Advanced BGP - BGP Route Filtering and Manipulation
8 important questions on Advanced BGP - BGP Route Filtering and Manipulation
When is BGP Route Filtering applied and which 4 options are provided by IOS XE?
The 4 options are:
1. Distribute list (Filtering of networks based on ACLs, ends with implicit deny)
2. Prefix list (prefix-matching specifications, ends with implicit deny)
3. AS Path ACL/Filtering (Filter with RegEx on AS_Path value, end with implicit deny)
4. Route Maps (Various options, end with implicit deny)
When doing Route Filtering in BGP, what is important to note about the Distribute List and Prefix List options?
In what configuration context is Distribute List filtering configured and of which 3 components is the command comprised of?
It starts with a (Neighbor ip-address) to indicate the neighbor
followed by (Distribute List ACL-name) to indicate the ACL
ended with (in/out) to indicate if it is and inbound policy or outbound.
Example: Neighbor 10.12.1.2 distribute-list ACL-Allow in
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In what configuration context is Prefix List Filtering configured and of which 3 components is the command comprised of?
Under the Address Family which is under the BGP AS context.
It starts with a (Neighbor ip-address) to indicate the neighbor
followed by (Prefix-List prefix-list-name) to indicate the list
ended with (in/out) to indicate if it is and inbound policy or outbound.
Example: Neighbor 10.12.1.2 Prefix-List Prefix-List-Allow in
How does AS Path ACL Filtering work?
ip as-path access list ACLNumber permit/deny REGEX
This is then added under the address family under the BGP AS with the command:
neighbor IP-Address filter-list ACLNumber in/out
What is the power of Route Maps compared to the other 3 BGP Route Filtering options?
And instead of just an ACL where each line is a statement each line can have different options. A Prefix List filtering in 1 line and a different Prefix List in the next line where there is also a Set statement for routes matching that specific list.
When changing BGP Route Filtering, why can it be important to refresh the BGP Connections and how is this done?
This can be done with a hard or soft reset with the command:
clear ip bgp ip-address [soft]
A hard reset tears down the BGP connection and removes the BGP Routes from the peer.
Append the Soft option for a soft reset which invalidates the BGP Cache and requests a full advertisement from the BGP Peer.
What is important to note about the BGP Reset command ip-address and soft option?
Some routers support a route refresh capability which would be negotiated when the session is established.
A BGP Reset command with the Soft option specified actually triggers a Route Refresh when it is supported.
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