Tag: bgp

Selective BGP Dampening and parameters tuning

Some time ago, I wrote about BGP Dampening and how this feature can improve the stability of the network. A lot happened since then and during my experience with different service providers I have seen that BGP dampening can help in the same measure at it can harm your network. An endless discussion can be started on this topic, but this is not what I want to do here.

One thing that I did learn is that fine tuning of any feature can help a lot in some cases making the difference between stable network and a total disaster. In regard to BGP dampening, I have the following scenario. Imagine that you would like to use BGP dampening, but only for some networks, which are proven to be more stable than others. I will base my example on the following scenario:

Task

Router 1 in the above scenario has three networks that are advertised into BGP:
L0 – 1.1.1.0 /24
L1 – 11.11.11.0 /24
L2 – 111.111.111.0 /24
Imagine this are being remote networks and that 11.11.11.0 /24 is very unstable. To simulate an unstable network that triggers BGP, shut / no shut multiple times.

On Router 2, we want to use the BGP Dampening feature, but only for this network. We know already that we can do something like:

conf t
router bgp 200
bgp dampening

This will enable the BGP dampening feature. We can even fine tune some parameters like:

conf t
router bgp 200
bgp dampening 15 750 5000 30

In this way we increase the limit at which a route will be dampened (5000) and decrease the maximum dampening time to 30 minutes. Unfortunately this parameters are applied globally and all routes (stable and unstable) will play by this rules.

Solution

Going back to the idea of this post, use of selective BGP Dampening, we can configure Router 2 like this:

conf t
access-list 11 permit 11.11.11.0 0.0.0.255
!
route-map DAM permit 10
match ip address 11
set dampening 15 750 2000 60
!
route-map DAM deny 1000

What we just did is to match the unstable prefix in an ACL. Use the ACL in a route-map with permit policy. Mandatory set the dampening parameters. They can be the same as original values, but if you don’t set anything here, you will meet the following error when trying to apply the BGP dampening.

%BGP-3-BADROUTEMAP: Bad parameters in the route-map DAM applied for Dampening

At the end we have a deny policy in the same route-map to avoid matching any other prefixes. We can not apply it to BGP:

conf t
router bgp 200
bgp dampening route-map DAM

We want to check that BGP Dampening feature is activated:

R2#sh ip bgp dampening parameters
 dampening 15 750 2000 60 (route-map DAM 10)
  Half-life time      : 15 mins       Decay Time       : 2320 secs
  Max suppress penalty: 12000         Max suppress time: 60 mins
  Suppress penalty    :  2000         Reuse penalty    : 750

By the way, if you check the output immediately after applying the BGP dampening feature, you might see the following error:

% dampening reconfiguration in progress for IPv4 Unicast

Verification

Let us see if there are any flaps on going:

R2#sh ip bgp dampening flap-statistics 
 
R2#

Now we can shut / no shut L1 interface on R1 to create an instability of this network. After doing so couple of times we can see that the BGP dampening is active:

R2#sh ip bgp dampening flap-statistics | b Net
   Network          From            Flaps Duration Reuse    Path
 h 11.11.11.0/24    10.1.12.1       1     00:00:22          100

If we continue to play with shut / no shut, soon we will see that 11.11.11.0 /24 is marked as dampened:

R2#sh ip bgp dampening dampened-paths | b Net
   Network          From             Reuse    Path
*d 11.11.11.0/24    10.1.12.1        00:06:29 100 i

Now I want to prove that the same BGP dampening policies does NOT apply to other networks like 111.111.111.0 /24. I will try to play the same shut / no shut game with L2 on R1. After 5 minutes of this game I can see the following:

R2#sh ip bgp dampening dampened-paths | b Net
   Network          From             Reuse    Path
*d 11.11.11.0/24    10.1.12.1        00:02:09 100 i
 
R2#sh ip bgp dampening flap-statistics | b Net
   Network          From            Flaps Duration Reuse    Path
*d 11.11.11.0/24    10.1.12.1       3     00:07:51 00:01:49 100 
 
R2#sh ip bgp | b Net  
   Network          Next Hop            Metric LocPrf Weight Path
*> 2.2.2.0/24       0.0.0.0                  0         32768 i
*> 3.3.3.0/24       10.1.23.3                0             0 300 i
*d 11.11.11.0/24    10.1.12.1                0             0 100 i
*> 111.111.111.0/24 10.1.12.1                0             0 100 i

You can see that 111.111.111.0 /24 does not appear in any dampening report.

