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20 votes
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Does anycast addressing add additional latency in any way?

Does anycast addressing, in itself, add any additional latency to network connections? No. one using unicast and the other using anycast Anycast is unicast. It is just that the same network is ...
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18 votes
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Can I announce prefix 161.117.25.0/24 even though I don't have all of /24 IPs?

That /24 is part of a larger block (161.117.0.0/16) which is assigned to Alibabacom Singapore and announced by AS45102 (which is Alibaba again). My guess is that you're a customer of them and have ...
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15 votes
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In which OSI/TCP-IP model layers do BGP, RIP protocols belong?

You have to remember that models like OSI are just that, models. They are theoretical. The real world doesn't fall neatly into these models. For the most part, routing is a layer-3 function, but, as ...
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14 votes

Use BGP to defend against a DDoS attack originating from remote AS

There are two things you could do with BGP: RTBH - Remotely-Triggered Black Hole First option is the radical one: Blackhole (stop traffic) for the IP getting attacked. Downside: The IP being targeted ...
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12 votes
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How is BGP "AS_SET" used?

My question then is, what would the purpose be of sending an unordered list? In what cases would you be better off sending an unordered list as opposed to an ordered list? as-set is commonly used ...
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11 votes
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When is BGP needed?

The generic statement that BGP is required when you need to route between two autonomous systems is fairly misleading. There are a lot of scenarios in which you may choose to use BGP, or it might ...
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11 votes
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Difference between default route, partial and full routing table for a BGP session

A default route is the route that will be used if there's no other route that matches the destination in the router's forwarding table. A full routing table is a table which contain all the routes the ...
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11 votes
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Why do IXPs need ASN?

In its most basic form, an IXP is nothing more than a large switch, allowing many networks to exchange traffic without having to interconnect with every other network on the IXP, thus reducing costs ...
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10 votes
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Local pref VS prepend AS-path for two eBGP router

Most of your confusion seems to be related to your other duplicate question, which is the same as 3. above...: prepend AS path usually running with route-map out to adjust outbound traffic from ...
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10 votes

OSPF or iBGP for interior routing protocol?

iBGP requires a full mesh or use of mitigation like confederations or route reflectors, BGP doesn't converge with anything like the speed of OSPF, etc. Each OSPF router would have a full ...
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10 votes
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Smallest IPv6 block advertised?

Yes. Nobody will advertise any network smaller than a /48. The same thing is in IPv4; no ISP will advertise any network smaller than a /24. The problem is the sheer number of routes that would ...
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9 votes
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how can I filter all the BGP routes from one specific AS?

There are a couple ways to do this, without having to specify every single prefix that you're receiving from AS6400 in a prefix-list (I would personally advise against doing this because as you ...
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9 votes
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Matching a community in a route-map

Your issue was caused by the generic permit (no match clause) attached to rule 20 of the default-import route-map. Route-maps usually act like ACL's and stop processing after the first match. In ...
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  • 1,470
9 votes

When is BGP needed?

Instead of describing where you would use BGP, let me describe why: Other protocols like EIGRP or OSPF assume the best way to get somewhere is the fastest link -- i.e. the link with the highest ...
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  • 63k
9 votes

OSPF or iBGP for interior routing protocol?

First off, I would dispute the assertion that some others have made that OSPF converges faster than BGP, but more on that later. To answer the OP question first. You want both. iBGP is designed to ...
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  • 2,476
9 votes
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Why is the BGP base on TCP 1027 rather than 179?

One side of the connection will have an arbitrary port number, the other will be on 179. Cisco Press "BGP Fundamentals" has a good explanation (link) the neighbor with the higher IP address ...
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9 votes

BGP traffic management

Load balancing with BGP is very tricky and the mechanisms available are quite crude. With BGP you can only route based on the destination IP address. There's no way to distinguish payload traffic and ...
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8 votes
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Filter the default route, but accept all other routes

You need to add a statement to explicitly allow those prefixes after denying the default-route, so the final prefix-list becomes: ip prefix-list no-default-route seq 5 deny 0.0.0.0/0 ip prefix-list ...
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  • 15.7k
8 votes

In which OSI/TCP-IP model layers do BGP, RIP protocols belong?

BGP is on top of TCP, so it would be Internet layer 4, OSI layer 7. Usually external BGP is done only between 2 directly connected peers, enforced by setting TTL flag on IP header, which is located at ...
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8 votes
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Why does iBGP require a full-mesh?

...why is a iBGP connection between two routers that don't have eBGP connections needed? They won't distribute anything, right? Not quite right. It's not that two routers connected via iBGP cannot ...
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  • 94.1k
8 votes
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RTBH vs FlowSpec

Typically, FlowSpec is only used within a specific network, while RTBH is used between a network and its upstream(s). The mains reasons for this are that only a few vendors support FlowSpec allowing ...
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8 votes
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What considerations should be made when determining whether to use the BGP "network" command vs Redistribution?

First - there are two distinctly different problems being solved with these commands: The network and redistribute commands are mechanisms to initiate routes into BGP, either statically (with a ...
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  • 6,036
8 votes

Difference between default route, partial and full routing table for a BGP session

A default route is the same in every routing table -- it is the route of last resort, when no other route matches. Since many organizations have only a single connection to the Internet, a default ...
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  • 63k
8 votes
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My BGP router does not advertise configured prefixes

Router 2 will not advertise networks that are not in its routing table. So if router 2 does not have a route to 2.0.0.0, it will not advertise it. This prevents BGP routers from "blackholing" ...
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8 votes
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BGP Best Path Selection Algorithm

By default, the shortest path will be the best path. If you notice, the first three are locally configured options that you can use to override any selections that would normally be made by the ...
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8 votes
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How much RAM you actually need to keep whole global BGP routing table?

Here is the info given but a production router that does IPv4 BGP only, as of 2021 October 5: 1547576 RIB nodes, using 142 MiB of memory 1687609 BGP routes, using 103 MiB of memory 8 Static routes, ...
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  • 18.4k
8 votes
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In practice, how are BGP Advertisement destinations determined and configured?

Some connection between the two ISPs is needed. Dedicated fibers (for example in a datacenter they're both present in) is very common. But there are other possibilities: a layer 2 connection (for ...
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7 votes

Use BGP to defend against a DDoS attack originating from remote AS

The RTBH method described by @Sebastian via Packetlife is helpful, but that method will only work if your uplink is not saturated by attack traffic. If your uplink is saturated, then the blackhole ...
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