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BGP

whats the difference between ip prefix list and distribute list and access list.....now if i have 2 netwroks and i advertise them to BGP neigbour with the network command why would i use prefix list in that i m not getting this idea......guru's help me required plz....thanks in advance i have seen lot of doc on that but not getting it plz can someone tell me in plain words.....thanks a million

5 REPLIES

Re: BGP

Hi,

We use prefix-lists or distribute-lists and access-lists to filter either incoming or outgoing routes, you might require to send routes to a neighbor but filter them and don't send them to another neighbor, and you might also require not to receive certain routes from certain neighbors.

In brief, the advantages of using prefix lists over access-lists are as follows:

. Significant performance improvement in loading and route lookup of large lists (less CPU intensive than access-list).

. They allow editing of the lists so that additional lines of code can be inserted anywhere in the list.

. More user friendly CLI. The CLI for using access lists to filter BGP updates is difficult to understand and use because it uses the packet filtering format.

. Greater flexibility.

How to Block One or More Networks From a BGP Peer:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00801310cb.shtml

HTH, please rate if it does help,

Mohammed Mahmoud.

New Member

Re: BGP

thanks, Mohammed. I also have been struggling with BGP, since failing the BSCI recently. I have another, related, question:

the configuration guide for prefix lists(http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800ca763.html#wp1001470)

says "Note The sequence number of an entry need not be specified when you delete the entry." -- but how then will you specify which entry to delete, if not by sequence number?

Re: BGP

Hi,

Sequence numbers are generated automatically unless you disable this automatic generation. If you disable the automatic generation of sequence numbers, you must specify the sequence number for each entry using the sequence-value argument of the ip prefix-list global configuration command.

Regardless of whether the default sequence numbers are used in configuring a prefix list, a sequence number need not be specified when removing a configuration entry, this is simply because when deleting the router doesn't delete using the prefix-list seq number, it uses the contents of the statement itself which can never be duplicated, the seq number main job is to be able to insert statements in between:

Router(config)#ip prefix-list test permit 192.168.99.0/24

Router(config)#ip prefix-list test permit 10.10.10.0/24

Router(config)#ip prefix-list test permit 1.1.1.0/24

Router#sh run | i prefix

ip prefix-list test seq 5 permit 192.168.99.0/24

ip prefix-list test seq 10 permit 10.10.10.0/24

ip prefix-list test seq 15 permit 1.1.1.0/24

Router(config)#no ip prefix-list test permit 10.10.10.0/24

Router#sh run | i prefix

ip prefix-list test seq 5 permit 192.168.99.0/24

ip prefix-list test seq 15 permit 1.1.1.0/24

When trying to enter the same network but with different sequence number you will fail, this means that the router can actually judge the contents of the statement rather that the seq number:

Router(config)#ip prefix-list test seq 8 permit 1.1.1.0/24

%Insertion failed - prefix-list entry exists:

seq 15 permit 1.1.1.0/24

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

Re: BGP

Nicely explained, deserves rating

Narayan

Re: BGP

Hi Narayan,

Thank you very very much, it really means a lot to be rated by one of the elite members of the forum :)

BR,

Mohammed Mahmoud.

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