Showing posts with label cisco. Show all posts
Showing posts with label cisco. Show all posts

Sunday, June 9, 2024

Cisco hidden command: bgp bestpath as-path multipath-relax

Hidden command

  bgp bestpath as-path multipath-relax

What for is this?

By default, Cisco does not do load-balance or distribute traffic between different ASs, this command allows it. Important, you must also use the maximum-paths command


router bgp 65001

 bgp router-id

 bgp log-neighbor-changes

 bgp bestpath as-path multipath-relax

 neighbor 2001:DB8:12::2 remote-as 65002

 neighbor 2001:DB8:12:10::2 remote-as 65002

 neighbor 2001:DB8:13:11::3 remote-as 65003


 address-family ipv4

 no neighbor 2001:DB8:12::2 activate

 no neighbor 2001:DB8:12:10::2 activate

 no neighbor 2001:DB8:13:11::3 activate



 address-family ipv6

 maximum-paths 3

 neighbor 2001:DB8:12::2 activate

 neighbor 2001:DB8:12:10::2 activate

 neighbor 2001:DB8:13:11::3 activate


Output after implementation:

     Network          Next Hop            Metric LocPrf Weight Path

 *m  2001:DB8::4/128  2001:DB8:12:10::2

                                                              0 65002 65004 ?

 *>                   2001:DB8:12::2                         0 65002 65004 ?

 *m                   2001:DB8:13:11::3

                                                              0 65003 65004 ?

 *m  2001:DB8:24:11::/64


                                                              0 65002 65004 ?

 *>                   2001:DB8:12::2                         0 65002 65004 ?

 *m                   2001:DB8:13:11::3

                                                              0 65003 65004 ?

 *m  2001:DB8:34::/64 2001:DB8:12:10::2

                                                              0 65002 65004 ?

 *>                   2001:DB8:12::2                         0 65002 65004 ?

 *m                   2001:DB8:13:11::3

                                                              0 65003 65004 ?

Thursday, February 1, 2024

A Much-Needed BGP RFC: AS Path Prepending


The Border Gateway Protocol (BGP) plays a critical role in building and maintaining Internet routing tables, so much so that it is considered the “glue” that holds the Internet together. In this context, a long-standing and very popular technique known as ‘AS Path Prepending’ has been devised as a key strategy for influencing route selection and optimizing an AS’s inbound and outbound traffic.

In this document, we will navigate through the IETF draft titled “AS Path Prepending” [1], which includes several ideas and concepts that are of great value to the community.

About draft-ietf-grow-as-path-prepending

This draft has been under discussion within the Global Routing Operation (GROW) Working Group since 2020 and is currently on version 10. The document has seven co-authors: M. McBride, D. Madory, J. Tantsura, R. Raszuk, H. Li., J. Heitz, and G. Mishra. It predominantly received support on the discussion list (including my own). You can read the draft here.

What is AS Path Prepending?

AS Path Prepending is a technique that involves repetitively adding one’s autonomous system identifier (ASN) to the list of ASs in a BGP route path (AS_PATH). Its goal is to influence route selection by making certain paths less attractive to inbound/outbound traffic. In other words, it consists of adding our autonomous system to the AS_PATH and therefore artificially “lengthening the path” to a prefix on the Internet.

In the figure above, without prepends, Router A prefers to go to C via B. However, when three prepends are added on B, router A decides to reach C via D.

Why is AS Path Prepending used and what is it used for?

AS Path prepending is used for multiple reasons. The main reason is undoubtedly traffic engineering, which in turn is used to influence an AS’s inbound and outbound traffic. It is very likely that the AS wishes to achieve one of the following goals:

  • to distribute traffic among two or more upstream providers, or
  • to have an upstream backup provider.
  • Whatever the case, the goal is traffic engineering.

To prepend or not to prepend, that is the question

Prepending is a bit like NAT in that it is often a necessary evil. As we will explain, its excessive and sometimes unnecessary use can become a vulnerability with significant implications for network stability.

What’s wrong with using AS Path Prepending?