I tried this in multiple scenarios and every time I got the expected result. If you test this and get different results, please let me know in comments and we can discuss.


BGP Conditional Advertisement

BGP Conditional Advertisement – it let the impression of a very complex task. Actually you will find it very easy to implement once you understand it.

In simple words I would say  that BGP conditional advertisement it’s a feature that let you advertise a prefix to one of your neighbors if mandatory condition is satisfied. To implement this feature you must configure at least two of the following three maps:

– Advertise-map -> this contains the selected prefixes which will be advertised if the mandatory condition is satisfied
– Exist-map -> condition to be satisfied (e.g. advertise the prefixes in the “advertise-map” only if the prefixes in this condition exist)
– Non-exist-map -> condition to be satisfied (e.g. advertise the prefixes in the “advertise-map” only if the prefixes in this condition doesn’t exist)

Let’s take an example. We have the following topology:

We have here a simple topology, with eBGP peering between R1 – R2 and R2 – R3. Each router has a Loopback interface with the following IP addresses:

R1 – L0 – 1.1.1.1 /24
R2 -L0 – 2.2.2.2 /24
R3 – L0 – 3.3.3.3 /24

These interfaces are advertised into BGP and they have full reachability:

R2#sh ip bgp sum | b Nei
Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
10.10.12.1      4   100       5       4        0    0    0 00:01:27        1
10.10.23.3      4   300       2       2        0    0    0 00:00:24        0
 
R2#sh ip bgp | b Net
   Network          Next Hop            Metric LocPrf Weight Path
*> 1.1.1.0/24       10.10.12.1               0             0 100 i
*> 2.2.2.0/24       0.0.0.0                  0         32768 i
*> 3.3.3.0/24       10.10.23.3               0             0 300 i
 
R2#sh ip route bgp
     1.0.0.0/24 is subnetted, 1 subnets
B       1.1.1.0 [20/0] via 10.10.12.1, 00:04:02
     3.0.0.0/24 is subnetted, 1 subnets
B       3.3.3.0 [20/0] via 10.10.23.3, 00:04:02

Task

An easy task would ask you to solve something like if 1.1.1.0 /24 is NOT in the BGP table of R2, then R2 should advertise its 2.2.2.0 /24 to R3.

Solution

Let’s analyze this request a little bit. The prefix to be advertised or not, depending of the satisfaction of the condition, is 2.2.2.0 /24. According to what I said before about the necessary “maps” for BGP conditional advertisement, this prefix will go into the “advertise-map”. R2 is the only router you need to configure to accomplish this task.

conf t
access-list 2 permit 2.2.2.0 0.0.0.255
route-map ADVERTISE permit 10
match ip address 2

OK, we have the advertisement map. What about the condition? The task request that 2.2.2.0 /24 should be advertised if 1.1.1.0 / 24 does NOT exist in the BGP table of R2. Using logical deduction we can say that “non-exist-map” is what we need to configure.

conf t
access-list 1 permit 1.1.1.1 0.0.0.255
route-map NOT-EXIST permit 10
match ip address 1

Let’s add everything together in BGP:

conf t
router bgp 200
neighbor 10.10.23.3 advertise-map ADVERTISE non-exist-map NOT-EXIST

Verification

On R2 check the BGP table:

R2#sh ip bgp | b Net
   Network          Next Hop            Metric LocPrf Weight Path
*> 1.1.1.0/24       10.10.12.1               0             0 100 i
*> 2.2.2.0/24       0.0.0.0                  0         32768 i
*> 3.3.3.0/24       10.10.23.3               0             0 300 i

We have the 1.1.1.0 /24 prefix in the BGP table. According to our task, 2.2.2.0 /24 should be advertised if 1.1.1.0 /24 does NOT EXIST. By analogy, if 1.1.1.0 /24 EXIST, the 2.2.2.0 /24 should NOT be advertised. You see? It’s just a tricky words game.

