BGP routes in edge route connected to ISP

[interfacedy]

Hi there is a router connected to ISP. BGP in the router can allow all bgp routes in from ISP or none of them. What is benefit if we allow all bgp routes into the router from ISP? I know some of them. Anyone can mention all of them? Thanks

[Flavio Miranda]

Hi

  There´s no benefit in receiving all the internet routing table unless you are an ISP itself. This will require a lot more expensive hardware that´s it.

[interfacedy]

From client perspective, why they like to get all bgp routes into client's edge router? thanks

[Richard Burts]

You provide no context for your question and I wonder if you are asking about some aspect that is not clear to us. But based on what we know so far I will give this answer: when you negotiate with an ISP and sign a contract for Internet service it will specify a type of service (send only a default route, send a default route plus limited other routes, send the full Internet routing table). Once you have completed the contract why would you want to accept less than what you contracted for? 

I agree with @Flavio Miranda that for customers with a single ISP that only a default route is the appropriate choice.

[M02@rt37]

Hello @interfacedy 

Clients may want to receive all BGP routes from their edge routers for:

-- Path Diversity: Receiving all BGP routes provides clients with a greater level of path diversity, which can help to ensure high availability and minimize disruptions. Having multiple paths to reach the same destination means that if one path fails, traffic can automatically be rerouted through another path.

-- Optimal Routing: Clients may be able to select the optimal path to reach their destination by having access to all available routes. With more routes to choose from, clients can optimize traffic flow based on factors such as cost, latency, and reliability.

-- Network Visibility: By receiving all BGP routes, clients have a more complete view of the Internet routing table, which can be useful for network analysis, troubleshooting, and security monitoring.

-- Business Requirements: Certain businesses may have specific requirements that necessitate receiving all BGP routes, such as content providers, financial institutions, and government agencies.

Overall, receiving all BGP routes provides clients with more control, visibility, and options when it comes to routing their traffic, which can be crucial for business operations and customer satisfaction.

[Joseph W. Doherty]

In whole, in my experience, almost all the benefits you describe, using full Internet route tables, at Internet edge, their actual real-world benefit is generally nil.

Something very interesting to "watch" is Cisco's PfR (configured to use best performing path to destinations) managing multiple CEs, in your own AS, with multiple full Internet route tables.  I.e. how often it overrides BGP's best path.

About the only BGP information that's usually useful is in knowing destinations which are directly connected to the same ISPs you're connected to too.

[MHM Cisco World]

• A default route is the route that will be used if there's no other route that matches the destination in the router's forwarding table.

• A full routing table is a table which contain all the routes the BGP neighbor is aware of.

• A partial table is a table filtered (with route map, community...) so that only some specific routes are exchanged.

If you are connected to the Internet through a single ISP, a default route is enough, since there's only one possible path, so there's no point in having 500,000 routes in memory that all point to the same next-hop.

If you are connected through several ISPs with BGP and want to always use the "best" path, then a full routing table makes sense. In this case the default route will (almost) never be used since the router knows every possible destination with a specific route.

However a full routing table take some memory and also more CPU power (and time) to perform a lookup for each destination among the 500,000 known routes.

When the vast majority of your traffic will take one path and only some specific destinations will take another path, a partial table is more efficient.

[interfacedy]

Thanks all for your reply. Its very nice explanation. Another reason for full bgp routes is client can use a lot public ip address within client internal network, right? Partial bgp routes also can do this. but default cannot. 

[Richard Burts]

I do not see any relationship between how many public addresses ip a client uses and whether they have full or partial (or even default) routes in their routing table.

[MHM Cisco World]

How many ISP we have is the key factor here'

You cannot use load balance with more than one isp and defualt route.

If Clinet use specific then you can use partial if client use wide range of public then full

Thanks 

MHM

[Joseph W. Doherty]

If only one ISP, only benefit would be you can drop packets to unknown Internet destinations.

If multiple ISPs, your traffic can select "best" path.  However, best path can be debatable, especially for destinations more than one AS hop away, or without any traffic engineering.

Of course, having full Internet route tables allow "bragging".  ; )

[Leftz]

@Richard Burts If its default route, its not easy to setup public ip address within internal network unless setup NAT within it

[Richard Burts]

Your comment about the complications imposed by configuring NAT are true. But how significant are they? VERY FEW organizations have enough Public IPs to be able to have Internet access without requiring NAT. How many devices are in your network that need access to Internet? How much would that many IPs cost from your ISP? For almost all organizations NAT is a requirement when they need Internet access.

Another way to look at the question is that there is no relationship between whether you receive just a default route, or a default route plus selected routes, or full BGP routes and whether NAT is required. 

[Kanan Huseynli]

Hi,

sometimes clients have not only IX session (internet exchange) , but also LX (local exchange). In this case, better to route directly to ISP1 for the ISP1 owned or connected subnets, rather that through ISP2. It works great, especially there is no eBGP between ISP1 and ISP2. Sometimes, client has 2 ISP connection on local country, but ISP1 reaches ISP2 via external country (due to lack of connection between ISP1-ISP2). To give better user experience for clients from local country, it worth to have LX session and route exchange.

For one ISP connectivity there is no meaning to have full bgp table. For multiple one getting full bgp view require more memory and if you don't have strict requirements, does not have specific meaning.

[Joseph W. Doherty]

As I described in an earlier posting, there's one advantage (meaning?) having a full Internet route table, with just one ISP, although in most cases the benefit doesn't exceed the "cost".