We all know that AS Path Prepending is a very common technique to influence BGP decisions. However, its excessive, incorrect, and sometimes unnecessary use can have negative consequences. For example:

  • Creation of suboptimal traffic flows. In other words, we may achieve our goal of distributing traffic in the immediate links; however, beyond our immediate upstream, traffic is not optimized to reach our autonomous system and vice versa.
  • Prefix de-aggregation. When implementing traffic engineering, it is very common to de-aggregate prefixes, which affects the Internet ecosystem.
  • In the event of a route-leak, under normal circumstances, the as-path of our advertisements would be shorter than that of the leaked route. However, if we artificially lengthen the path by prepending, the as-path of the leaked routes may be shorter than those we are legitimately announcing for our legitimate prefix, which would have lower preference, leading to potential route hijacking, attacks, and a long etcetera.
  • Memory consumption. As expected, these AS Path Prepends are learned by BGP Speakers, thus increasing their memory usage. To this I would add that prepending introduces a small additional CPU usage penalty for each prefix.

Given that AS Path Prepend is no longer recommended, what alternatives are available?

There are many techniques for performing traffic engineering in BGP. I will mention some that appear in the draft:

  • Leveraging BGP communities. In addition to the well-known BGP communities, I recommend talking to your BGP peers to optimize traffic. There are numerous BGP communities implemented by providers, which might certainly benefit your setup.
  • Announcing more specific routes to your main upstreams.
  • Manipulating the AS Origin Code. Remember that this attribute is also found in the BGP route selection algorithms.
  • Using Multi Exit Discriminator (MED), a non-transitive attribute that can be used with excellent results for manipulating inbound traffic when we have several links to the same provider
  • Using Local-Preference, another non-transitive attribute, perfect for influencing the traffic that leaves our autonomous system

This is all well and good, but I still need to use AS Path Prepending. Any suggestions?

The draft mentions the best current practices when using AS Pat Prepending, which I will summarize below:

  1. Only use AS Path Prepending if it is absolutely necessary.
  2. Due to some traffic manipulation techniques, when using AS Path Prepending, we may not see significant changes in the traffic distribution, which is why it is important to talk to our peers to see if they will honor the prepends.
  3. Use local-preference on our network.
  4. Don’t prepend ASNs that you don’t own.
  5. Don’t prepend if you are connected to a single ISP using a single link, i.e., single homed (this one is not included in the draft).
  6. If we prepend a prefix, it might not be necessary to use that prepend for all our peers.
  7. There is no need to use more than five prepends. The reason for this is that more than 90% of path are five ASs or fewer in length.

Final Considerations

The use of AS Path Prepending is a valuable strategy but should be used only when necessary and with caution, following best practices. Excessive use of prepends may cause unforeseen events that may affect our autonomous system from a traffic and a security perspective.

We invite you to read the full draft (available here and to join the discussion on the LACNOG mailing list.

We also encourage you to comment on this post to let us know if you are prepending your ASN, as well as why and what you are using this for.



Saturday, July 24, 2021

Solved: GNS3 - Private Vlan - non-operational - Cisco


  In summary: private vlan are not working in GNS3, not even using IOU or VIRL


  User IOU i86bi-linux-l2-adventerprisek9-15.2d.bin


IOU3#show vlan private-vlan 

Primary Secondary Type              Ports

------- --------- ----------------- ------------------------------------------

500     501       community         Et0/1, Et0/2, Et1/0

500     502       isolated          Et0/0, Et0/3, Et1/0

Good luck!,

Monday, July 8, 2019

BGP: To filter or not to filter by prefix size. That is the question


In order to write these post the R+D team and the WARP team joint together after analyzing some security incidents related with BGP, network accessibility, network hijacks and network visibilities.

As you probably know, in the BGP world, there are dozen of ways to filter prefixes. The goal of this post is to show some recommendations in order to have a more stable network, keep the visibility of your prefixes as much as possible and of course reduce the calls to the NOC


Many ISPs around the world can not (or do not wish) to receive the full routing table (DFZ) which by the time of writing this text is about 750.000 prefixes (IPv4)

  • The above description could be due to some -not limited- of the following causes:
  • The routers does not have enough RAM to learn all the prefixes (please also note that there could be also several BGP session at the same time)
  • The network admin wants to save CPU cycles in their devices
  • The upstream providers is not announcing the full routing table
  • The network admin wishes to keep his network simple and easy

Anyhow, in the end of the story, the router is not learning the full routing table.