R2#sh ip bgp neigh 10.10.23.3 adv | b Network
   Network          Next Hop            Metric LocPrf Weight Path
*> 1.1.1.0/24       10.10.12.1               0             0 100 i
 
Total number of prefixes 1

We advertise only one network. The 2.2.2.0 /24 is not advertised to R3:

R3#sh ip bgp | b Net
   Network          Next Hop            Metric LocPrf Weight Path
*> 1.1.1.0/24       10.10.23.2                             0 200 100 i
*> 3.3.3.0/24       0.0.0.0                  0         32768 i

To check that the conditional advertisement really works, stop R1 from announcing 1.1.1.0 /24 in BGP. You can just shutdown the interface.
Check if the L0 of R1 is in the routing table / BGP table of R2:

R2#sh ip bgp | b Net
   Network          Next Hop            Metric LocPrf Weight Path
*> 2.2.2.0/24       0.0.0.0                  0         32768 i
*> 3.3.3.0/24       10.10.23.3               0             0 300 i

1.1.1.0 /24 is not in the BGP table of R2, then 2.2.2.0 /24 should be advertised to R3:

R2#sh ip bgp neigh 10.10.23.3 adv | b Network
   Network          Next Hop            Metric LocPrf Weight Path
*> 2.2.2.0/24       0.0.0.0                  0         32768 i
 
Total number of prefixes 1

On R3:

R3#sh ip bgp | b Net
   Network          Next Hop            Metric LocPrf Weight Path
*> 2.2.2.0/24       10.10.23.2               0             0 200 i
*> 3.3.3.0/24       0.0.0.0                  0         32768 i

Remember that usually in the exams, the tasks related to BGP Conditional Advertisement are more word tricks than complex. As an example, base on the above topology, resolve the following:
– If 1.1.1.0 /24 and 2.2.2.0 /24 are both in the BGP table of R2, both prefixes should be advertised to R3
– If 1.1.1.0 /24 is not in the BGP table of R2, then 2.2.2.0 /24 should not be advertised to R3
– If 1.1.1.0 /24 and 2.2.2.0 /24 are not in the BGP table of R2, then none of them should be advertised to R3

If you don’t understand the trick, let me know and I will explain.


Cisco: 2-Bytes / 4-Bytes ASN BGP scenarios

In my last post I wrote some basics about the new 4-bytes AS number which is supported on new Cisco IOS images. Now I would like to give you a brief overview about different BGP peering scenarios that you can meet in the real world. Even if the configuration examples in this article are on a Cisco hardware with the latest IOS, the BGP scenarios can be applied to any other platform.

OK, let take the following topology (the high quality PDF file can be downloaded here):

2bytesAS-4bytesAS-BGP

You can see that I think of 3 main scenarios when it comes to have in the same topology 2-Bytes only and 4-Bytes ASN BGP routers:
– 2-Bytes support ASN peering with a 4-Bytes ASN, which has a number greater that 65535 (if you remember from the previous article, this can be wrote as ASPLAIN or ASDOT)
– 4-Bytes ASN peering with a router that support 4-Bytes ASN, but in fact has a number smaller than 65535 (e.g old telecom provider which has has the ASN already asigned as 3456)
– 2-Bytes ASN peering with the same 4-Bytes supported ASN, but with a number smaller than 65535, as in the above scenario

I chose this particular scenarios because it will be interesting to see how we configure a 2-Bytes ASN BGP router which has a 4-Bytes ASN peer or how the same 2-Bytes ASN BGP router will have the AS-Path containing the 4-Bytes ASN router.

For the R1 and R3 routers I will use a C3640 with an IOS image that support only 2-Bytes ASN, and for R2 and R3 a C7200 with the newest IOS image that can support 4-Bytes ASN. All routers have a Loopback interface that will be the BGP router-id an also announced into BGP (e.g. R2 will have interface Loopback100 with IP address 2.2.2.2/32). The AS number that I will use (ignore the fact that I use public ones, it’s just to be more easy to follow):

R1 – ASN 1111
R2 – ASN 2.2 (ASPLAIN 131074)
R3 – ASN 3333
R4 – ASN 4444

I will skip all the boring point-2-point configuration and present only the BGP config.

#### R1-C3600 ####

router bgp 1111
bgp router-id 1.1.1.1
neighbor 10.10.12.2 remote-as 23456
network 1.1.1.1 mask 255.255.255.255

As you can see in BGP peering between R1 and R2 we have to use the remote-as number 23456, due to the fact that R1 is not supporting 4-Bytes ASN. If you read my previous post, or from other sources, you know that any BGP router that does not support 4-Bytes ASN, will use a transitory number for peering with 4-Bytes ASN BGP routers and this number is 23456.

