© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-1 Scaling Service Provider Networks...

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-1

Scaling Service Provider Networks

Scaling IGP and BGP in Service Provider Networks


• Overview • Common Service Provider Network• Route Propagation in Service Provider Networks • Scaling Service Provider Routing Protocols• Scaling Service Provider Addressing• BGP Policy Accounting• Summary

Outline


• Runs BGP or static routing with customer• Exchanges routes with other service providers via BGP• Runs IBGP between its own BGP speakers• Runs one instance of IGP (OSPF or IS-IS)

– IGP used for internal routes only

Common Service Provider Network


Common Service Provider Network (Cont.)


• Networks are divided into POPs.• Different types of media are concentrated at the POP.• Optimal routing between POPs is desired.



• POP routers use BGP or static routing with customer routers.• The provider core IGP is a single instance of IS-IS or OSPF.• The core IGP is used only within the service provider backbone.



Route Propagation in Service Provider Networks

• BGP route propagation– BGP carries customer routes.– BGP carries other provider routes.

• IGP route propagation– IGP is responsible only for the next hop.

• Do not redistribute BGP into IGP.– IGP performance and convergence time suffer if a large

number of routes are carried.– No IGP is capable of carrying full Internet routes.– A full Internet routing table has exceeded 110,000 routes.


Routing Information Exchange with Other Service Providers

• BGP is used to exchange routing information between Internet service providers.


Routing Information Exchange with Customers

• The provider edge router redistributes static customer routes into BGP.

• BGP carries customer routes.


Next-Hop Resolution

• The core IGP of the service provider should carry information only about backbone links and loopback addresses.


Scaling Service ProviderRouting Protocols

IGP responsibilities• Carrying route to BGP next hop• Providing optimal path to next hop• Converging to alternate path so that BGP peering is

maintained BGP responsibilities• Generating BGP update • Scaling BGP policies• Scaling IBGP mesh• Reducing impact of flapping routes


Scaling IGP

• Loopbacks and internal links carried only• Good addressing structure within the POP required• Loopback addresses taken out of a different address space

and not summarized• Summarization of internal link addresses on POP level• Optimal routes to loopbacks needed only (with proper

summarization)


Scaling BGP

• BGP policy scaling– The AS routing policy should be unitary and easy to maintain.– This goal is achieved by reusing the same configuration in all EBGP-

speaking routers.

• IBGP mesh scaling– Avoid unnecessary duplicate updates over a physical link.

• Updates and table size scaling– Route summarization is the key to scalability.


Private vs. public addresses • Private addresses on links break traceroute when run from

inside a firewall.• Private addresses on loopbacks call for careful external

routing.• MPLS with TTL propagation disabled solves the traceroute

issue.• Otherwise, use public addresses in service provider

networks.

Scaling Service Provider Addressing


Scaling Service ProviderAddressing―Example

• Assign addresses to allow for route summarization.


BGP Policy Accounting


Summary

• The service provider network usually consists of a network core that interconnects edge devices connecting customers or other service providers and that are located at various POPs.

• Service providers use an IGP to carry internal routes and to provide optimal routing between POPs, the information that is needed for IBGP sessions to be established, and the addresses that are required for BGP next-hop resolution.

• In scaling IGPs and BGP in service provider networks, the IGP is responsible for carrying routes to the BGP next hops, providing an optimal path to the next hop, and converging to an alternate path in the case of lost links or routers; the BGP is responsible for generating BGP updates about reachable and unreachable networks, implementing and scaling the BGP routing policy, and reducing the impact of individual flapping routes through route summarization.


Summary (Cont.)

• Using private addresses on the service provider router loopback interfaces is possible, but you must take care not to advertise any private addresses to any other autonomous systems. You can prevent the announcement of any private addresses by using prefix-lists that are applied on outgoing updates to external neighbors.

• BGP policy accounting measures and classifies IP traffic that is sent to, or received from, different peers. Policy accounting is enabled on an input interface, and counters based on parameters such as community-list, AS number, or AS path are assigned to identify the IP traffic.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—6-1 Scaling Service Provider Networks...

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