Not learning the full routing table can bring many partial inconveniences that in the end brings connectivity problems, users complaints, issues accessing some web sites and more.


Please try to imagine the following case

  1. I have a router (property of EXAMPLE) in Internet that is ONLY learning a partial DFZ
  2. The routers mentioned in “1” is only learning “big network”, over /20. I mean, the router learns /20, /19, /18, etc. (of course, we are talking about IPv4)
  3. Based in the configuration indicated in number “2”, the router is not going to learn prefixes such as /21, /22, /23 nor /24
  4. So far so good. However, somewhere in Internet, some people hijacked an /21 to the company ACME (hijack, bad configuration, whatever)
  5. ACME decides to perform more specific prefix advertisements, so, he takes his /21 and announces 8 /24 prefixes to the DFZ.
  6. Because of the filters configured by EXAMPLE, he will never learn the legitimate /24 prefixes advertised by ACME
  7. EXAMPLE will keep learning the hijacked /21 which obviously will bring connectivity problems aim to the legitimate owner of the network


The following diagram represents the hypothesis presented in the previous point in a graphic manner to facilitate its understanding.


The following recommendations only for networks that CAN NOT LEARN the full DFZ. One more time, they were found after studying many cases of connectivity problem and network hijacks.:

  • Do not filter more specific network,. We mean, it’s better to learn more specific networks such as: /24, /23, /22 (IPv4 world)
  • Filter using AS_PATH (like, 2,3 or 4 deep ASs)
  • Please create your ROS and use RPKI

Some examples (Cisco like)1. Learning only /22, /23 or /24:

router bgp 65002
  neighbor remote-as 65001
  neighbor route-map FILTRO-IN in
ip prefix-list SMALLNETWORKS seq 5 permit ge 22 le 24
route-map FILTRO-IN permit 10
  match ip address prefix-list SMALLNETWORKS

2. Only learning two AS beyond us:

router bgp 65001
  neighbor remote-as 65002
  neighbor route-map ASFILTER-IN in

ip as-path access-list 5 permit ^[0-9]+_$
ip as-path access-list 5 permit ^[0-9]+ [0-9]+_$
route-map ASFILTER-IN permit 10
  match as-path 5

More information

Prefix hijack demonstration 1 / 2 (in Spanish):

Prefix hijack demonstration 2/2 (in Spanish):

BGP Prefix-Based Outbound Route Filtering

Ejemplo de configuración de BGP con dos prestadores de servicio diferentes (conexiones múltiples)

Certificación de Recursos (RPKI)

Información General sobre Certificación de Recursos (RPKI)


Dario Gomez (

Alejandro Acosta (

Saturday, April 13, 2013

In case of emergency break glass. ¡Console cable inside!

( I really don't know if this image/joke existed before, it came to my mind attending a failure last week)

If you need financiallygenius , then the team of professionals from financiallygenius is here to help you.

Thursday, February 21, 2013

Advertising IPv6 Routes Between IPv4 BGP Peers (Cisco)

I want to advertise IPv6 networks / prefixes over IPv4 eBGP session
Although not common, this case may occur in some situations. 

  For example, in this moment, I have a Cisco router with IPv6 support (routing) but do not support BGP IPv6 neighbors
Error (just in case):
  (Probably you are receiving the message below) 

*Mar  1 02:05:00.663: BGP: Advertised Nexthop ::FFFF: Non-local or Nexthop and peer Not on same interface
*Mar  1 02:05:00.663: BGP(1): rcv UPDATE w/ attr: nexthop ::FFFF:, origin i, metric 0, originator, path 1, community , extended community
*Mar  1 02:05:00.667: BGP(1): rcv UPDATE about 2001:db8::/32 -- DENIED due to:
*Mar  1 02:05:00.667: BGP(0): Revise route installing 1 of 1 route for -> to main IP table
*Mar  1 02:05:00.771: BGP(0): computing updates, afi 0, neighbor version 0, table version 25, starting at