#### R2-C7200 ####

First let’s be sure that the IOS is supporting 4-Bytes ASN. Simplest way:

router bgp ?
<1-65535>    Autonomous system number
<1.0-XX.YY>  4 Octets Autonomous system number

I think it’s obvious that it supports. A small note. On all Cisco platforms (usually C3600, C7200) with IOS that I tried testing 4-Bytes ASN BGP, I saw that only ASDOT format is supported. I don’t know about other software platforms (like NX-OS) or something high-end hardware if they support ASPLAIN. Now for the configuration, R2 is peering with R1 and R3:

router bgp 2.2
bgp router-id 2.2.2.2
neighbor 10.10.12.1 remote-as 1111
neighbor 10.10.23.3 remote-as 3333
network 2.2.2.2 mask 255.255.255.255

#### R3-C7200 ####

R3 supports 4-Bytes ASN, but due to the fact that it’s and “old telecom provider” it had been assigned a 2-Bytes ASN, so I will preserve this form:

router bgp 3333
bgp router-id 3.3.3.3
neighbor 10.10.23.2 remote-as 2.2
neighbor 10.10.34.4 remote-as 4444
network 3.3.3.3 mask 255.255.255.255

The difference between R3 and R1 is that R3 support for “remote-as” a 4-bytes ASN number (again only ASDOT representation). So, in it’s peering with R2, the remote ASN is 2.2

#### R4-C3600 ####

R4 only support 2-Bytes ASN, but the difference from R1 is that it has to peer with a 4-Bytes supported ASN router that in fact has a 2-Bytes AS number.

router bgp 4444
bgp router-id 4.4.4.4
neighbor 10.10.34.3 remote-as 3333
network 4.4.4.4 mask 255.255.255.255

This is the basic configuration in this topology. Let’s take now the IP address 1.1.1.1 which is advertised into BGP from R1. If you will check the AS-Path on R4 and R3, you will see a big difference. Have a look into the screen capture below to see the real environment for the above topology:

2b4b-bgp

4-bytes Autonomous System Number

Last week I received a form from APNIC with a new AS numbers. When I had a look through papers I saw there something strange: AS 123456 (I replaced the original with this number). 6 digits. First I thought that there is a mistake or something, then I recall the new 4-bytes ASN. If for IPv6 the things seems to be moving slower, than for the new format of AS numbers, it seems that the things are going faster. So faster that by January 1, 2010 all BGP speaker must support this feature, according to Cisco. I didn’t understood if they refer in the document for their products or it is something that is mandatory globally. No matter how, the things are moving quite fast in this direction.

Since I have to implement a BGP configuration with this 4-Bytes ASN, I started to search with Google friend about the standards and I was surprised that there is not to much to search after. Of course there is the official RFC, some other documentation, but not real examples how to configure, troubleshoot and so on. That’s why I said it’s nice to put something together for a general understanding of what is and how does it work this 4-Bytes ASN. I assume here that reader has a basic understanding of what ASN and BGP is.

RFC 4893 is the reference for “BGP Support for Four-octet AS Number Space”. Currently the Autonomous System number is encoded as a two-octet (2-bytes) entity in BGP, meaning 16bits and this was defined in RFC 4271. The new system is using a four-octet (4-bytes) , meaning 32bits. Currently the ASN 2-bytes include a range from 1 – 65535, used in decimal plain text when configuring the BGP. The expansion from 2-bytes to 4-bytes give us 4,294,967,295 AS number which can be written in ASPLAIN or ASDOT format.

Why two formats? Mainly due to different opinions about how the 4-bytes number should be represented:

ASPLAIN representation

The RIPE NCC assigns and registers 4-byte AS Numbers in ASPLAIN format.
ASPLAIN defines the 4-byte AS Number as a basic 32-bit integer.
“2-byte only AS Numbers” refers to AS Numbers in the range 0 – 65535
“4-byte only AS Numbers” refers to AS Numbers in the range 65536 – 4294967295”
“4-byte AS Numbers” refers to AS Numbers in the range 0 – 4294967295
Advantages:
– IETF preferred notation
– continuation on how a 2-Byte AS number has been represented historically
– does not break AS-PATH REGEX
– APNIC reached consensus to adopt ASPLAIN for assignment and representation of 4-byte AS Numbers
– routers vendors appear to be supporting ASPLAIN, which will require no conversion from allocation to configuration
Disadvantages:
– long number to remember
– All existing 4-byte only assignments have been made in ASDOT

