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37

(Note that these two terms are often used interchangeably, which can lead to some confusion.) Looking Glasses A looking glass is usually a website (most often CGI) that interfaces with routers that are owned and operated by a single ISP or other network operator. Most of the time these are publicly accessible, but there may be instances where they're not. ...


26

Can anyone explain me what is the need of IBGP communication for the routes, when we have the IGP protocols (OSPF, RIP) for internal communication? Scalability1: Imagine that you're receiving 500,000 EBGP routes in more than one location2, and you need to influence the per route exit point in your AS. BGP can handle many more routes than IGP protocols. ...


20

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 in two different places, and the routing protocol chooses the closest network to which it send traffic destined for that network. It seems like having to ...


19

These days it's usually at /48, and it's very unlikely to ever be longer. There are still AS' filtering longer than /32 though, but they should all have a default at this point (Verizon were a hold-out, but I believe they've given up). There are also AS' filtering on allocation lengths, so if you got a /32 they might require you advertise it as such (based ...


19

TCP "keepalive" is a feature that is not present on all operating systems. It is not in the TCP standard itself and it's not reliable. RC1122 specifes it, but explicitly states: Implementors MAY include "keep-alives" in their TCP implementations, although this practice is not universally accepted. If keep-alives are included, ...


18

To conserve memory at destination, it was not important to micro-optimize forwarding path in the past. This is quote from RFC4456: One of the key component of the route reflection approach in addressing the scaling issue is that the RR summarizes routing information and only reflects its best path. While scaling is always important, clearly there are ...


18

Every time you see .+_, that regular expression represents a single BGP autonomous system. This as-path list denies BGP as paths equal to or longer than 12 paths long. The regular expression works because . represents any character, + is a wild card which optionally repeats the previous character an unlimited number of times, and _ represents the space ...


17

Yes, this can be done by BGP and is called AS prepending. The objective is to change the best route taken by BGP, since the shortest AS path is preferred by BGP. In your case there might be another path with one AS 2 only, thus BGP will select the other route, not this one. Example would be a site with 2 WAN links to the internet, if we prepend our AS ...


17

TCP/IP sockets establish an end-to-end connection through the network, between two specifically addressed end points. BGP uses TCP/IP to communicate between routers (any devices exchanging routing information.) The information exchanged is used by the BGP peers, to better choose the way they choose where to send, (aka, next-hop) packets that they need to ...


17

I think your study guide is a little outdated. Currently we are at about 500k routes on the Internet. Geoff Huston is collecting stats weekly. You can find his reports here. If you want to see for yourself goto http://www.routeviews.org/ and get access to some real systems on the internet.


17

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 been assigned a number of IP addresses to use from that /24. However, that does not give you the right to advertise them under your own ASN in BGP. If you do so, ...


16

bgp bestpath as-path multipath-relax was introduced by CSCea19918. Normally eBGP load-balancing requires the candidate routes to be equal-cost paths; i.e. identical BGP attributes: same Weight same Local-Pref same AS-Path (both the AS numbers, and the AS path length) same Origin same MED different next-hop As you mentioned, this command relaxes the same ...


16

This question is akin to asking, "Why can't I use a screwdriver to hammer nails?" You could use OSPF in certain inter-AS situations if you REALLY, REALLY wanted to/needed to. There would be many "gotchas" and caveats to keep in mind when utilizing OSPF as an EGP. Just like you could hammer nails with a screwdriver. However, you would risk breaking the ...


15

You're kind of thinking about it the wrong way, but I'll try to explain. When you purchase bandwidth from an ISP, this is called transit (colloquially in the industry). Assuming you've got yourself some PI space (1.0.0.0/8, for example), you're paying your ISP to get your bits from your network to other networks. So say your AS is 6500, and your ISP is 3356....


15

It is unlikely (but not impossible) for the communities you tag on your routes to propagate past your upstream. Most providers will strip the communities out before readvertising routes up/downstream. If you want to influence multiple remote ASes in this manner, you would need to use AS_PATH prepending. Prepend 2-3 times to your backup/passive provider, and ...


