Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

BGP Issue

Q:-- Which of the following statements is true about the BGP synchronization command?

Ans:-

A. Synchronization must be enabled when implementing a multi-homed BGP connection to

multiple ISPs.

B. If it is turned ON, a prefix learned from IBGP neighbor is valid only if a non-BGP (IGP)

route exists for that prefix.

C. Synchronization is necessary when peering with an EBGP neighbor.

D. Synchronization improves BGP routing convergence.

E. Synchronization can be turned off if all the transit routers in an Autonomous system are

running a fully meshed IBGP.

One more thing I want to know what is difference between synchronisation rule and split horizon rule.

What is the relation between synchronisation rule and fully meshed ibgp peers.

Thanks

Mahi

1 ACCEPTED SOLUTION

Accepted Solutions
Purple

Re: BGP Issue

Hi Mahi,

As I see it, there are two answers to that question:

B - this is stating that a router will only use and advertise a prefix learned via IBGP if it also has a route to the destination that is learned via an IGP

E - this is also correct. If all routers in the AS are running fully-meshed IBGP, then there is no need for the IGP to carry BGP prefixes

The synchronization rule is there simply to ensure that a router can actually route to BGP destinations that it is advertising. There is no point advertising a route for a destination that will result in packets for that destination being dropped by some intermediate router in an AS. Note that synchronisation is no longer in favour and is disabled by default - it's not a good idea to redistribute the internet routing table into an IGP !

Split horizon, on the other hand, simply means that you do not advertise a prefix to the router from which you learned it in the first place. Why tell someone something they already know ? !

Pls do remember to rate posts...

Paresh

7 REPLIES
Purple

Re: BGP Issue

Hi Mahi,

As I see it, there are two answers to that question:

B - this is stating that a router will only use and advertise a prefix learned via IBGP if it also has a route to the destination that is learned via an IGP

E - this is also correct. If all routers in the AS are running fully-meshed IBGP, then there is no need for the IGP to carry BGP prefixes

The synchronization rule is there simply to ensure that a router can actually route to BGP destinations that it is advertising. There is no point advertising a route for a destination that will result in packets for that destination being dropped by some intermediate router in an AS. Note that synchronisation is no longer in favour and is disabled by default - it's not a good idea to redistribute the internet routing table into an IGP !

Split horizon, on the other hand, simply means that you do not advertise a prefix to the router from which you learned it in the first place. Why tell someone something they already know ? !

Pls do remember to rate posts...

Paresh

New Member

Re: BGP Issue

Hi Paresh,

Thanks for your reply..

But i do not agree with your split horizon answer.

According to me split horizon rule says that a IBGP peer will not send a route learned by IBGP to another IBGP peer.

It is not the case that we are sending a route back to the same router which has passed that route/generate the route already.. i think it is different one.

what you will say...

Thanks

Mahi

Purple

Re: BGP Issue

Mahi,

My definition of split-horizon is the tradionally accepted definition of it - it applies to BGP as well as all other path/distance vector routing protocols.

I wasn't quite sure about the context in which you asked the question. But in terms on IBGP split-horizon, you are absolutely correct that split-horizon means that you don't advertise IBGP routes to other IBGP neighbors.

Paresh

New Member

Re: BGP Issue

Hi Paresh,

OK thanks...

Your synchronisation concept solved my query..

Thanks a lot...

Mahi

Purple

Re: BGP Issue

No probs at all, Mahi.

One more thing you should know about synchronisation. When you use an IGP such as OSPF, you need to ensure that the router-ID that you use for OSPF is the same as you use for BGP. If you don't, you will see that the routes are considered unsynchronised and are therefore, not considered valid.

Paresh

New Member

Re: BGP Issue

Hi Paresh,

I am not able to understand your concept. can you explain it little bit.

Regards

Mahi

Purple

Re: BGP Issue

Ok.. I'll try and explain.

Consider a setup with 2 IBGP peers, A and B. Now say A learns a prefix R via EBGP and then advertises it to B via IBGP. According to the rule of synchronisation, B can only install that prefix in its routing table or advertise it to its EBGP peers if there is also an IGP route to R in its routing table. Now, when using OSPF as the IGP, router B has to make sure that the router from which that OSPF route was learned was the same router that advertised it that route via iBGP. To make that comparison, it will compare the BGP ID of the BGP peer with the router ID in the OSPF LSA that advertised that same route. If they match, the route is installed. If it does not, synchronisation fails.

Pls do rate the post if it helps...

Paresh

263
Views
3
Helpful
7
Replies
CreatePlease to create content