ASDOT representation

The full binary 4-byte AS number is split two words of 16 bits each. It is proposed to identify 4-byte AS Numbers using a syntax of <high
order 16 bit value in decimal>.<low order 16 bit value in decimal>:
“2-byte only AS Numbers” refers to AS Numbers in the range 0 – 65535
”4-byte only AS Numbers” refers to AS Numbers in the range 1.0 – 65535.65535
“4-byte AS Numbers” refers to AS Numbers in the range 0.0 – 65535.65535
Advantages:
– easy to read and remember
Disadvantages
-require conversion from ASPLAIN to ASDOT
-hard for regular expressions

What’s happening if in a BGP peering one router supports the new format and the other one only the old one.  The new reserved ASN 23456 is used for backward compatibility between 4-bytes and 2-bytes BGP speakers. So, if your router advertise BGP with a 4-bytes as number (doesn’t matter in which representation ASDOT or ASPLAN), the peer which does not support the new format, will translate the 4-bytes ASN into 2-bytes ASN 23456. A graphical representation of the AS path from 4-bytes to 2-bytes in BGP would be:
4bytes-2bytes-as

OK, I hope you understand the basics of 4-bytes ASN. For me, some challenge was to understand to transform the 4-bytes ASN from ASPLAIN to ASDOT. In every document that I saw on the Internet there was the same example: AS 65546 in ASPLAIN is 1.10 in ASDOT, but without no explanation. See below how I understood that the conversion takes place. If I understood it wrong, please let me know, to correct it here. After all I’m not an 4-byte ASN expert, I just try to help as much as I can.

So let’s take the number 65546.

1. 65546 / 65535 = 1 (integer) which will be the parte in front of the . (dot) in ASDOT representation.

2. 65546 – ( 65535 * 1) = 11 (see how much rest remains after 65353 going once in 65546)

3. 11 – 1 = 10 which will be the part after . (dot)

4. You obtain 1.10

As a general rule, you have an ASPLAIN number. You take 65535 and see how many times it goes, with integer in the ASPLAIN number (1 time, 2 times, 3 times…depending). This will be your decimal number before dot in ASDOT format . Then you multiple the 65535 with the integer obtained in the first step and you deduct from the ASPLAIN number. From the rest after the second operation, you deduct the the decimal you have in front of the dot. The rest in decimal that comes after the dot .

What confused me is that the number in the example was so close to the last 2-bytes ASN which is 65535.

Now for the last example, let me take a random higher number 194534 (the example from the first line) and to obtain the ASDOT format.

1. 194534 / 65535 = 2 (integer)

2 194534 – ( 65535 * 2) = 194534 – 131070 = 63464

3 63464 – 2 = 63462

4 ASDOT = 2.63462

An online converter from ASPLAIN do ASDOT you can find here: http://as4.nullroute.se/index.php

For some more detailed explanation I would like to ask you to download the Cisco and Juniper documents regarding 4-bytes ASN implementation in BGP.

If you have any useful information about this topic or if something is wrong in my post, please comment and share your knowledge.

Cisco Security Advisory: Cisco IOS XR Software Border Gateway Protocol Vulnerabilities

Official document on Cisco Security Advisory website

Summary

Cisco IOS XR Software contains multiple vulnerabilities in the Border Gateway Protocol (BGP) feature. These vulnerabilities include:

Cisco IOS XR Software will reset a BGP peering session when receiving a specific invalid BGP update.

The vulnerability manifests when a BGP peer announces a prefix with a specific invalid attribute. On receipt of this prefix, the Cisco IOS XR device will restart the peering session by sending a notification. The peering session will flap until the sender stops sending the invalid/corrupt update. This vulnerability was disclosed in revision 1.0 of this advisory.

Cisco IOS XR BGP process will crash when sending a long length BGP update message.

When Cisco IOS XR sends a long length BGP update message, the BGP process may crash. The number of AS numbers required to exceed the total/maximum length of update message and cause the crash are well above normal limits seen within production environments.

Cisco IOS XR BGP process will crash when constructing a BGP update with a large number of AS prepends.

If the Cisco IOS XR BGP process is configured to prepend a very large number of Autonomous System (AS) Numbers to the AS path, the BGP process will crash. The number of AS numbers required to be prepended and cause the crash are well above normal limits seen within production environments.All three vulnerabilities are different vulnerabilities from what was disclosed in the Cisco Security Advisory “Cisco IOS Software Border Gateway Protocol 4-Byte Autonomous System Number Vulnerabilities” on the 2009 July 29 1600 UTC.

