BGP Communities

If we used just prefix-lists and distribute lists to filter BGP updates it would be a very manual intensive job due to the size of most BGP networks and the fact that you would have to configure each router one at a time! Today I learned that you can group routers running BGP into groups that can share the same filtering information. Therefore you would only need to configure one of the routers in the group for all of the other routers to know what updates should be filtered and what shouldn't.

"BGP communities function allows routers to tag routes with an indicator (the community) and allows other routers to make decisions (filter) based on that tag. BGP communities are used for destinations (routes) that share some common properties and that, therefore, share common policies; routers, therefore, act on the community, rather than on individual routes. Communities are not restricted to one network or autonomous system, and they have no physical boundaries."

the community attribute is considered an optional transitive attribute. If a router receives an update with community attribute information but doesn't use that attribute, it will ignore it but pass it along to other BGP neighbor peers. The community attribute consists of 32-bits, 16 for the Autonomous System number (AS) and the other 16 identifies the community number.

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Creating Prefix-Lists for BGP Routing 2

I spent this early afternoon finishing up the BSCI BGP Appendix section on prefix-lists for BGP, I mainly created a lab that specifies that that the network 172.30.0.0 /24 in the AS 65500 only shows as the supernet 172.0.0.0/8 in the AS 65000 BGP table as shown above. Tomorrow I will learn a little bit about BGP communities and go over what I've learned!

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Creating Prefix-Lists for BGP Routing

I spent a good bit of my morning learning and configuring BGP prefix-lists which I will wrap up tomorrow most likely. Prefix-lists provide greater flexibility over access-lists due to the fact you're allowed more granular control of where you want input your statements inside the prefix list. This differs from the standard access-list where one no command on the ACL requires you to recreate the access-list completely! I'm still not entierly sure how prefix-lists differ from ip access-list commands which allows you to enter sequence number states like prefix-lists. I do know that you can control exactly how you want a neighbor BGP autonomous sysstem (AS) to know about external routes by using the le and ge commands.

The le and ge values are used in a prefix-list statement to create a range of the prefix length to be matched more specifically compared to the network/length commands used in the prefix-list statements. Prefix lists do provide the advantage of being less performance intensive due to not requiring the amount of route lookup processing sometimes required by large access-list tables.

As you can see in the above lab I worked earlier, the prefix list tells AS_65000 to only let AS_65002 know about the 172.16.0.0 /16 external network instead of the more specific 172.16.10.0 /24 and 172.16.11.0 /24 routes.

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BGP Summary and Aggregated Routes

I spent this morning briefly covering how to summarize routes in BGP using CIDR Aggregated Routes. BGP specifically uses the Atomic Aggregate attribute which is considered one of the well-known discretionary attributes. BGP also uses the optional transitive attribute called an Aggregator which specifies the BGP ID and the AS that performed the aggregation in BGP updates. If you aren't careful when planning which routes to summarize your AS could easily claim routes that it really doesn't own which could upset other AS's in the BGP system! AS's doesn't really use aggregation as much as they could because some are multihomed to many ISP's and would rather make sure that all of the routes that own are being advertised without being summarized into one route.

Policy Based Routing 2

This morning I created another PBR lab that I was able to wrap my head around a lot easier than yesterday. As you see in the above image, there are 3 routers in which specific LAN traffic from Router C should be routed out of Router A's Serial 0/0/1 interface. It was good to get some more hands on with route-maps the past few days. I'm going to work some more labs throughout the day most likely on BGP. My lab guide book should be here in another weeks so you should be seeing a ton of new labs from me here shortly!

Policy-Based Routing

Now that I finished the main book for BSCI, I'm now reviewing everything I learned and will spend most of my time creating labs and touching up the details. But before I do to much, Cisco was kind enough to include 5 extra Appendix PDF files to learn about some technology in even more detail. This is mainly appendixes on how to manipulate packets and even more BGP no surprise! I hear that in order to fully be perpared for the BSCI you have to dig even deeper than what the Self Study Guide book provides. This includes everything from reading white papers, CBT's, and creating labs for pretty much ANYTHING related to the exam.

Today I learned a little bit about Policy Based Routing (PBR) which is basically route-maps on steroids. Similar to how there are access-lists and then extended access-lists (access-list on steroids), PBR allows you to maniplulate routes in a more granular manner. Tomorrow I'll be finishing this appendix up and moving to the last few that are left.

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