Course contents

Module 1: Communities and MultihomingLesson 1.2 (2 of 3 in this module)2 of 11 in the Advanced BGP course

Multihoming designs

One or two routers, one or two ISPs, full table or default only, and what each design costs.

Advanced · 20 min read

You'll learn to compare default-only, partial and full-table multihoming, avoid becoming a transit network, and choose a design for a given site.

Why two ISPs

A second provider protects a site against one ISP's outage and gives some freedom to choose paths. It also introduces choices: how much of the internet should your routers learn, and which router connects to which provider?

Three common ways to connect to two ISPs
Three common ways to connect to two ISPsThree columns compare default route only, partial routes and full tables for a dual-ISP site.Default onlyRouter memory: tinyExit choice: weakControl inbound: yesFits: small branchPartial routesRouter memory: moderateExit choice: goodControl inbound: yesFits: mid-size siteFull tablesRouter memory: largeExit choice: bestControl inbound: yesFits: large edge

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More routes give finer control but need more memory and more care with filters. Most small sites start with default routes.
Single-homed, dual-homed and multihomed
Single-homed, dual-homed and multihomedThree ways to connect to the internet. Single-homed uses one link to one provider, so any failure cuts you off. Dual-homed uses two links to the same provider, which survives a link failure but not a provider failure. Multihomed uses links to two different providers, which survives either failing and is the case where BGP is normally used.Single-homedYour ASISP ADual-homedYour ASISP AISP AMultihomedYour ASISP AISP BOne link, one providerTwo links, one providerTwo providers: use BGP

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BGP earns its place once there are two or more providers. With one provider a default route is usually enough.

The three common designs

Default routes only. Each ISP sends 0.0.0.0/0 and nothing else. The router holds two defaults and picks one by local preference. Memory use is tiny and failure is handled by BGP removing the default from a dead session. You cannot choose better exits for particular destinations.

Partial routes plus a default. Each ISP sends its own routes, its customers' routes and a default. You can steer traffic to nearby networks through the best provider and fall back to the default for the rest.

Full tables. Each ISP sends the whole internet table, hundreds of thousands of routes. You pick the best exit for every destination. Routers need enough memory and the policy must be maintained carefully.

ip prefix-list DEFAULT-ONLY seq 10 permit 0.0.0.0/0 router bgp 64500 address-family ipv4 unicast neighbor 192.0.2.1 prefix-list DEFAULT-ONLY in neighbor 192.0.2.5 prefix-list DEFAULT-ONLY in

R1 and R2 accept only a default route from each provider. The filter protects you if a provider starts sending more than agreed.

Avoiding transit and unwanted routes

Whatever you accept, advertise only your own prefixes to each provider, as in the filtering lesson. For default-only designs, also ensure that the router does not originate a default into BGP toward a provider.

Transit leak against a filtered edge
Transit leak against a filtered edgeWithout outbound filters, routes learned from one ISP can be passed to the other.No filterISP A routes go to ISP BCompany becomes transitOutbound filterOnly own prefixes sentCompany stays an edgevs

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The company would then carry other networks traffic.

Choosing a design

Choosing a design
Choosing a designSite; Likely design; Reason. Branch office: Default only Little memory, simple failover Data centre with many users: Partial routes Good control of key destinations Large content or hosting edge: Full tables Per-destination control and traffic engineeringSiteLikely designReasonBranch officeDefault onlyLittle memory, simple failoverData centre with manyusersPartial routesGood control of key destinationsLarge content orhosting edgeFull tablesPer-destination control and trafficengineering

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Read each row across: the left box names the item, the others describe it.

Failure behaviour in each design

Each design reacts differently when a provider link fails. With default routes only, the default from that provider disappears and traffic uses the other. With partial or full tables, specific routes through the failed provider disappear and the next best path takes over.

What happens when ISP A fails
What happens when ISP A failsThe session drops, its routes are removed and the best path moves.ISP A link downsession closesRoutes removedfrom ISP ABest path recomputedISP B routes usedTrafficflows via ISP B

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Convergence depends on detection speed, so BFD helps here.

Check your understanding

Predict · scenario 1

A small branch has two ISPs and modest routers. Which design fits best?

Predict · scenario 2

What protects against accidentally becoming a transit AS?

Predict · scenario 3

What is a drawback of full tables?

FAQ

Do I need the full internet table?
Rarely at a small or medium site. A default route from each ISP gives resilience. A full table lets you choose the best exit per destination, at the cost of memory and more policy to maintain.
What is a partial table?
The ISP sends its own and its customers' routes plus a default. You get good control for nearby destinations without storing the whole internet.
Why is transit a risk?
If you advertise routes learned from one ISP to another, you invite traffic across your links that is not yours. Outbound filtering prevents it.