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20 votes
Accepted

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 ...
Ron Maupin's user avatar
  • 99.8k
18 votes
Accepted

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 ...
Teun Vink's user avatar
  • 17.4k
12 votes
Accepted

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 ...
JFL's user avatar
  • 19.7k
11 votes
Accepted

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 ...
Teun Vink's user avatar
  • 17.4k
9 votes
Accepted

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 ...
jonathanjo's user avatar
  • 16.3k
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 ...
Teun Vink's user avatar
  • 17.4k
8 votes
Accepted

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 ...
Teun Vink's user avatar
  • 17.4k
8 votes
Accepted

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 ...
rnxrx's user avatar
  • 6,114
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 ...
Ron Trunk's user avatar
  • 67.6k
8 votes
Accepted

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" ...
Ron Trunk's user avatar
  • 67.6k
8 votes
Accepted

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 ...
Ron Maupin's user avatar
  • 99.8k
8 votes
Accepted

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, ...
JFL's user avatar
  • 19.7k
8 votes
Accepted

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 ...
Teun Vink's user avatar
  • 17.4k
7 votes
Accepted

Why is MBGP needed?

MBGP or Multiprotocol BGP extensions The first BGP specification was published in 1989, well before IPv6 was created and only shortly after multicast was added to IPv4. Even BGP-4 doesn’t support IPv6,...
Noction's user avatar
  • 166
7 votes
Accepted

Best practice for advertising routes in BGP

You are conflating two different issues: Route-maps can act as a policy filter, selecting which prefixes are advertised to neighbors and which are accepted from neighbors. Advertise-maps allow you ...
Ron Trunk's user avatar
  • 67.6k
7 votes
Accepted

Why are iBGP routes not propagated to other iBGP neighbors?

The reason is to prevent routing loops. BGP routers don't advertise routes learned from iBGP neighbor A on to a second iBGP neighbor C. This is specified in the BGP RFC 4271 §9.2 paragraph 2. To ...
Ron Maupin's user avatar
  • 99.8k
7 votes
Accepted

How is traffic forwarded though IXPs if route servers do not forward any traffic

In the IXP, the routers still have connections to each other, so traffic is sent between the routers. You may be confusing the (logical) TCP connections used by BGP to exchange routing information ...
Ron Maupin's user avatar
  • 99.8k
7 votes

Why Cisco BGP neighbor command need the remote-as number?

cisco documentation says that: "the configuration does not overtly define peers as iBGP or eBGP. Instead, each router examines its own ASN as defined in the router bgp command, and compares that value ...
noissue's user avatar
  • 71
7 votes

BGP convergence issue

There are two issues here: BGP keepalives are 60 seconds, and the hold down timer is 3 times that. So that's your lower limit, unless you work with your carrier and adjust your timers. You both ...
Ron Trunk's user avatar
  • 67.6k
7 votes

BGP | Advertise a summary prefix out to customer location

This appears to be a Cisco router. Cisco has the aggregate-address command. You can advertise the aggregate address and all the individual addresses: aggregate-address 8.0.0.0 255.0.0.0 or only the ...
Ron Maupin's user avatar
  • 99.8k
7 votes
Accepted

How to block advertisement of a prefixes recieved from 1 BGP neighbor to another

As mentioned by Ron, you can use no-export (Don't advertise to any eBGP peers) or no-advertise (Don't advertise to iBGP or eBGP peers) to achieve this. However, this may strain scalability issues if ...
ditrapanij's user avatar
  • 1,307
7 votes
Accepted

Can I announce the same block of /24 given to me by ARIN at two different locations?

You can announce it from both locations, but the result will be that traffic to the IP block will be delivered to whichever location provides the "best" route for the traffic, which may or ...
Peter Green's user avatar
  • 13.3k
6 votes
Accepted

Multi-homing and BGP routing with less than /24

Most transit provider will refuse to announce tough BGP a prefix longer than /24. (I.E. /25 for example) It's related to the size of the routing tables in the routers. You have to understand that ...
JFL's user avatar
  • 19.7k
6 votes
Accepted

A more specific BGP route vs. a less specific static route, which is selected?

A more specific route is what is used to forward packets. The two prefixes you have are really that, two separate prefixes, just as if they were in completely different network blocks. You are trying ...
Ron Maupin's user avatar
  • 99.8k
6 votes

Should we require a 16-bit ASN?

If you're planning to do a lot with BGP communities which are used for signalling between your own ASN and your upstream or downstream peers, a 32-bit ASN might be less useful since the second 16 bits ...
Teun Vink's user avatar
  • 17.4k
6 votes
Accepted

iBGP AS path prepend propogation

I think you are confusing routers and ASes. To BGP, an AS is a hop, not a router. You can try to influence a neighbor AS for which peer it uses to send traffic to your AS, but the neighbor AS is free ...
Ron Maupin's user avatar
  • 99.8k
6 votes
Accepted

Why I Can't Summarize (aggregate) These Subnets in BGP router?

Generally speaking, route summarization hides information about more specific routes. It also hides the fact that you may not have a route to all addresses in your summary. So yes, you can summarize....
Ron Trunk's user avatar
  • 67.6k
6 votes
Accepted

Null 0 in ISP Core Router running bGP?

ip route 0.0.0.0 0.0.0.0 null0 will drop all unknown destinations. Without a more complete configuration it is difficult to hypothesize its full purpose. A popular ISP service is to provide a ...
TDurden's user avatar
  • 1,241

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