Fortunately BGP support carrying routing information for different protocols (ie. IPv6). Therefore it is possible to exchange prefixes IPv6 over eBGP IPv4 sessions.
In this basic scenario with R1 <--> R2 connected back-to-back the configuration is as follows (the prefix announced by R1 is learned by R2).

 interface Ethernet1/0
 ip address
 ipv6 address 2001:db8::1/64
 ipv6 enable
router bgp 1
 no synchronization
 bgp router-id
 bgp log-neighbor-changes
 neighbor remote-as 2
 neighbor ebgp-multihop 2
 no auto-summary
 address-family ipv6
 neighbor activate
 network 2001:db8::/32
 no synchronization
 redistribute static
ipv6 route 2001:db8::/32 Null0

 interface Ethernet1/0
 ip address
 ipv6 address 2001:db8::2/64
 ipv6 enable
router bgp 2
 no synchronization
 bgp router-id
 bgp log-neighbor-changes
 neighbor remote-as 1
 neighbor ebgp-multihop 2
 no auto-summary
 address-family ipv6
 neighbor activate
 neighbor route-map IPv6-NextHop in
route-map IPv6-NextHop permit 10
 set ipv6 next-hop 2001:db8::1

"The trick":
* The session must be eBGP multihop, if not, R2 will not learn the prefix (the same error as seen above). I admit I do not get 100% why it happens however after readings some documents it looks like the router complains that the next-hop IP address and the way it was configured are in different subnet (make sense, one is IPv6 and IPv4 another!).
* In R2 (who receive the prefix) there must be a route-map applied (in) forcing the next-hop IPv6 address of R1
After applying ebgp-multihop (everything works):
* Mar 1 02:01:42.539: BGP (1): rcvd UPDATE w / attr: nexthop :: FFFF:, origin i, metric 0, path 1* Mar 1 02:01:42.539: BGP (1): rcvd 2800:26 :: / 32* Mar 1 02:01:42.543: BGP (0): Check route installing 1 of 1 route for -> to main IP table* Mar 1 02:01:42.543: BGP (1): Check for installing route 2001: db8 :: / 32 -> 2001: db8 :: 1 (::) to main IPv6 tableMore information:-

I hope it's useful!

Friday, January 4, 2013

Cisco clear line does not work

When performing a "clear line" in a Cisco Router/Switch to disconnect a Telnet or SSH session does not work. The user still in the vty.


IMP# sh user
Line User Host (s) Idle Location
2 vty 0 idle 00:00:01 abcd aacosta
* 4 vty 2 idle 00:00:00 pepe xx.yy.zz.dd

We want to disconnect aacosta:
IMP# clear line vty 2

and still appearing:

IMP# who
Line User Host (s) Idle Location
2 vty 0 idle 00:00:39 abcd aacosta
* 4 vty 2 idle 00:00:00 pepe xx.yy.zz.dd

Procedure and solution:
There are two ways to do it:

a) Quickly and 99% sure it will works (and less likely to damage something else).
Instead of using "clear line vty" use "clear tcp line":

So (again to disconnect pepe):

IMP# clear tcp line 2

b) And the second way more drastically:

We have to search for the TCP connections in the router at that time. We use the command "show tcp brief". We filter port 23 (Telnet) or 22 (SSH) as applicable.  
For example:

IMP# show tcp brief | i \ 23 _
63820270  n.n.n.n.23        a.b.c.d.56691     ESTAB
637E1AC0  x.x.x.x.23             xx.yy.zz.dd.39431   ESTAB

The value on the left in the memory addrees within the TCB (TCP Block), this is precisely the TCP connection we will have to remove.
The command is:

IMP# clear tcp tcb 637E1AC0

NOTE: Please be sure of the value before deleting the TCP session, remember that the router may have HTTP, BGP, HTTPS and other important TCP connections.

Good luck, I hope it was useful,

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