BGP Multihoming

steve-o87

Hi guys,

I've been brushing up on my BGP knowledge(aswell as others)!
I just wanna know - Is multihoming used solely for the purpose of redundancy? Also, I read that BGP can sometimes be used in place of an IGP - Why? When?

One final thing - Are the BGP message types covered heavily in the exam? I understand the four main types but the actual fields and sub fields are a bit overwhelming to say the least!

Thanx

mwgood

Multihoming is also used for load balancing.

Using BGP instead of an IGP would be it's iBGP implementation. I'm sure there would be many reasons for doing so. One I can think of off the bat is an MPLS VPN - since it uses multi-protocol BGP extensions.

I don't remember anything of the BGP message types covered on the exam.

From http://www.cisco.com/web/learning/le3/current_exams/642-801.html

The BGP section is described as follows:

Describe and implement BGP for enterprise ISP connectivity
Describe the foundational concepts of eBGP and iBGP
Configure and verify BGP operation
Manipulate BGP path selection using Local Preference and MED attributes

BubbaJ

mwgood wrote:

Using BGP instead of an IGP would be it's iBGP implementation. I'm sure there would be many reasons for doing so. One I can think of off the bat is an MPLS VPN - since it uses multi-protocol BGP extensions.

There really aren't any reasons for using BGP as an IGP. Using MPLS is not a reason. It works much, much better in conjunction with an IGP.

One problem without an IGP is that you would have to use directly connected interfaces for your neighbors since you wouldn't have an IGP to describe links to other (loopback) interfaces (less reliable than loopbacks). BGP doesn't converge nearly as fast as an IGP since it was not designed that way. You risk large, long-lasting routing blackholes.

There is a very goo reason that the administartive distance for iBGP is 200.

mwgood

BubbaJ wrote:

There really aren't any reasons for using BGP as an IGP. Using MPLS is not a reason. It works much, much better in conjunction with an IGP.

http://www.cisco.com/en/US/products/sw/iosswrel/ps1830/products_feature_guide09186a00800e977b.html#xtocid234857

I was referring to routing using iBGP instead of the IGP - not ditching an IGP altogether.

BubbaJ

mwgood wrote:

I was referring to routing using iBGP instead of the IGP - not ditching an IGP altogether.

Then I misunderstood. Using it as iBGP is what is required for transit ASs like are found in large ISPs. I am very familiar with MPLS. What you use it for is optimized crossing of your AS by outside traffic. The outside traffic can see only 1 hop, not your entire network structure, and you can do better traffic engineering since the routers only need to look at the label instead of the IP header. It also helps to keep different customers' traffic separate by setting up different VPNs for each customer. Each customer can think the other side of your AS is a directly connected segment of their own.

This is not using BGP in place of an IGP as the original poster asked about. Your AS still uses an IGP for its own routing. You don't want to burden the internal routers with the whole Internet routing table - only the edge routers which can be large enough to handle it. It would be very expensive to equip all your routers this way.

Ching01

Is multihoming used solely for the purpose of redundancy

MULTIHOMING is not really a redundancy you can use it also to separate your traffic like if you have 2 ISP and the other one handles the IRC and the other ISP handle the video conferencing but the problem with the multihoming is that your EBGP router will cosume much processing imagine two EBGP routers passing you an updates.

I can say that BGP is good when you know how to implement like using confederations and route reflector.

Off topic , Is BSCI exam difficult?

EdTheLad

Ching01 wrote:

Off topic , Is BSCI exam difficult?

Is it difficult, i say yes,not because the information is complex, its just it takes some time to distinguish the differences in the protocols,i found i would learn rip then igrp,eigrp then change alot in OSPF, start forgetting the difference between the distance vectors,look over DVs, then on to ISIS then forget ospf fine details and alot of time is wasted trying to remember the little but important difference.If you have some experience in the field then this removes this gumble of protocols.
Now on to the exam, when i took it i was very confident walking in, but with the time constraint, you dont have much time to think, the questions are long winded with alot of discriptions and its hard work to focus.Really i think an extra 30 mins should be added as the only people who have time on this test are the braindumpers.

Ching01

Oh , thanks for the info just wondering do I need to memorize all the attributes use in BGP? Some I can remember but others not.

I'll apply it on my lab next week 2 more chapters on my Sybex and 3 days watching the CBT I hope I could pass BSCI exam.