Affected Products

The “Cisco IOS XR Software will reset a BGP peering session when receiving a specific invalid BGP update” vulnerability affects all Cisco IOS XR Software devices after and including software release 3.4.0 configured with BGP routing.

The other two vulnerabilities affect all Cisco IOS XR Software devices configured with BGP routing.

Workarounds

Cisco IOS XR Software will reset a BGP peering session when receiving a specific invalid BGP update.
There are no workarounds on the affected device itself. Co-ordination is required with the peering neighbor support staff to filter the invalid update on their outbound path. The following procedure explains how to help mitigate this vulnerability:

Using the peer IP address in the log message that was generated when the Cisco IOS XR Software device received the invalid update; capture the notification message hex dump from the CLI command show bgp neighbor and contact the Cisco TAC, who can assist with a decode. Details on how to contact Cisco TAC are contained within the “Obtaining Fixed Software” section of this advisory.

For illustrative purposes, the following example shows a log message generated by an affected device when it receives an invalid/corrupt update message:

RP/0/RP0/CPU0:Aug 17 13:47:05.896 GMT: bgp[122]: %ROUTING-BGP-5-ADJCHANGE : neighbor 192.168.0.1 Down - BGP Notification sent: invalid or corrupt AS path

These details can be captured and provided to Cisco TAC to decode the update message. show bgp neighbors [ip address of neighbor from above log message]:

RP/0/RP0/CPU0:CRS#show bgp neighbors 192.168.0.1        

<capture output and provide to Cisco TAC>

Working with Cisco TAC, the decode of the above will display the AS path in a manner illustrated below.

ATTRIBUTE NAME:  AS_PATH

 AS_PATH: Type 2 is AS_SEQUENCE
 AS_PATH: Segment Length is 4 (0x04) segments long
 AS_PATH: 65533  65532 65531 65531

Working cooperatively with your peering partner, request that they filter outbound prefix advertisements from the identified source AS (in this example 65531) for your peering session. The filters configuration methods will vary depending on the routing device operating system used. For Cisco IOS XR Software the filters will be applied using Routing Policy Language (RPL) policies or with Cisco IOS Software via applying route-maps that deny advertisements matching that AS in their AS-PATH. Once these policies are applied, the peering session will be re-established.

For further information on Cisco IOS XR RPL consult the document “Implementing Routing Policy on Cisco IOS XR Software” at the following link: http://www.cisco.com/en/US/docs/ios_xr_sw/iosxr_r3.0/routing/configuration/guide/rc3rpl.html#wp1118699.

For further information on Cisco IOS route maps with BGP, consult the document “Cisco IOS BGP Configuration Guide, Release 12.4T” at the following link: http://www.cisco.com/en/US/docs/ios/12_2sr/12_2srb/feature/guide/tbgp_c.html.

Cisco IOS XR BGP process will crash when sending a long length BGP update message
While the long length BGP update message can be caused by any number of attributes, then most common would be the AS Path. Limiting the number of AS numbers in the AS Path Attribute should mitigate this vulnerability. The following shows an example of filtering on AS paths within Cisco IOS XR Software:

route-policy maxas-limit
# Check number of AS Numbers in AS Path attribute.
# If greater than 100 drop the update.
# If less than 100 pass the update.
  if as-path length ge 100 then
        drop
  else
        pass
  endif
end-policy

router bgp 65533
 neighbor 192.168.0.1
  remote-as 65534
   address-family ipv4 unicast
     policy maxas-limit in
     policy maxas-limit out

For further information on Cisco IOS XR RPL consult the document “Implementing Routing Policy on Cisco IOS XR Software” at the following link: http://www.cisco.com/en/US/docs/ios_xr_sw/iosxr_r3.0/routing/configuration/guide/rc3rpl.html#wp1118699.

Cisco IOS XR BGP process will crash when constructing a BGP update with a large number of AS prepends
There is no workaround for this vulnerability, other than reducing the number of AS path prepends configured within Cisco IOS XR.

I must admit that after reading the document above, a big question sign appeared in my head about the BGP functionality on IOS XR and if we can rely on this in a working environment. I’m sure that Cisco will mitigate this issues as the currently workarounds are just a temporary solution.