© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-1

BGP Transit Autonomous Systems

Forwarding Packets in a Transit AS

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-2

Outline

• Overview • Packet Forwarding in a Transit AS• Recursive Lookup in Cisco IOS Software• Routing Protocols in a Transit AS• BGP and IGP Interaction• Problems with BGP and IGP Interaction• Summary

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-3

All core routers need external routers for proper packet forwarding.• Redistributing can overload IGP resources.• IBGP is preferred for scalability.

Packet Forwarding in a Transit AS

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-4

• Routes learned via BGP do not have an outgoing interface associated with them in the routing table.

• Recursive lookup is performed to forward IP packets toward external destinations.

Packet Forwarding in a Transit AS (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-6

Recursive Lookup in Cisco IOS Software (Cont.)

• Traditional Cisco IOS software switching mechanisms perform recursive lookup when forwarding the first packet.– Fast switching, optimum switching.

• CEF precomputes the routing table.– All recursive lookups are performed while the routing

table is built.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-7

With IBGP running on all core routers, is an IGP still needed in the core?• An IGP is needed to resolve BGP next hops and perform

fast convergence after a failure in the core network.

Routing Protocols in a Transit AS

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-8

Routing Protocols in a Transit AS (Cont.)

• Core routers need to run BGP and an IGP.• BGP carries all external routes.• The IGP propagates BGP next hops and other core

subnets only.• All customer routes are also carried in BGP.– Reduces IGP topology database– Removes customer-caused route flaps from IGP; IGP

becomes more stable

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-9

BGP and IGP Interaction

Ideally, there will be no interaction between BGP and the IGP.• BGP carries external and customer routes.• The IGP carries only core subnets.• The IGP is not affected by external route flaps.• BGP is not affected by failures internal to the network as

long as the BGP next hop remains reachable.• The only link between BGP and the IGP should be the

recursive lookup.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-10

Sometimes, BGP and the IGP will propagate the same route.• Usually stems from bad network design.• In this case, routes are determined in EBGP/IGP/IBGP order

based on administrative distances of the routes.

Routing Protocol Default Administrative Distance

EBGP 20 IGP 90 – 170 IBGP 200

BGP and IGP Interaction (Cont.)

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If an IGP route is learned through EBGP, the EBGP route will take precedence.• Potential causes include bad network design, routing

problems, or denial-of-service attack.• Protect IGP routes with inbound prefix-list filters at AS

edges.• Routers should never accept information about local subnets

from an external source.

Problems with BGP and IGP Interaction

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-12

Summary

• All core routers need external routers for proper packet forwarding.• A recursive lookup is performed in BGP to resolve the forwarding

path reference of the next-hop attribute.• Packet forwarding to external destinations benefits from the high-

speed convergence offered by an IGP; therefore, an IGP is still needed inside a transit AS.

• The IGP should provide reachability toward BGP next-hop addresses only if they are not disturbed by external updates from other autonomous systems (those are handled by BGP).

• IP packets could be erroneously forwarded out of the local AS if an external AS accidentally (or by intent: DoS) feeds the local AS with EBGP routes that should be local.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-13