eBGP Question

Answered Question
Nov 25th, 2008

Hello Cisco Networkers..

I have a pretty cool question.

I am by far not considered a expert in BGP but here is the question.

If you have a eBGP relationship between 2 ISP providers, both peers in the relationship should pass the FIRST (FULL INTERNET ROUTING SWITCHING TABLE) is this correct? I think so!

So, if they pass the FIRST (FULL INTERNET ROUTING SWITCHING TABLE) is the included???

The reason is because that I see from the "show ip bgp neighbor x.x.x.x received-routes" command that they are passing me the route as well as the FIRST Table...

If I have the FIRST table I should need the default, is this correct..

Please let me know if the should be passed or not between to ISP's..


I have this problem too.
0 votes
Correct Answer by csco10716389 about 8 years 1 month ago

Your Query is correct if u have a single ISP active currently then you don't need any default route injected in your network as you already have the Full routing table.And this default route may cause your AS to be a transit AS for other destination via the ISP.so better is to block the default route injection.

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Overall Rating: 4.4 (5 ratings)
guruprasadr Tue, 11/25/2008 - 02:43

HI Mavrick, [Pls RATE if HELPS]

The situation changes case-by-case basis.

In most of the cases, only the Internet Routing table is received and the not the default.

Any ISP / Network, if the route to the destination is not available in the routing-table means the next preferred path is IGP Default.

So, if your exit point is only the connected Service Provider / Partner means, you can still validate to receive the default from the same.

Otherwise, ask the other ISP to stop advertising the default (or) apply the DENY control-list by stopping the default acceptance.

Hope I am Informative.


Best Regards,

Guru Prasad R

Mavrick25 Tue, 11/25/2008 - 03:17

Thanks guruprasadr ,

(Pls Note: I will rate this post)

Now, the case is that we are peering only with one other ISP (which is a trusted AS, it's an ISP that has acquired us).

Anyhow, at the moment we are not peering with any other providers, so allowing the default to be passed into our AS should be ok.. the only concern is if we lose the links...

If we lose the links we would lose the default..

But, I guess my question again remains..

Why have the default, if you have the full routing table..?? There shouldn't be a situation when there is no route to a network within an ISP.. We should have the full routing table!

Let me know..

Thanks again

marikakis Tue, 11/25/2008 - 04:55


Having a default inside the network of a large SP does not make much sense. Which of the numerous exit points should the default point to? It looks like random routing with low possibility of hitting the actual destination and more possibility of forwarding useless traffic only to be dropped at some further point. So, large SPs normally run their cores "default-free".

If you can easily detect an exit-point that can serve as your default, this might be it. If you do not like them sending you the default (e.g. you would rather manage your defaults from inside your network or do not want a default at all), discuss it with them. (I was about to suggest you ask them to stop sending it and always filter on your side as a safeguard, but they have acquired you, and it is not clear to me who is setting the policies in this scenario). Note here, that if you lose the links and most of your traffic is supposed to go towards those links, it doesn't matter much if they were sending you the default or not. If links are down, no traffic passes with or without a default.

A slight advantage of having a default of your own appears in failure scenarios. For example, if your single exit-point to the Internet flaps a couple of times and then is ok, your outgoing traffic will flow directly towards your upstream before your router completes receiving the full routing table from your upstream (quicker recovery from failure). This can also work similarly in cases where (due to some temporary issue) you are not receiving all the routes, while your upstream does have them.

Kind Regards,


Correct Answer
csco10716389 Tue, 11/25/2008 - 04:59

Your Query is correct if u have a single ISP active currently then you don't need any default route injected in your network as you already have the Full routing table.And this default route may cause your AS to be a transit AS for other destination via the ISP.so better is to block the default route injection.

tcordier Tue, 11/25/2008 - 05:58

I do not understand how you can become a transit AS if you a default route? As long as you do not the default to your provider (which the provider will probably not accept), there is not much of a chance you will become a transit AS?

- thomas

tcordier Tue, 11/25/2008 - 05:37

I agree with Maria that it is useful to keep the default you receive from your ISP. It makes it easier to generate a default in downstream routing protocols you run at your end. It also protects you from situations where your router has, e.g. due to a memory issue, problems holding the full routing table. In such an event your traffic would still flow to the ISP and be forwarded to the destination, while it would be dropped without the default.

As to the question whether or not the default is part of the Internet routing table: if you check Internet route servers which are supposed to hold the full Internet routing table, they will mostly - if not all - have a default route, but static. They do not advertize the default in BGP. So I would say, no, it is not.

- Thomas

guruprasadr Tue, 11/25/2008 - 06:32


Again, i recommend to have the default route available in your network since you have only one exit point.

In-case, as recommended earlier this will help to avoid any kind of Memory issue at your Peering Router (ASBR).

In addition, if the match to the specific destination is not available in the Routing table means (either if failure in receiving the full-routing table / problem with the Other T1 ISP's) the default route will help in routing the packet to the next-hop atlest.

Hope i am Informative. Pls RATE all Informative POST.

Best Regards,

Guru Prasad R

ullasupendran Tue, 11/25/2008 - 07:29

Hey guru ,mari and all

I have a small query on this.If we have only one exit outside through ISP and we recieve a default route , why do u need full routing table from your ISP and burden your router? I feel keep it simple by accepting only the default route and advertise it to your internal routers(if any). Let me know your views on this .


guruprasadr Tue, 11/25/2008 - 07:39

HI Ullas, [Pls RATE if HELPS]

There are many difference in having the Full Routing table and the Default itself.

In case of having the Full Routing Table, the best path selection is carried out at the your Router point (considering as CE) for the destionation address.

In case of having the Default only, the egress traffic from CE will by default fall into the PE Router and the ISP takes the Decision Control in giving the BEST path the destination address.

Say for Eg, if the connected PE / ASBR at ISP side is facing some issues means (either the CPU Spike / Memory Issue / Others), the ISP will stop giving the full routing table to the PE / ASBR to safe-gaurd the Router itself. In that situation, across the entire ISP Cloud the packet will again follow the DEFAULT path (available in the ISP IGP Cloud) and will lead into the High Latency to the destination addresses.

So, if your Router (considered as CE) have enough Hardware capability means, accept the FULL Internet Routing and have the decision for the Packet forwarding from your Router Point.

Hope I am Informative. Pls RATE if HELPS

Best Regards,

Guru Prasad R

ullasupendran Tue, 11/25/2008 - 07:48


In this scenarion my only exit point is my upstream ISP.So even if i have a full routing table or default route ,i will be sending the packets to ISP. Always my best path is to that one link to ISP.so why i need full routing table?


tcordier Tue, 11/25/2008 - 07:45

If you have only one ISP with a single connection, there is no need to receive the full routing table, and I do not see added value. The only advantage of a full routing table is that traffic to "illegal" addresses (private ranges, BOGON ranges) will be dropped at your router, and not pass the link. Such traffic can sometimes be substantial, e.g when a host is hit by a virus which generates traffic to illegal IP addresses. Then again, you could implement outbound filters for that, or let your ISP worry.

- thomas


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