Triple ISP with Cisco IOS XR

[Marc du]

Hi Cisco Experts

Greeting

I lost my way with RPL config , calling all of you to help  me  how the RPL config  looks like for the below scenario , 2 X  ASR 9006

0utbound policy
===============

1. AS 52X accepts local routes from the 3 providors , along with default for the rest of the internet routes
2. Traffic destined to ISP-1 goes via ASR-1
3. Traffic destined to ISP-2 goes via ASR-1
4. Traffic destined to ISP-3 goes via ASR-2
5. all other traffic prefer ASR-1 ISP-1
6. if link to ISP-1 goes down traffic should re-route it via ASR-1 ISP-2
7. if link to ISP-2 goes down traffic should re-route it via ASR-2 ISP-3



Inbound policy
==============

1. Traffic is destinet to PI Subnet 91.X.X.X/24 from Internet shoud come Via ASR-1 ISP-1
2. Traffic is destinet to PI Subnet 92.X.X.X/24 from Internet shoud come Via ASR-1 ISP-2
3. Traffic is destinet to PI Subnet 93.X.X.X/24 from Internet shoud come Via ASR-2 ISP-3
4. if one ISP or Link Fails , other ISP should route traffic back to AS 52X from internet

Thanks all

Sayed hasan moubarak

[Marc du]

Hi Alexander

thanks for ur reply

here you go for inbound policy result 

RP/0/0/CPU0:XR-1#   sh bgp 0.0.0.0/0
Mon Jun 20 23:16:22.089 UTC
BGP routing table entry for 0.0.0.0/0
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                 13          13
Last Modified: Jun 20 23:16:01.243 for 00:00:21
Paths: (2 available, best #1)
  Advertised to peers (in unique update groups):
    192.168.21.2    
  Path #1: Received by speaker 0
  Advertised to peers (in unique update groups):
    192.168.21.2    
  300
    192.168.31.3 from 192.168.31.3 (30.30.30.30)
      Origin IGP, localpref 250, valid, external, best, group-best, multipath
      Received Path ID 0, Local Path ID 1, version 13
      Origin-AS validity: not-found
  Path #2: Received by speaker 0
  Not advertised to any peer
  500
    192.168.51.2 from 192.168.51.2 (50.50.50.50)
      Origin IGP, localpref 250, valid, external, multipath
      Received Path ID 0, Local Path ID 0, version 0
      Origin-AS validity: not-found
RP/0/0/CPU0:XR-1#   

====================================================

Bro , what does  bgp bestpath as-path ignore and  bgp bestpath as-path multipath-relax , in my situation as my 1st link to ISP1 its 1GB while the link to ISP2 its 512

===================================================

bro so for as-path prepending the value should be different for each prefix per ISP

91.x.x.x came via ISP1

92.x.x.x came via ISP2

93.x.x.x came via ISP3

===============================================

if i  didn't use IP SLA , what would be the issue ?

thanks a lot

[xthuijs · Accepted solution]

that looks good now! multipath is enabled and both default routes will be used on a per flow bases. and if there is good distribution this will be near 50%.

if you want to take the bw into consideration and ratio it 2:1 you need to use dmz link bandwidth to help BGP understand the bandwidth of each link so instead of 50/50 we do 66/33. You can read here how to set up dmz link bw.

rule 5 of BGP best path selection is the as-path. the shorter path will win.

in many cases for loadbalancing over multipath, the different available paths have different lengths of the AS path showing a shorter AS hop count towards the destination. With the as-path ignore we can omit looking at the AS-PATH as a tie breaker for a preferred route.

the AS relax would override the stnadard rule for mpath that both AS-paths should match from all routes. this is also a silly rule, since like in your case you have different ISP's, and they will have different as-path sets and or lengts, each of those knobs will override either of these two rules for as-path consideration.

xander

[Marc du]

Hi Alexander

thanks for ur reply

1 ) How to verify " bgp bestpath as-path ignore " and "bgp bestpath as-path multipath-relax" are working ?

2)if i  didn't use IP SLA , what would be the issue ?

thanks a lot

[xthuijs]

1) if you do a show bgp <prefix>/<mask> and you see that the as-paths are different and the multipath indicator is not set, you knoww what the issue is :)

2) the ipsla helps with the conditional advertisement based on SP availability.

you don'tneed it, per-se, it is a nice to have.

cheers!!

xander

[Marc du]

Hi Alexander

thanks for ur reply

1 )for DMZ link bandwidth how the RPL config looks like 

ASR-1 Link to ISP-1 Bandwidth 1 GB

ASR-1 Link to ISP-2 Bandwidth 512 MB

both ISP1 and ISP2 connected to ASR-1 while ISP3 connected to ASR-2

2) "How to verify " bgp bestpath as-path ignore " if its working or not

Thanks a lot

[Marc du]

Hi Alexander

Bro can you please answer the above Question

thanks

[xthuijs]

for q1, follow this guide 

for q2, the only way to verify that is that if you have a prefix that has 2 paths for which the weight and local pref are the same, then AS-PATH is the next selection criteria. If you then see a path that is chosen that happens to have a longer as-path then you the as-path override kicked in. there is no indicator in show bgp that spells it out, so it comes down to manually evaluating 2 paths, and determine by running the best path algorithm manually what the selection would be.

Noting that in order the selection is: weight, local-pref, as-path, origin, med, e over i, igp next hop metric, oldest, routerID.

xander

[Marc du]

Thanks bro Alexander for your good Help