is to inject the route into IGP; its like saying whats the use of a rim if your driving on the tire, the tire won't work without the rim.

Now as to explain why its important....


I had something typed out but its much easier to just google a link =D....

what is the bgp rule of synchronization « Pete's Packet


I don't think it explains why the packets are sent to the non IBGP peer; its b/c IBGP has an AD of 200 and almost all if not all IGP protocols have a lower AD.


Keep in mind this mostly refers to Transit AS like a SP; you don't want your AS to be a transit AS (at least I don't!) and most of the time you setup redundancy between your ISP links so you end up with IBGP full mesh links anyways (there are other options).... most people do not inject BGP into their IGPs unless they need to waste money on more expensive routers than necessary =D

Mrock4

The short version is, if you have a gap in 'understanding' in your BGP network where one router (let's say a transit router) doesn't know about a route, you could end up blackholing' traffic without synchronization turned on.

ISP A

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RTR A (BGP border router & OSPF)

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RTR B (Only OSPF..older router)

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RTR C (BGP border router & OSPF)

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ISP B

Say traffic was sent from ISP A to RTR A destined for a network off of ISP B. If the IGP did not know about the route, as it reached RTR B, it would be dropped. Synchronization ensures all nodes are on the same page, so to speak, to avoid these situations. It's not used very often these days since folks can afford to run BGP on all routers (whether that's in a full-mesh or not).

networker050184

xXErebuS wrote: »

The use of BGP is to inject the route into IGP;

This is not the use of BGP. You do not want to inject your BGP routes into IGP unless you are doing something small scale like an MPLS VPN. The current BGP table size is above 450,000 routes! You don't want that in your IGP table.

The purpose of BGP is external reach ability. An IGP is for internal reach ability.

xXErebuS

networker050184 wrote: »

This is not the use of BGP. You do not want to inject your BGP routes into IGP unless you are doing something small scale like an MPLS VPN. The current BGP table size is above 450,000 routes! You don't want that in your IGP table.

The purpose of BGP is external reach ability. An IGP is for internal reach ability.

Read the entire sentence... notice its a ";" and not "."

"The use of BGP is to inject the route into IGP; its like saying whats the use of a rim if your driving on the tire, the tire won't work without the rim."

I'm saying its incorrect; basically you can't say whats the use after you were to inject it....cant have the cart before the horse....

llllvllll

By default synchronization is off.

sh run | begin router

it will show the synchronization details.

When Synchronization is turned off.

R1 , R2 and R3 will run both iBGP as well as IGP.Same with R4,R5 and R6
In iBGP area R1,R2 & R3. R2 will recieve all the updates from R1 as well as R3 but it wont further send each other info to its neighbour.

Ex :- R2 recieve some updates from R3 but it wont further send those updates to R1.(iBGP properties)
So we can use IGP to communicate b/w iBGP neighbours.

Now , when synchronization is ON.

In this case R1, R2 and R3 wont be sending their info to their iBGP neighbours and they have to use IGP to communicte with each other.

Reduces router CPU usage as only IGP is being used.

In short when synchronization is on :- we can only use IGP but when its off we can use both iBGP and IGP.

Mrock4

llllvllll wrote: »

In short when synchronization is on :- we can only use IGP but when its off we can use both iBGP and IGP.

This is incorrect. When synchronization is ON, the router must "synchronize" the route, or know about it in both IGP and BGP. When synchronization is OFF, it's assumed you're either running a full-mesh topology, or BGP on every node, and therefore there is no need to run IGP.

From Cisco's site: "A BGP router with synchronization enabled does not install iBGP learned routes into its routing table if it is not able to validate those routes in its IGP."

llllvllll

@Mrock4 -- >

So we have to use IGP, right? :P
what i have noticed is that when sync is ON then BGP router does not share their info to each other even to their BGP neighbours. In that case we have to use IGP in order to make them communicate with each other.

Mrock4

You are right that with synchronization on you will need routes to synchronize with the IGP before they can be shared. I think the wording you used in your prior post sounded a bit odd, that's all.

Danielh22185

Good discussion. I learned a good deal from this thread!