15

which peer will send the open message first? Normally, the speaker that opens the socket sends the first OPEN message. But it actually doesn't matter (ref the DelayOpen timer), because BGP also provides a way to delay the OPEN message so the opposite peer can send first: Option 1: DelayOpen Description: The DelayOpen optional session attribute ...


14

You can indeed request a Provider Independent (PI) assignment from the local RIR through an LIR. Routing a block of IPv6 address space is done with BGP in the same way as a block of IPv4 address space. The block is only a bit bigger :-) For IPv4 it is common that a block smaller than a /24 (so a /25 or longer prefix) will not be commonly routed by ISPs. In ...


14

Yes. Option 1 is likely your best bet for simplicity's sake, but you can use the confederation method if you don't want to change the ASN. You can also do neighbor x.x.x.x local-as <as> in the BGP config but this prepends an ASN of your choosing onto the path, rather than replace the ASN in the path, so updates from one router to the other would be ...


14

Assuming you have no IGP configured (such as EIGRP / OSPF / ISIS / RIP), then the simplest explanation is that R3 hasn't got a route to 192.168.1.0/24's next-hop when the iBGP update arrives at R3. loop0(R1)s0/0 <-----------> s0/0(R2)s0/1 <-------------> s0/1(R3) AS 1 AS 2 AS 2 -----...


14

BGP message length in of 2 bytes. So why can't the max bgp message size be 65535? why is it 4096? Official Answer Quoting Tony Li when he responded to this question on the IDR List: 1a) Having a fixed size is good because it makes the protocol implementations easy. There is no point to having complexity in an implementation if it provides no benefit. ...


14

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 is no longer reachable. Benefit: The rest of your network stays up. Packetlife has a nice explanation on how it work and how to do it. The second option builds ...


14

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 you pointed out, BGP uses a layer-4 protocol to communicate with other BGP speakers in order to do what is normally considered a layer-3 function. Many network ...


13

As the others stated, RPKI would be the way to go, but it's not there yet. At exchange points we generally put a max-prefix limit on every session. Additionally we use the following rules: No default route No bogons, more exactly this list: route-filter 0.0.0.0/8 orlonger reject; route-filter 127.0.0.0/8 orlonger reject; route-filter 10.0.0.0/8 orlonger ...


13

Routing protocols do not "achieve" L3 connectivity. They populate the routing (forwarding) table of the router with information learned from other routers. BGP is an "application" that runs over TCP/IP. In other words a BGP router uses TCP/IP to communicate with other BGP routers to exchange routing information. In order for BGP to work, you must ...


12

Is there a clear definition between site A and site B? If so then I would look to define a policy on the edge routers to inject a community when receiving routes from the carrier MPLS. Once this community has been put on the prefixes (say 100:1 for site A and 100:2 for site B) you could then add a policy to each of site A's routers to increase the LP for ...


12

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 when aggregating routes downstream of an autonomous system; so the use case for an unordered list is bgp aggregation. EXAMPLE: In the example below, AS65500 ...


11

I don't guess there's any real reason it can't work just fine. You, of course, have the normal loss of information anytime you redistribute from one protocol into another, but there's an argument to be made that you get that when you use network statements as well, so that's kind of a wash. The main benefit that I see in this is that redistribution would ...


11

At the moment (until RPKI is more widespread), we generally just filter common bogons and apply a max-prefix filter to exchange peers. We also filter certain ASNs, ones we are certain will never show up in most peering sessions, such as Level3 or Cogent, or should not be transited over an exchange. We usually find that most of the common route leaks are not ...


11

You didn't mention vendor, but if Cisco IOS, you can use: router bgp 43792 bgp bestpath as-path multipath-relax address-family ipv4 maximum-paths 2 ! ! multipath-relax is needed as normally you'll only multipath with same as-path routes. I'm personally strongly against carrying default routes via dynamic routing-protocols, there simply isn't any ...


11

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 of cabling and route rports. As it only provides layer 1 and 2 connectivity, the IXP does not require to have an ASN. However, IXPs often do a bit more: they ...


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