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Query on Transit AS


We have 2 router ( A & B) connected with different ISP ( C & D) via EBGP with different Public AS no provided by ISP

In between 2 router( A & B) we are running IBGP so that any ISP link fail, traffic can take IBGP path to internet via alternate ISP

How to ensure, that my IBGP Link / My Internal Private AS not become transit AS in between ISP C & D

What are the commands/area to check for the same & how to prevent the same

Also How to check in my ISP facing router whether I am getting Full internet Routing table or default table or partial routing table ?



Query on Transit AS


The easiest way not to become transit area is to filter all the outgoing announces.

One way could be to filter everything, but local prefixes.

Another one is to allow only locally originated:

ip as-path access-list 1 permit ^$

route-map ALLOW_LOCAL_ONLY permit 10

match as-path 1

router bgp xxx

neig a.b.c.d route-map ALLOW_LOCAL_ONLY out

If you have only 2 routers and you receive only default gateway (that is typical situation) and want them to run in active/passive mode, then it would be easier to:

  • run HSRP on internal interface;
  • track default route and decrement HSRP priority if it's not in RIB;
  • LAN should use static toward HSRP address.

So, no need for IBGP as additional troubleshooting area.

IBGP is worth to use if your ISP facing routers are far from each other or you are doing load-sharing/load-balance.

Super Bronze

Re: Query on Transit AS


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As Mikhail described, you just filter what you send to your eBGP peer.  (NB: Normally an ISP will filter what you provide them.  I.e. if you did try to act as a transit, the ISPs shoud precluded.  However, you still shouldn't send the them the transit routes.)

If you look at the routes received from you eBGP peers, you may be able to determine if you're getting the full Internet set of routes, or not.  The reason I say may, if the ISP filters out "some" (e.g. couple hundred or less) you may not be able to tell you're not receiving the the whole Internet route set.  Conversely, if you're only getting a default just some, you can tell your only working with partial.

If you have two different ISP, you can also compare what each is giving you.

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